Gt Hoa Phan Tich

I HC THI NGUYN KHOA KHOA HC T NHIN V X HI

GIO TRNH HA HC PHN TCH

(DNG CHO CC H KHNG CHUYN HA)

TS. Nguyn ng c

Thi Nguyn, thng 5 nm 2008

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DANH MC CH VIT TT

1. AAS: Php phn tch ph hp th nguyn t 2. CBHH: Cn bng ha hc 3. LTDKL: nh lut tc dng khi lng 4. HCL: n canh rng (Holow Cathod Lamp) 5. HSKB: Hng s khng bn 6. TTCB: Trng thi cn bng

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M U

Ha hc Phn tch l mt mn khoa hc c lp, n l chuyn ngnh ring ca Ha hc. Trong ha hc gm c 4 chuyn ngnh: ha V c, ha Hu c, ha phn tch, Ha l th ha Phn tch ng vai tr quan trng v n l mt mn ha hc thc nghim c xy dng trn nn tng ca ha hc V c ha Hu c v Ha l, n gm c phn tch nh tnh v phn tch nh lng. Phn tch nh tnh lm nhim v pht hin thnh phn nh tnh (s c mt) ca cc cht hay hn hp cc cht, cn phn tch nh lng lm nhim v xc nh hm lng c th ca cht c trong mu phn tch (thng tnh thnh phn trm).

gii quyt nhim v ca phn tch nh tnh ngi ta thng dng hai phng php phn tch ha hc nh: phng php H2S, phng php Axit - baz hoc cc phng php phn tch ha l: phn tch ph pht x nguyn t, phn tch hunh quang, phng php quang k ngn la... gii quyt nhim v ca phn tch nh lng ngi ta cng dng cc phng php phn tch ha hc: phn tch khi lng, phn tch th tch hoc cc phng php phn tch ha l: o mu, phn tch ph hp th nguyn t, cc phng php phn tch in ha, cc phng php phn tch sc k...

Ha hc phn tch rt quan trng khng nhng trong cc ngnh Ha hc ni ring m cn trong cc ngnh Sinh hc ni chung: Y hc, Nng nghip, Lm nghip, Dc hc... Chnh v vy ngghen tng ni: Khng c phn tch th khng th tng hp.

V quan trng nh vy nn mi sinh vin mun hc tt mn hc ny phi hc tt cc mn: Ha i cng, ha V c, ha Hu c v Ha l, v cc mn ny c s cho mn ha hc Phn tch.

phn tch mt i tng no , ngi lm phn tch phi thc hn cc bc sau:

1. Xc nh cc vn cn gii quyt chn phng php phn tch thch hp.

2. Chn mu i din v chuyn mu t dng rn sang dung dch. 3. Tch cc cht, l cng vic cn thit xc nh i tng chnh trong

mu c chn lc v chnh xc cao. 4. Tin hnh nh lng cc cht bng phng php phn tch chn trn. 5. Tnh ton nh gi tin cy ca n. Chng ti son gio trnh ny nhm mc ch trang b cho sinh vin cc ngnh:

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Sinh hc, Khoa hc Mi trng, Y hc, Nng hc c kin thc c bn nht v Ha phn tch, gip cho h c vn kin thc trong qu trnh hc tp trn gh nh trng cng nh sau khi ra trng c th bt tay vo cng vic chuyn mn ca h, iu kin lm vic vi nhng cng vic lin quan n ha Phn tch.

Khi bin son gio trnh ny khng th trnh khi nhng thiu st. Chng ti mong c s ng gp kin ca cc bn ng nghip v cc em sinh vin.

Thng 5 nm 2008

Tc gi

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PHN TH NHT C S L THUYT HA PHN TCH

CHNG 1: DUNG DCH CHT IN LY - CN BNG HA HC

1.1. CHT IN LY V S IN LY

1.1.1. nh ngha v s in ly v cht in ly

- S in ly l qu trnh phn t phn ly thnh ion cn cht in li l cht c kh nng phn li thnh ion khi ha tan vo nc lm cho dung dch dn c in.

V d: NaCl, HCl ha tan trong nc hay trong dung mi phn cc khc.

V d:

Tng qut cho mt cht in ly c cng thc AmBn th:

s

1.1.2. Cht in ly mnh, cht in ly yu

1.1.2.1. Khi nim

Cht in ly mnh thc t phn ly hon ton. a s cc mui tan (NaCl, KCl, NaNO3, K2SO4, Na2CO3).Cc kim mnh (KOH, NaOH) u thuc loi ny.

Cc cht in ly yu trong dung dch phn ly khng hon ton (dung dch NH3,CH3COOH, HCOOH, dung dch axit ccbonic....).

Trong dung dch cht in ly mnh nng ln c dn in nh, dn in tng khi pha long dung dch. Dung dch cht in ly yu c dn in nh v nng ln th dn in khc nhau khng ng k nhng khi pha long dung dch dn in tng ln mnh.

1..2.2. Cc i lng c trng cho s in li

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c trng cho kh nng phn ly ca cc cht trong dung dch, ngi ta dng hai i lng l in ly v hng s in ly.

a. in ly l t s gia phn nng in ly v phn nng ban u.

T gi tr ngi ta tm phn loi:

2%: Cht in ly yu (cc axit yu, cc baz yu).

2% 30%: Cht in ly trung bnh (HF, H SO3 nc 1).

30%: Cht in ly mnh cc axit mnh, cc baz mnh, cc mui trung tnh.

b. Hng s in ly (K): Thc cht l hng s cn bng ca phn ng phn ly, l t s gia phn tch s nng ca sn phm in ly v phn tch s nng cha in ly.

Ngi ta chng minh c rng: gia v K c mi quan h vi nhau qua

h thc.

Trong C l nng ban u ca cht in ly.

T ta thy in ly t l nghch vi nng , nng cng cao, in ly cng gim v ngc li.

1.2. CN BNG HA HC

1.2.1. Trng thi cn bng

Gi s ta c cn bng:

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Khi thm 2 milimol Fe2+ vo 1 lt dng dch cha 1 milimol I3-, mu gim nhanh, ngha l phn ng xy ra theo chiu nghch.

Ngc li khi thm 2 milimol Fe3+ vo 3 milimol I- th mu tng ln, phn

ng xy ra theo chiu thun.

V d dn ra ch r mi quan h v nng cc cht trng thi cn bng ha hc. Mi quan h ny bin i di tc dng ca mt s yu t nhit , p sut nng (theo nguyn l chuyn dch cn bng Lsatlie). Mt trng thi cn bng c c trng bng 1 hng s cn bng.

1.2.2. Cc phng php biu din hng s cn bng.

Gi s c cn bng ha hc

Gi v1 l tc ca phn ng thun v v2 l tc phn ng nghch.

Theo nh lut tc dng khi lng v tc phn ng ta c:

Trong k1, k2 l hng s tc ca phn ng thun v nghch. Ti trng thi

cn bng ca phn ng ta c v1 = v2 hay qQpP2nNmM1 .C.Ck.C.Ck =

Ngi ta gi K l hng s cn bng ca phn ng, k hiu l KC. phn bit

nng cc cht trng thi cn bng v nng cc cht trng thi bt k, ngi ta k hiu nng cc cht trng thi cn bng qua du mc vung [ ].

Do :

Nu phn ng trn l ca cc cht kh v gi pM, pN, pP, pQ l p sut ring phn

ca cc cht M, N, P, Q th ngi ta cn chng minh c hng s cn bng tnh theo p sut (Kp).

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Nu phn ng M, N, P, Q thc hin trong dung dch v gi NM, NN, NP, NQ l

nng phn mi ca cc cht M, N, P, Q th ta c:

Gia KC v KP v KN c mi quan h vi nhau nh sau:

Trong n = (nP+ nQ) (nM + nN) v ch p dng c khi M, N, P, Q l cc

cht kh.

Ngoi cc cch biu din trn y, ngi ta cn biu din hng s cn bng thng qua cc hm nhit ng.

V d: vi phn ng:

Bit H0 v S0 phn ng, cho nhit phn ng l T. Gi G l th ng

nhit ng p ca phn ng th:

trng thi cn bng:

1.2.3. Biu din nh lut tc dng khi lng i vi mt s dng cn bng thng gp

1.2.3.1. Cn bng axt - baz

Cn bng phn ly ca Axt:

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Kt gi l hng s phn ly axit (gi tt l hng s axit).

Cn bng phn li ca baz:

Kb gi l hng s phn li baz (hng s baz).

1.2.3.2. Cn bng to phc

1, 2 l hng s to thnh tng nc ca cc phc cht Ag(NH3)+ v Ag(NH3)2+

1.1, 1.2 l hng s to thnh tng hp ca cc phc cht Fe(OH)2+ v Fe(OH)2+

1.2.3.3. Cn bng to thnh hp cht tan

Ts l tch s tan ca AgCl

1.2.3.4. Cn bng phn b

KD gi l hng s phn b ca I2

1.2.3.5. Cn bng ha tan ca cht kh - nh lut Henri

Khi p sut ring phn ca kh bng 1 at th hng s Henri K chnh l tan

ca kh.

1.2.4. T hp cn bng

Trong cc bng tra cu v cn bng ngi ta cho gi tr hng s ca cc cn

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bng n gin. Trong thc t chng ta thng gp cc cn bng phc tp c t hp t cc cn bng ring l. Sau y l mt s v d t hp.

1.2.4.1. Biu din cn bng theo chiu nghch

Qu trnh thun:

Qu trnh nghch:

Nh vy hng s cn bng ca qu trnh nghch bng gi tr nghch o hng s

ca cn bng ca qu trnh thun.

V d:

Nh vy, 1 c gi l hng s bn ca phc v K, c gi l hng s khng

bn ca phc.

1.2.4.2. Cng cn bng

Cho:

Tnh hng s cn bng ca M + 2A ' MA2 1.2 (c) Cn bng (c) thu c khi ta cng v theo v ca cc cn bng (a), (b), ng

thi loi b s hng MA cng c mt hai v.

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Hng s cn bng t hp thu c khi cng cc cn bng vi nhau bng tch

cc hng s cn bng ring l.

1.2.5. Tnh gn ng khi h ch c mt cn bng ch yu

Trong trng hp n gin thng gp ch c mt cn bng duy nht th c th t hp nh lut bo ton nng vi nh lut tc dng khi lng (LTDKL) tnh nng cn bng cc cu t.

V d l: Trong dung dch MX c nng C mol/1 ch c cn bng.

Theo nh lut bo ton nng i vi MX, ta c:

Theo biu thc LTDKL p dng cho (a) ta c:

Thay (c) v (e) vo (d) ta c:

Gii phng trnh (g) cho php nh gi X v t (c), (e) ta tnh c nng

cn bng cc cu t trong dung dch.

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cho tin ta ghi s tnh ton nh sau:

Cn bng: MX ' Mn+ + Xn- (K) Nng ban u (mol/1) C O O

Nng phn li: (C) (mol/l) - x + x + x

Nng ln: ([ ]) (mol/l) c - x x x

Theo LTDKL:

Nu x

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1.3. HOT

Theo thuyt v cht in li mnh ca bai - Huken, vi nhng dung dch long v rt long khong cch gia cc con cch xa nhau, lc tng tc gia cc ion l khng ng k, c tnh bng cng thc:

Trong F l lc ht gia cc con, ql v q2 l din tch hai ion, r l khong

cch gia cc ion, E l hng s in mi ca dung mi.

Cn vi dung dch m c hn, xung quanh mt cation cn c nhiu anion v ngc li hin tng bao bc ny to thnh lp "kh quyn" ion:

Do lc tng tc F gia cc ion b gim hn v c hiu ng chn gia cc

con vi nhau v nng th hin hot tnh ha hc ca chng b gim xung v nng c hot tnh ha hc thc s ny c gi l nng hot ng, gi tt l hot v k hiu l a(active concentration).

Nh vy a = f.c, trong f c gi l h s hot , v a < c nn f < 1.

Da trn l thuyt ng hc phn t, nh lut phn b Bolztman, cc ng thit lp phng trnh tnh f:

Trong Zi l in tch ln th i, l lc ion.

Lc ion t c tnh theo cng thc.

Trong : i l ion th i; n l tng s ion c mt trong dung dch.

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Nu 0,01 0,2. Ta tnh lgf theo (1.12).

V d: Tnh hot ca ion Ca2+ trong dung dch bao gm CaCl2 0,01M v NaNO3 0,01M.

Gii:

Tnh h s hot f:

1.4. CC LOI PHN NG S DNG TRONG PHN TCH TH TCH

1.4.1. Phn ng trung ha

- Axit mnh v baz mnh:

- Axt yu v baz mnh:

- Axt mnh v baz yu:

- Baz mnh v axit a chc:

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1.4.2. Phn ng oxy ha kh

1.4.3. Phn ng kt ta

1.4.4. Phn ng to phc

1.5. NNG DUNG DCH.

1.5.1. Cc loi nng dng trong phn tch th tch

Nng dung dch l: lng cht tan trong mt n v th tch hay mt n v khi lng.

1.5.1.1. Nng % khi lng (C%) ch ra c bao nhiu gam cht tan trong lng dung dch. V d dung dch NaOH 40% tc l trong lng dung dch NaOH c 40g NaOH nguyn cht.

Trong a l s gam cht tan v b l s gam dung mi.

1.5.1.2. Nng mol /l (gi l nng mol, k hiu l M) ch ra c bao nhiu mt cht tan trong 1 lt dung dch. V d dung dch H2SO4 0,5M ngha l trong 1 lt dung dch H2SO4 c 0,5mol axt:

Trong a l s gam cht tan, p l khi lng v V l s lt dung dch.

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1.5.1.3. Nng ng lng gam/ lt, cn gi l nng ng lng, k hiu l N ch ra c bao nhiu ng lng gam cht tan trong mt lt dung dch. V d dung dch KMnO4 0,1N ngha l trong 1 lt dung dch c 0,1 ng lng KMnO4.

Trong a l s gam cht tan, D l ng lng gam v V l s lt dung dch.

1.5.1.4. chun T l s gam (hay miligam) cht tan trong 1mililt dung dch.

1.5.1.5. chun (A) theo cht c xc nh (cht nh phn). TNB ch ra c bao nhiu gam cht nh phn B tc dng vi lu dung dch chun A. V d, /ClAgNO3T =

0,004 (g/ml) ngha l 0,004g cht nh phn Cl- tc dng va vi lu dung dch chun AgNO3.

1.5.1.6. Mi lin h gia cc loi nng :

Nu gi nng % ca dung dch l C%, khi lng ring ca dung dch l d, P l phn t gam cht tan, D l ng lng gam cht tan, ta c:

1.5.2. Cc bi ton v nng dung dch

1.5.2.1. Pha ch dung dch t lng mui rn tinh th

V d: Hy pha 2 lt dung dch Axt oxalic 0,1N t Axt tinh khit H2C2O4.2H2O.

p dng cng thc:

Ta tnh ra lng gam axt cn thit pha dung dch l:

a = 0,1.63.2 = 12,6g

Vy, ta cn chnh xc 12,6g axit tinh khit, ha tan hon ton vo nc ri chuyn ton b lng dung dch ny sang bnh nh mc 2 lt, nh trng cc ha tan axt ba ln, mi ln 10ml H2O, ht vo bnh 2 lt. Cui cng thm nc n vch mc, o u.

1.5.2.2. Pha ch dung dch t dung dch gc (quy tc ng cho)

a. Vi nng phn trm: Gi s phi trn dung dch 1 c nng % l C1, th tch V1, khi lng ring l d1 vi dung dch 2 c nng % l C2, th tch V2,

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khi lng ring l d2 c dung dch c nng %C2. Ngi ta chng minh c:

Cc v d:

1. Trn 500ml dung dch HNO3 30% vi 500ml dung dch HNO3 10% thu c dung dch HNO3 bao nhiu % (cho d1 = 1,2g/ml, d2 = l,05g/ml).

p dng (1.19) ta c:

2. Cn bao nhiu ml dung dch HNO3 25% (d = 1,19) trn vi dung dch HNO3

5 %(d = 1,04) c 1 lt dung dch HNO3 10%

p dng (1.19):

Hy pha 2 lt dung dch HCl 0% t axt c 36% (d = 1,19 g/ml).

y l trng hp pha long dung dch trong dung dch th 2 l H2O c nng % bng khng v khi lng ring l 1 g/ml. p dng (1.19) ta c:

Vy ta ong 448ml axt c, chuyn ton b sang bnh 2 lt, trng bnh bng

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nc ct v thm nc ct n vch mc, ri y kn np bnh, o i o li trn u dung dch.

b. Vi nng M, N.

Gi s phi trn V1 ml dung dch th nht c nng mi l M1 (hoc nng ng lng N1) vi V2 ml dung dch th hai c nng mi l M2 (hoc nng N2) c dung dch c nng l M (hoc N) th tch l V = V1 + V2 (ml). Khi ta c:

Cc V d:

1. Pha 2 lt dung dch H2SO4 0,5 M vi 1 lt dung dch H2SO4 0,1 M c dung dch bao nhiu M.

p dng cng thc (1.20) thay s vo ta c:

2. Cn bao nhiu mililt dung dch NaOH 0,5N trn vi 1 lt dung dch NaOH

0, l c 2 lt dung dch NaOH 0,2N. p dng (1.20) ta c:

V1 = 600ml

3. Pha 2 lt dung dch HCl 0,1N t axt HCl 0,5M. y cng l s pha long dung dch trong nc l dung dch th hai c nng

bng khng. Do p dng (1.20) ta c:

Vy cn ong chnh xc 0,4 lt dung dch HCl 0,5M, chuyn ton b sang bnh

nh mc 2 lt trng bnh chuyn ht HCl sang bnh nh mc thm nc n vch mc, o u bng cch y np kn vo o ln o xung.

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CU HI - BI TP CHNG 1

1.1. Vit phng trnh phn ng dng ion thu gn xy ra (nu c) trong cc dung dch nc sau:

1.2. Trn 20,00 ml dung dch Ba(OH)2 0,0211M vi 40ml dung dch Na2SO4

0,0153M. Hy Cho bit nng gc, nng ban u v nng cn bng ca cc ion trong dung dch (coi phn ng xy ra hon ton).

1.3. Vit biu thc nh lut tc dng khi lng i vi cc cn bng sau:

1.4. Cho bit nng cn li ca dung dch H2SO4.

1.5. Hy pha 2 lt dung dch H2SO4 0,1N t axt c 98% (d= 1,84g/ml).

1.6. Tnh lc ion ca dung dch gm NaNO3 0,01M, NaOH 0,02M v BaCl2 0,005M.

1.7. Tnh h s hot ca ion OH- trong dung dch NaOH 0,001 M v NaCl 0,009M.

1.8. Tnh in li ca dung dch HCN 0,10-4M, cho Ka = 10-9.35.

1.9. Tnh nng cn bng ca cc ion trong dung dch gm Fe3+ 0,1 M, H+ 1,00M v I- 0,100M. Bit rng cn bng ch yu xy ra:

1.10. Tnh nng cn bng trong dung dch gm CH3COOH 0,10M v

CH3COONa 1,00M. Bit cn bng ch yu xy ra trong dung dch:

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CHNG 2: PHN NG AXIT - BAZ

2.1. AXT BAZ

2.1.1. Quan im axit v baz ca Bronsted - Laury

Axit l cht cho proton, baz l cht nhn proton.

Axt:

Baz:

Trung tnh:

Lng tnh:

Ngi ta chng minh c [H+][OH-] = [H3O+][OH-] = 10-14 250C theo

qui c quc t: pH = - lg(H+) v POH = lg(OH-) (2.1)

2.1.2. nh ngha phn ng axit - baz.

Phn ng axit - baz l phn ng gia mt axit v mt baz l nhng phn ng trung ha nghin cu trn y, to ra sn phm l mt mui axit hoc trung tnh v nc.

mnh axit c c trng bng hng s phn li. Khi mnh baz c

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c trng bng hng s phn li Kb.

Trong dung dch nc bao gi cng tn ti mt cp axt baz lin hp,

Cp: V: Cp:

c gi l cc cp axt - baz lin hp. Vi 1 cp axt baz lin hp nu axt

cng mnh th baz cng yu v ngc li. Mi quan h gia Ka v Kb cng c biu din bng h thc sau:

2.2. PHNG TRNH BO TON PROTON

Phng trnh bo ton proton ca mt dung dch axit - baz l phng trnh biu din s trao i proton ca cc axit baz c trong dung dch qua cc nng cn bng ca cc cu t trong dung dch: s mol proton cc axit cho lun bng s mol proton cc baz nhn. Sau y l mt s th d:

V d l: Phng trnh bo ton proton ca nc nguyn cht.

Trong nc nguyn cht ch c mt cn bng trao i proton: H3O v H+.

S mol proton nc nhn l [H3O+] bng s mol proton [OH-] cho v 1 phn t

H2O khi cho i 1 proton th bin thnh ion OH- nn phng trnh bo ton proton l [H3O+] = [OH-]

V d 2: Xt dung dch HCl nng C mol/l.

Trong dung dch c hai axit l HCl v H2O v 1 baz l H2O. Trong xy ra cc qu trnh sau:

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S mol proton H2O nhn bng s mol proton HCl cho v H2O cho:

V d 3: Xt dung dch hn hp HCl C1M + CH3COOH C2M.

Trong dung dch c axit mnh HCl, axt yu CH3COOH v baz yu H2O, axit yu H2O nn xy ra cc qu trnh sau:

S mol proton nc nhn bng s mol proton 3 axit cho nn:

V d 4: Xt dung dch NH3

l dung dch ca 2 baz (H2O, NH3) v mt axit (H2O):

S proton cc baz nhn bng s proton cc axit cho:

V d 5: Xt dung dch hn hp CH3COO- v CN-.

Trong dung dch c 3 baz v 1 axit, nn xy ra cc cn bng

Tng s mol proton m cc baz nhn bng s mol proton nc cho nn

phng trnh bo ton proton l:

V d 6: Xt dung dch H3PO4

Dung dch gm hai axit: a axit (H3PO4), n axit v n baz (H2O) nn trong dung dch c cc cn bng sau:

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Phng trnh bo ton proton ca dung dch ny:

V d 7: Xt dung dch a baz CO32- (Na2CO3) cc Cn bng trong dung dch

ny:

Phng trnh bo ton proton ca dung dch:

V d 8: Xt dung dch NH4CH3COO.

Trong dung dch ny c cc cn bng sau:

Phng trnh bo ton proton ca dung dch l:

2.3. TNH pH TRONG CC DUNG DCH NC

Gi s c h gm: mt axt yu HA c hng s Ka v nng Ca mt baz yu NaA (mui ca axit yu v baz mnh) c nng Cb.

Ngoi cc phng trnh (1, 2, 3) ta cn c:

Phng trnh trung ha in: [H+] + [Na] = [CH3COO-] + [OH-] (4)

Phng trnh bo ton nng : [HA] + [A-] = Ca + Cb

Bng cc php bin i ton hc, t 5 phng trnh trn, ngi ta chng

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minh c:

y l phng trnh tng qut tnh [H+] trong dung dch, n l phng trnh

bc 3 ca [H+]. Do tnh pH trong cc dung dch nc, ta phi dng cch gii gn ng.

2.3.1. Dung dch axt mnh c nng Ca.

2.3.2. Dung dch baz mnh c nng Cb.

2.3.3. Dung dch axit yu c Ka, Ca.

V l mt Axt yu nn [OH-]

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2.3.5. Dung dch gm 1 axit yu v 1 baz yu (dung dch m)

2.3.5.1. nh ngha: Dung dch m l dung dch c gi tr pH khng thay i hoc thay i rt t khi thm mt lng nh axit mnh hoc mt lng nh baz mnh vo dung dch , k c khi pha long dung dch.

V d h m axtat 1 M:

Thm 10ml HCl 0,1M vo pH = 4,7

Thm 10ml NaOH 0,1 M vo pH = 4,9

2.3.5.2. Cc loi dung dch m

- m axtt: CH3COOH / CH3COONa

- m cacbonat: NaHCO3/ Na2CO3

- m Amoni: NH4Cl / NH4OH

- m pht pht: Na2HPO4 / Na3PO4 (hoc NaH2PO4 / Na2HPO4; H3PO4 / NaH2PO4).

Nhng h m ch c mt cp axt baz lin hp c gi l m n, cn h c 2 cp axt baz lin hp c gi l m a.

2.3.5.3. Cng thc tnh pH ca dung dch m

V dung dch c mt axt yu v mt baz yu nn t (2.5) ta c:

26

2.3.6. Dung dch axit a chc: H3PO4, H2SO3, H2CO3.

Loi axit a chc ny phn li theo nhiu nc ?

Nu nc (1) cch xa nhiu nc (2) hng vn ln th khi tnh ton vi axt 2 nc

ngi ta khng tnh n nc 2, coi nh nh mt n axt yu m pK1= pKa ca n axt yu:

Vi axit ba nc ngi ta tnh pH nc 1 NaH2PO4 theo cng thc sau:

Cn nc th 3 c tnh (Na3PO4):

2.4.CC V D TNH pH CA DUNG DCH:

V d l: Tnh pH ca dung dch HCOOH 0,1M. Cho Ka= 10-4. Ta p dng (2.8).

V d 2: Tnh pH ca dung dch NH3 0,1 M. Cho KNH3 = 10-4,8. p dng (2.9)

ta c:

V d 3: Tnh pH ca dung dch thu c khi thm 15ml HCl 0,1M vo 20ml

dung dch NH3 0,1M. Cho KNH3 = 10-4,8

Phn ng xy ra: HCl + NH3 = NH4Cl

Nng hai cht u bng nhau v u l 0,1 M nn th tch tiu tn phi tng ng nhau. Do th tch HCL trung ha NH3 l 15ml, lng NH3 cn l

27

5ml, y l mt baz yu, dung dch cn c NH4Cl to thnh l 15ml v l mt axt yu. Dung dch thu c l mt h m.

p dng (2.10):

Thay pKa = 14 - pKb v

vo ta c:

V d 4: Tnh pH ca dung dch thu c khi thm 20ml KOH 0,1 M vo 25ml

dung dch HCOOH 0,1 M. Cho Ka = 10-4

Phn ng xy ra:

Cng tng t bi trn, ta thy dung dch sau phn ng cn d HCOOH l 5ml,

lng mui HCOOK to thnh l mt baz yu c nng tng ng nng KOH thm vo. Do dung dch thu c l h m.

p dng (2.10), ta c:

2.5. CN BNG AXIT - BAZ TRONG DUNG MI KHNG NC

2.5.1. Phn loi cc dung mi

V nguyn tc, mi cht lng u c th dng lm dung mi. C cc dung mi phn t trong tn ti ch yu cc phn t khng mang in (v d nc, ru, amoniac lng, cc axit cc boxylc, cc amin...) c cc dung mi ion, trong ch tn ti cc cation v anion v loi dung mi kim loi l kim loi nng chy ( nhit cao).

Cc dung mi phn t ng vai tr quan trng, trong cc dung mi lng tnh l quan trng hn c.

2.5.2. Phn ng Axt - baz trong dung mi lng tnh

Trong cc dung mi lng tnh c qu trnh t phn proton ca cc phn t dung mi. Chng hn vi dung mi HS.

28

Tch s ion ca dung mi.

Phn ng t phn proton ca mt s dung mi lng tnh.

Nc:

Ru etylic:

Axit focmic:

Cng nh trong nc, trong dung mi lng tnh HS, ta c:

V pKi = pH + pS

Trong dung mi nguyn cht pH = pS =1/2 pKi

Dung dch s c phn ng axt khi [H2S+] > [S-] hoc pH < pS, s c phn ng baz khi [H2S+] < [S-] hoc pH > pS

Ni chung trong dung mi ny pH c xc nh trong phm vi pH = 0 cho n pH = PK(HS). Trong bng (2.1) c cho c tnh axit - baz ca mt s dung mi lng tnh.

Bng 2.1. c im axit - baz ca mt s dung mi lng tnh. Dung mi

(HS) Ionlioni Ionliat Ki Thang pH pH trung

bnh H2SO4

HCOOH

CH3OH

C2H5OH

H3SO4+

HCOOH+

CH3OH+

C2H5OH2+

HSO4-

HCOO-

CH3O-

C2H5O-

10-3,8

10-6

10-16,7

10-19

0 - 3,8

0 - 6,0

0 - 16,7

1,9

3,0

8,35

29

Trong nc HCL l mt axt mnh, nhng trong dung mi axt axtc bng c tnh baz rt yu nn HCl li l mt axit yu.

i vi cc dung mi c hng s in mi th thng xy ra s lin hp gia

cc ion tch in ngc du. Chng hn, trong dung dch axt HA, cn xy ra s lin hp gia cc ion H2S+ v A- to ra cc cp ion H2S+A- (hay H+A-) khc vi phn t HA. Do trong cc dung mi ny lc axt ca nhiu cht gim i.

trong H2O ( = 78,5) axit axetic c pKa = 4,76; cn trong ru (etanol = 24,2) th pKa ca axt axetic bng 10,3, ngha l lc axit gim i nhiu.

Hng s phn li ca axt Ka ph thuc vo hng s in mi ca dung mi, ph thuc vo in tch v kch thc cc ion v nng cc ion.

Bng 2.2. cho ch s hng s axt v ch s hng s baz ca mt s cp axit - baz trong etanol v trong axit axetic bng.

Bng 2.2. Ch s hng s axit v baz

Cp Axit Baz Dung mi pKa pKb HNO3/ NO3-

HCOOH/HCOO-

CH3COOH /CH3COO-

HClO4/ClO4-

HCl/Cl-

HNO3/NO3-

C2H5OH

C2H5OH

C2H5OH

CH3COOH bng

CH3COOH bng

CH3COOH bng

3,6

9,2

10,3

2,9

5,0

9,4

15,4

9,5

8,7

11,5

9,4

5,0

Vic nh gi pH trong cc dung dch khng nc cng tng t nh trong dung dch nc.

30

CU HI V BI TP CHNG 2

2.1. Tnh [OH-], pH, pOH i vi dung dch axt c nng [H+] = 0,0010M, coi h s hot cc cu t bng 1.

2.2. Thm 300ml H2O Vo 200ml dung dch HCl c nng ln H+ 0,10M. Tnh pH ca dung dch sau khi pha long.

2.3. Tnh pH, pOH v [H+], [OH-] ca dung dch HCl 1,0.10-3M.

2.4. Trn 200ml dung dch HCl c pH = 2,0 vi 300ml d HNO3 c pH = 3,0. Tnh pH ca dung dch thu c.

2.5. Nh 1 git HCl 3,4.10-3 M vo 300,00 ml nc. Tnh pH ca dung dch, bit th tch 1 git l 0,03ml.

2.6. Tnh [H+], [OH-], pH ca dung dch thu c khi thm 20,0 ml dung dch NaOH l,00.10-3M vo 80,00 ml dung dch HCl 2,5.10-4 M.

2.7. Tnh pH ca dung dch HCOOH 0,1M.

2.8. Tnh nng ca dung dch CH3COO- phi c trong dung dch sao cho pH

= 3,0.

pH= 3,0

31

CHNG 3: PHN NG TO PHC

3.1. KHI NIM V PHC CHT

3.1.1. nh ngha - phn loi

Phc cht l nhng hp cht c thnh phn phc tp, bn trong dung dch nc, ngha l kh nng phn li km.

V d: Cho 1mmol (NH4)2SO4. Fe2(SO4)3.12H2O ha tan vo th tch xc nh nc v cho vo lng KCNS d, dung dch nhn c c mu mu.

Cho 2mmol K3[Fe(CN)6] ha tan vo th tch nc nh trn v cho vo lng KCNS d dung dch nhn c s c mu hng. iu chng minh K3[Fe(CN)6] bn hn (NH4)2SO4 Fe2(SO4)3. 12H2O. Kh nng phn li ra Fe3+ t do ca [Fe(CN)6]3- l rt yu v n l phc cht, cn (NH4)2SO4.Fe2(SO4)3.12H2O l mui kp.

Phc cht c hai loi, l phc cht v c v phc cht hu c.

- Phc cht v c: K3[Fe(CN)6], K4[Fe(CN)6], NH4+ [Ag(NH3)2]Cl, CdCl3-, CdCl42-...

- Phc cht hu c: +Me2+ to vi mui 2 ln th Na ca EDTA (Etylen Diamin Tetra Axetic axit):

Pb2+ to vi Dietyl Dithio Cacbamat Natri:

Phc [DDC]Pb/2 c cng thc nh sau:

32

3.1.2. Cch gi tn phc cht:

S ch s phi t + tn phi t + tn ion trung tm + tn ion cu ngoi.

- [Cu(NH3)4]SO4: Tetramino ng (II) sun pht.

- K3[Fe(CN)6]: Kali hexa xyanua st (III)

Tn ca phi t: X-halogien nua; OH - hyroxo; NO3- nitrato; H2O - aqu; NH2(NH3- NH) amino; CO - cacbonyl.

3.1.3. Hng s bn v khng bn ca phc cht:

Trong :

c gi l hng s khng bn nc 1 th:

l HSKB nu 6, cn

c gi l hng s khng bn tng hp.

Ngc li:

th

c gi l hng s bn nc 1 (3.4)

cn:

33


v


Cn phn ng:

c gi l hng s bn tng hp (3.7)

i vi phc nhiu nhn th hng s to thnh tng hp c th thm ch s cho bit s lng ln trung tm, ch s ny sau ch s phi t. V d:

3.2. TNH NNG CN BNG CA CU T TRONG DUNG DCH PHC CHT

Gi s c cc phn ng to phc xy ra theo s :

Nng cc dng ng:

Nh vy khi bit c dng tn ti ch yu ca phc cht, ta c th tnh c

nng cn bng theo nh lut tc dng khi lng hoc theo nh lut bo ton

34

nng i vi ion kim loi v i vi phi t.

3.2.1. Phi t rt d so vi ion trung tm.

t c iu phi tnh ton bng nhng phng php gn ng thch hp.

Khi phc to thnh s c th c s phi tr cao nht (v CL >> CM).

Ta c nng cn bng phi t bng nng ban u ([L] CL)

V d 1: Tnh nng cn bng cc cu t trong dung dch AgNO3 0,01M, NH3 1,00 v NH4NO3 1,00M. Cc qu trnh xy ra trong dung dch:

Xt cc iu kin gn ng.

Tnh gn ng pH ca dung dch.

T cn bng (3) ta c:

gi tr pH ny: [NH3] = 1 = CNH3

Nh vy c th b qua s proton ha ca NH3.

T (4) ta c:

Nh vy s to phc hiroxo cng xy ra khng ng k.

T (1), (2) ta c:

Vic so snh (5) v (6) vi [NH3] CNH3 = 1 cho thy.

35

Vy phc tn ti trong dung dch ch yu l Ag(NH3)

T lp lun trn ta c th tnh gn ng nh sau:

- Xc nh thnh phn gii hn:

Tnh CB theo s Kamar:

Vi x > +2CdC , cc gi tr lg khng qu ln v khng chnh lch nhau nhiu.

Do c th coi [Cl-] ClC nhng khng th coi 1 dng phc no l chim u th

(1 2 3 4).

36

Khi tnh gn ng, phi k s c mt ca tt c cc dng phc cloro ca Cd2+

p dng nh lut bo ton nng ban u i vi ion Cd2+ ta c:

p dng nh lut TDKL cho cc cn bng (1) (4) v thay cc gi tr tnh

c ca cc dng phc vo (5) ta c:

Sau khi t hp ta rt ra:

Chp nhn [Cl-] ClC = 1. Gii (7) ra ta c:

Vic kim tra bng cch thay cc kt qu tnh c vo phng trnh (5) cho

thy:

+2CdC = 9,975.10-3M so vi gi tr ban u ( +2CdC = 0,010) th sai s l khng

ng k (-0,25%).

Nu thay cc kt qu tnh c vo phng trnh nh lut bo ton nng i vi ion Cl- th:

Suy ra:

Sai s:

C th chp nhn c, nu ch rng sai s ca cc hng s cn bng c th

dao ng trong khong 1 - 10%.

Cc kt qu tnh ton cng cho thy trong h Cd2+(010M) + Cl-(1,0M) cc dng phc tn ti mc ngang nhau, trong nng CdCl2 hi ln hn, c nng CdCl42- li b nht.

3.2.2. Ion trung tm rt d so vi phi t (CM>> CL)

Trng hp ny thng gp khi pht hin nh tnh cc ion.

37

V d: Thm 1 git (0,03 ml) dung dch NH4CNS 0,10M vo 1,00ml dung dch FeCl3 0,100M khi c mt HCl 1 M (coi th tch thay i khng ng k khi thm thuc th). Tnh nng cn bng cc cu t trong dung dch.

Xt cc iu kin gn ng.

V nng ln H+ ln nn c th b qua s phn li ca ion NH4+ v s to phc hidroxo ca Fe3+.

V +3FeC >> -SCNC nn c th coi phn ng to thnh phc FeSCN2+ l ch

yu:

Thnh phn cc ion:

Ngoi phc FeSCN2+ trong dung dch cn cc qu trnh:

38

Vic kim tra cho thy:

u rt b so vi [FeSCN2+]. Vy cch tnh gn ng trn l hon ton ph hp.

3.3. NH HNG CA pH V CA CC CHT TO PHC PH N NNG CN BNG CA PHC. HNG S KHNG BN V HNG S BN IU KIN

Trn y chng ta nghin cu cn bng to phc trong iu kin ln trung tm v phi t ch tham gia vo mt phn ng duy nht l phn ng to phc gia chng vi nhau. Trong thc t cc trng hp rt t gp m ta thng gp cc trng hp trong ion trung tm hoc phi t hoc c hai thng tham gia vo nhng phn ng ph khc. V d: cation Cu2+ v anion etylen - iamintetraaxetat (vit tt l EDTA) v c k hiu l Y4- khng nhng ch phn ng vi nhau to thnh phc CUY2- m Cu2+ cn c th to phc vi cc phi t khc nh OH-, NH3, v Y4- cn c th tham gia phn ng vi ion H+. V vy, nhng ion v phn t ny (OH-, H+, NH3) l nhng yu t lm thay i s to phc ca Cu+ v EDTA. Trong trng hp ny ngi ta thng dng hng s bn hoc khng bn iu kin phn nh nhng iu kin c th trong phn ng to phc c nghin cu xy ra ng thi n gin ha vic tnh ton cc nng cn bng.

Hng s bn iu kin.

Gi s ion kim loi Mn+, ngoi vic tham gia phn ng to phc chnh vi anion EDTA Y4-:

cn tham gia phn ng ph vi phi t L to nn cc phc ML, ML2 MLm v anion Y4- cn tham gia phn ng vi cc ion H+ to thnh HY3-, H2Y2-, H3Y-, H4Y. Trong iu kin ny hng s bn iu kin MY ca phc MYn-4 c nh ngha nh sau:

Trong : [M] l tng nng cn bng cc dng tn ti ca Mn+, tr phc

MYn-4, tc l:

39

[Y,] l tng cc nng cn bng ca cc dng tn ti ca Y4-, tr phc MYn-4,

tc l:

Nu k hiu hng s bn tng nc ca cc phc ML, ML2 ln lt l 1,

2, m v cc hng s axit tng nc ca H4Y ln lt l K1, K2, K3, K4 th ta c:

Thay (3.12, 3.13) vo (3.11) ta c:

Do hng s bn iu kin ch l hng s trong nhng iu kin xc nh. Ti

nhng iu kin xc nh, gi tr hng s bn iu kin ca mt phc cng ln th phc cng bn tc l trong iu kin phc to thnh cng hon ton. Hng s khng bn iu kin K'MY l nghch o ca hng s bn iu kin.

V d 1: Tnh hng s bn iu kin ca phc MgY2- trong dung dch c pH =

11,0. Bit hng s ca phc l MgY = 108,7. Hng s bn ca phc MgOH+: +MgOH = 102,58, H4Y c pK1 = 2,0; pK2 = 2,67; pK3 = 6,27; pK4 = 10,95;

t l hng s bn iu kin ca phc MgY2- ti pH = 11, ta c:

40

Trong

Hy tnh Mg(OH) v (H)

Hng s bn iu kin ca phc MgY2- trong dung dch c pH = 11 l

Tc l c nh hn cht t v gi tr ny kh ln chng t trong dung dch

c pH = 11 v khng cn cht no khc to phc vi ion Mg2+ th ion Mg2+ to vi EDTA thnh phc kh hon ton.

V d 2: Tnh nng cn bng pH = 11 ca cc cu t trong dung dch Mg2+ c nng ban u 0,01M v EDTA c nng ban u l 0,02M. Trong th d trn, ta tnh c hng s bn iu kin ca phc MgY2- trong dung dch c pH = 11 l = 108,28.

Phng trnh bo ton khi lng ca Mg2+:

v ca EDTA l:

T hai phng trnh trn suy ra:

Thay [MgY2-] = 0,01 - [Mg] v [Y] = 0,01 + [Mg] vo biu thc ca hng s

bn iu kin:

41

Gi s [Mg']

42

Nh vy trong dung dch m NH3 + NH4Cl c nng NH3 kh ln, c pH = 9, ion Ni2+ va to phc vi EDTA va to phc vi NH3 nhng khi tnh MY ta thy gi tr hng s rt ln chng t Ni2+ to phc nh lng vi EDTA.

3.4. PHC CHT CA CC ION KIM LOI VI AXIT ETILENDIAMINTETRAAXETIC.

Trong s cc cht to phc vng cng (Chelate) vi cc ion kim loi th nhm thuc th hu c c tn chung l complexon c ng dng rt rng ri trong ha phn tch, cc complexon l nhng dn xut ca cc axit aminopolycacboxilic.

V d: Nitrylotriaxetic hay complexon I.

Axit etylendiamintetraaxetic (EDTA) hay complexon II.

c vit tt l H4Y. V EDTA tng i kh tan trong nc nn trong thc t

ngi ta thng dng mui inatri ca n tan nhiu trong nc. Mui Na2H2Y c gi l complexon III hay Trilon B.

Cc complexon to c phc bn vi hng chc ion kim loi. Cc phc ny tha mn cc iu kin ca nhng phn ng dng trong phn tch th tch, v vy ngy ny ngi ta s dng rt rng ri phng php chun to phc dng cc complexon c gi l phng php chun complexon.

Trong phn ny chng ta s bn ti bn ca cc phc complexon c gi l cc complexonnat.

Complexon III to phc vi hu ht cc ion kim loi. Bng sau y ghi li hng s bn ca cc phc v gi tr l ca cc hng s

43

Bng 3.1. Hng s to thnh ca cc complexonat kim loi

Cation KL Phc conplexonat lg

Ag+

Mg2+

Ca2+

Sr2+

Ba2+

Mn2+

Fe2+

Co2+

Ni2+

Cu2+

Zn2+

Cd2+

Hg2+

Pb2+

Al3+

Fe3+

V3+

Th4+

AgY3-

MgY2-

CaY2-

SrY2-

BaY2-

MnY2-

FeY2-

CoY2-

NiY2-

CuY2-

ZnY2-

CdY2-

HgY2-

PbY2-

AlY-

FeY-

VY-

ThY

2,1.107

4,9.108

5,0.1010

4,3.108

5,8.107

6,2.1013

2,1.1014

2,0.1016

4,2.1018

6,3.1018

3,2.1015

2,9.1016

6,3.10211

1,1018

1,3.1016

1,3.1025

7,9.1025

1,6.1023

7,32

8,69

10,70

8,63

7,76

13,79

14,33

16,31

18,62

18,80

16,50

16,46

21,80

18,04

16,13

25,10

15,90

13,20

EDTA l axit 4 ln axit, cc hng s axit tng nc ca n l:

V phn to phc vi ion kim loi y l anion Y4- nn s to phc ca ion

kim loi ph thuc vo bn cht ca ion kim loi (th hin cc gi tr hng s bn nhit ng trong bng 3. 1) v vo pH ca dung dch.

i vi mi ion kim loi ta c th tnh c pH kim loi to phc mt

44

cch nh lng vi EDTA.

Ta hy tnh pH ca dung dch phi bng bao nhiu khi trn hai th tch bng nhau ca dung dch ton kim loi Mn+ v dung dch EDTA c nng bng nhau Co th 99,9% lng ion kim loi to phc complexonat l MYn-4.

Khi trn hai th tch bng nhau ca dung dch EDTA v ca dung dch Mn+ th

nng ca chng gim i mt na, bng 2

0C . Theo nh lut bo ton khi lng

th:

trong : [Y'] l tng nng cc dng tn tn ti ca EDTA tr phc MeYn-4.

T (a) v (b) suy ra: [Mn+] = [Y'] (c)

Khi 99,9% lng in kim loi to phc MYn-4, tc l:

Thay cc gi tr ca [MYn-4], [Mn+], [Y'] vo biu thc ca hng s bn iu

kin ', ta c:

Nh vy, mun cho 99,9% lng ion kim loi to phc complexonat th hng

s bn iu kin ca phc phi bng C

2.106 . Ta hy tnh pH ' bng gi tr .

Ta bit, theo nh ngha:

Gi s ta tm iu kin to phc hon ton khi nng ban u v EDTA

u bng nhau v bng 2.10-2M (nng thng gp trong thc tin). Trong trng hp ny:

Vi mi phc, ta s tm c mt gi tr (H) ' = 108.

45

Phng trnh biu din i lng (H) l phng trnh bc 4 i vi [H+]:

Nn khi bit (H) ta phi gii phng trnh bc 4 tm pH l vic cc k kh

khn. Do , ta nn v th biu din s ph thuc ca (H) vo pH ca dung dch.

Mun vy ta thay cc gi tr [H+] ln lt bng 10-1, 10-2... v 10-12 vo phng trnh (d) trn ri tnh cc gi tr (H) tng ng. Cc kt qu tnh c tp hp trong bng 3.2 sau y.

Bng 3.2. Cc gi tr (H) cc pH khc nhau

pH 1 2 3 4 5 6 (H) 1017,13 1013,43 1010,6 108,44 106,45 104,66

pH 7 8 9 10 11 12 (H) 103,32 102,27 101,28 100,45 100,07 1

Bng 3.1 ghi gi tr ca cc phc complexonnat cc kim loi khc nhau. T cc gi tr , ta tm c gi tr (H) cho ' = 108. Dng th trn ta c th tnh c pH ng vi gi tr . l pH ti thiu khi trn hai th tch bng nhau ca dung dch in kim loi 2.10-2M vi dung dch EDTA 10-2M th 99,9% lng ln kim loi i vo phc vi EDTA.

Bng 3.3 l cc gi tr (H) ca cc phc kim loi = 108 tc l cc gi tr (H) cn thit cc phc c to hon ton.

Bng 3.3. Cc gi tr (H) cc phc complexonat kim loi c ' = 108.

Phc complexonat (H) AgY3-

MgY2-

CaY2-

SrY2-

BaY2-

MnY2- FeY2-

CoY2-

10-0,68

190,69

102,70

100,63

10-0,24

105,8

106,33

108,31

46

NiY2-

CuY2-

ZnY2-

CdY2-

Hg2-

PbY2-

AlY-

FeY-

VY-

ThY-

1010,62

1010,8

108,5

108,46

1013,8

1010,04

108,13

1017,1

1017,9

1015,2

Hnh 3.1. Gin s ph thuc ca (H) vo pH ca dung dch v trn ng cong biu din s ph thuc c cc chm biu th gi tr phc kim loi c ' = 108. T im ta tm c gi tr pa tng ng trn trc honh l gi tr pH ti thiu 99,9% lng ion kim loi to phc complexonat.

T th ta thy cc ion Fe (III), In (III), Th (IV), Se (III) v Hg (I) to bn phc trong khong pH = 1 - 2. Cc ion Ni (II), Cu (II), Pb (II), Zn (II) v l (III), Cd (II), Co (II) to phc hon ton trong khong pH = 2 - 5. Cc kim loi kim th to phc hon ton trong khong pH = 8 - 10.

Hnh 3. 1. Gin s ph thuc ca (H) vo pH.

pH ti thiu 99.9% lng ion kim loi to phc complexonat t dung dch ion kim loi nng 2.10-2M.

47

3.5. NG DNG PHN NG TO PHC TRONG HA HC PHN TCH

3.5.1. Phn tch nh tnh cc cht

Cc phn ng mu c trng thng c s dng pht hin cc ion.

a. Cc phc ca ion kim loi vi amoniac. V d Cu(NH3)42+ mu xanh m dng pht hin ion Cu2+, phc Co2+ vi NH3 c mu vng CO(NH3)62+ b oxi ha bi khng kh hoc H2O2 to thnh phc CO(NH3)63+ c mu hng c trng.

Phc thioxyant kim loi, v d Fe(SCN)n(3-n) (n = 1 -5) c mu mu c trng c dng pht hin Fe3+. Phc Co(SCN)42- c mu xanh t bn trong nc nhng bn vi dung mi hu c (axeton, cc ancol, ete etylic) v vy phn ng gia Co2+ vi SCN- khi c mt dung mi hu c c dng pht hin Co2+.

b. Cc thuc th hu c c dng pht hin ion kim loi kh phong ph: Pht hin Ni2+ bng imetyl glyoxim, Al3+ bng Alizarin hoc Aluminon, Fe2+

bng O - phenantrolin hoc , ' dipiridin....

3.5.2. Xc nh nh lng cc ion kim loi.

a. EDTA l thuc th quan trng c dng xc nh nh lng nhiu ion kim loi.

b. Phn ng mu gia nhiu thuc th hu c vi cc ion kim loi c dng nh lng trc quang hoc chit trc quang cc ion kim loi. V d nh lng Ni2+ bng imetylglyoxim, Fe3+ bng SCN-, Al3+ bng Aliuminon, Pb2+ bng ithizon v Cu2+ bng ietyl ithiocacbamat... Cd2+ bng Amoni Pirolidin ithio cacbamt (APDC).

3.5.3. Ha tan cc kt ta kh tan, tch cc ion.

Nhiu thuc th to phc c s dng ha tan cc kt ta. V d AgCl tan trong NH3 do to phc Ag(NH3)2+; Cu(OH)2 tan trong NH4Cl do to phc Cu(NH3)42+; - cng o c tnh ny m ngi ta c th s dng cc thuc th to phc tch cc ion, v d dung NH3 d tch hn hp Fe3+, Al3+ v Cu+ do Fe3+, Al-3+ c chuyn vo kt ta Hydroxit Fe(OH)3, Al(OH)3, cn Cu2+ c gi li trong dung dch di dng phc Cu(NH3)42+.

3.5.4. Che cc ion cn tr

Mt trong cc ng dng quan trng ca phc cht trong phn tch ha hc l che cc ion cn tr. Khi mt thuc th c kh nng phn ng vi ion B cng c mt trong dung dch cha ion phn tch A th ta ni n ion B cn tr phn ng ca A vi thuc th. loi tr nh hng ca B ta phi kt ta B vi mt thuc th thch hp.

48

Mt bin php tch cc l "che" ion B, ngha l khng cn tch B m chuyn n sang to phc vi mt thuc th khc n khng cn tr phn ng chnh. Cht c a vo lm trit tiu hoc km hm phn ng cn tr c gi l cht che.

V d: ion Fe3+ cn tr n phn ng gia Co2+ v SCN- ta che n bng cch cho Fe3+ to phc vi F- v phc FeF63- bn hn nhiu phc Fe(CNS)nn-3 trn. Cht che phi c kh nng to phc bn vi ion cn tr n khng cn tc dng vi thuc th chnh dng nh lng, ion cn xc nh, mt khc bn ca phc gia cht che v ion cn nh lng phi rt b khng gy cn tr cho phn ng chnh. Chng hn, vic tm ion Cd2+ trong dung dch cha ion Cu2+, Co2+, Ni2+ bng Na2S khng thc hin c v cc ion ny cho kt ta sunfua mu en khng cho php nhn ra kt ta CdS mu vng. che cc ion ny ngi ta dng KCN, v cht che ny to ra cc phc bn Cu(CN)42-, Co(CN)42-; Ni(CN)42- khng phn ng vi Na2S, trong khi phc Cd(CN)42- km bn, Cd2+ phn ng vi Na2S.

49


3.1. M t cc qu trnh xy ra trong dung dch nc ca cc cht sau y: [Ag(NH3)2]Cl, [Cu(NH3)4]SO4, K2[HgI4], FeF3.

3.2. Nh rt chm dung dch NH3 vo dung dch CuSO4 th mi u c kt ta xanh lam xut hin, sau kt ta tan v dung dch to thnh c mu xanh da tri. Nu thm tip tng git HCl long vo dung dch th mu xanh da tri bin mt, dung dch hi c ri trong sut v c mu xanh nht, gii thch hin tng. Vit cc phng trnh phn ng ion.

3.3. Nh HCl vo dung dch AgNO3 c kt ta trng, thm dn NH3 vo hn hp th kt ta tan. Thm chm tng git HNO3 vo hn hp thu c th li c kt ta trng xut hin. Gii thch hin tng, vit cc phng trnh phn ng ion.

3.4. Tnh nng cn bng trong dung dch Pb2+ 0,1 M v CH3COO- lm pH = 2,0, +COO3PbCH

= 0,56.

3.5. Tnh nng CH+ lm gim nng Ag(NH3)2+ 0,1M xung cn 1,0 10-6 M, Ag(NH3)+ = 103,32.

3.6. Thm 1ml dung dch NH4SCN 0,10M vo 1ml dung dch Fe3+ 0,01M v F- 1,00M. C mu ca phc xut hin hay khng ? Bit rng mu ch xut hin khi CFeCN2+ > 7.10-6M v dung dch c axit ha va s to phc Hiroxo ca Fe(III) khng xy ra. Cho CFe3+ (= 0,0 1) < CF (= 1).

3.1. Hng s bn tng cng ca cc phc to bi ion Hg2+ v ion 1- ln lt l: 1.1 = 109,05, 1.2 = 1017,33, 1.3 = 1019,74 v 1.4 = 1021,0. Tnh cc hng s bn v khng bn tng nc ca cc phc .

3.8. Tnh nng cn bng ca cc ion v phn t trong dung dch Cd(ClO4)2 10-3M v KI 1 M. Trong dung dch c HClO4 Cd2+ khng to phc c vi OH- m ch to phc vi I-; cc phc c hng s bn tng cng ln lt l 102,88, 103,92, 105,00, 106,11.

3.9. Tnh nng cn bng ca cc ion trong dung dch gm bc nitrat 10-3M v amoniac c nng cn bng l:

a) 10-3M v b) 10-2M

Cc phc ca Ag+ v NH3 c logarit hng s bn tng nc l 3,32 v 3,92.

3.10. Tnh nng cn bng ca ion v phn t trong dung dch HgCl2 10-2M.

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Phc ca Hg2+ v Cl- c logart hng s bn tng cng l 6,74 v 13,22.

3. 11. Tnh nng cn bng ca ion Fe3+ v FeY- trong dung dch hn hp Fe3+ 10-2M v Na2H2Y 10-2M c pH = 2,0, FeY = 1025,1.

3.12. Cn phi thm NH3 vo dung dch AgNO3 10-2M n nng cn bng no gim nng Ag+ t do xung 10-8M. Phc ca Ag+ v NH3 c logant hng s bn tng cng ln lt l 3,32 v 7,24.

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CHNG 4 PHN NG KT TA

4.1. IU KIN TO THNH CHT KT TA

Phn ng kt ta l phn ng ha hc xy ra khi mt trong cc sn phm to thnh l cht rn t tan. Cht rn t tan c gi l cht kt ta.

4.1.1. Tch s tan (T)

Khi thm dung dch NaCl vo dung dch AgNO3, s c cn bng sau:

Qu trnh dng li khi c s cn bng gia qu trnh ha tan v qu trnh kt

ta:

Theo nh lut TDK th:

Vi dung dch bo ha K = const v c gi l tch s ha tan, k hiu l T.

Vy T = [Ag+] [Cl-]

Theo hot , ta c:

Tng qut cho qu trnh:

Hoc coi hot ng bng nng th:

Nu: (dung dch bo ha)

(dung dch qu bo ha)

(dung dch cha bo ha).

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4.1.2. tan (S) ca kt ta, quan h gia S v T.

tan (S) ca cht tan l nng mol/l ca cht trong dung dch bo ha.

Gia S v T c mi quan h:.

V d: Tnh TMg(OH)2 200C, bit trong 100ml dung dch bo ha nhit

ny c cha 0,84mg Mg(OH)2.

4.2. CC YU T NH HNG N TAN

4.2.1. nh hng ca ion chung

Ion chung l ion c trong thnh phn ca kt ta. Nu thm ion chung vo dung dch bo ha, tch s nng ion s tng ln, dung dch tr nn qu bo ha v s xut hin kt ta, tc l tan gim i.

V d l: Tnh SPbSO4 trong nc nguyn cht v trong dung dch Na2SO4 10-2M, cho TPbSO4 = l,6.10-8.

Gi tan ca PbSO4 trong nc nguyn cht l S.

Theo cng thc tnh tan ta c:

tan ca PbSO4 trong dung dch Na2SO4 10-2M.

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S = 16.10-6 mol/l kt qu ny cho thy gi thit trn l ng. Vy tan ca

PbSO4 trong dung dch Na2SO4 l 1,6.10-6 v thy rng tan ca n nh hn trong H2O rt nhiu, c th ti

V d 2: Tch s tan ca CaC2O4 200C bng 2.10-9. Hy so snh tan ca n trong H2O v trong dung dch (NH4)2 C2O4 0,1 M.

Gi tan ca CaC2O4 trong nc nguyn cht l S, theo cn bng.

Ta c:

Theo cng thc:

- tan ca CaC2O4 trong dung dch (NH4)2 C2O4 0,1 M

Gi tan ca CaC2O4 trong dung dch (NH4)2 C2O4 0,1M l S th:

Gi s: S

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Chng hn kt ta AmBn trong dung dch c ion H+ c kh nng phn ng vi B v c cht to phc L c kh nng to phc vi A, trong dung dch c cc ion bng:

A to phc vi L:

- B phn ng ph vi H+

K1, K2 l hng s khng bn ca phc HB, H2B cng l hng s phn li ca axit

HB v H2B.

- Nu gi S l tan ca kt ta th:

T cc hng s K 1, K2 ta tnh c [HB], [H2B].... theo [B] v [H+] Ta c:

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V d: Tnh tan ca CaF2 trong dung dch HCl 10-2M cho KHF = 6.10-4 v TCaF2 (trong nc) = 4.10-11.

Trong nc:

V Ca2+ khng tham gia phn ng ph vi F nn [Ca2+] = S.

Cn F- tham gia phn ng ph: H+ + F- = HF

4.2.3. nh hng ca cht to phc ph

Cc cht to phc ph c mt trong dung dch c th lm hn ch hoc ngn cn qu trnh kt ta do s to phc vi kim loi: Tnh cht ny c dng che cc ion cn tr.

V d Khi lm kt ta Cd2+ bng H2S c mt ca ion Cu2+ th ion Cu2+ s cn tr qu trnh kt ta CdS do s to thnh kt ta CuS mu en. "che" ion Cu2+ ngi ta cho KCN d vo dung dch. y xy ra cc qu trnh sau:

Phc Cu(CN)43- bn hn rt nhiu so vi phc Cd(CN)42- nn khi cho H2S li

qua dung dch th ch c kt ta CdS c to thnh.

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K1 kh ln, phn ng (4) xy ra d dng. Trong khi phn ng kt ta Cu2S

xy ra kh khn:

Qua v d trn ta thy qu trnh kt ta ph thuc vo nhiu yu t: hng s bn

ca phc to thnh gia ion kim loi v cht to phc, tch s tan ca kt ta, tnh cht axit - baz ca thuc th lm kt ta v ca cht to phc. Khi tnh ton gn ng c th p dng nh lut tc dng khi lng cho cn bng ch yu hoc tnh theo hng s iu kin.

V d 2: Trn 1,00ml hn hp m (A) gm NH3 2,0M + NH4NO3 2,0M vi 100ml dung dch B gm FeCl3 2,0.10-3M + NaF 0,20M. C kt ta Fe(OH)3 hay khng? Cho

Sau khi trn ta c:

Cc qu trnh xy ra:

Trong dung dch B, Fe3+ tn ti ch yu di dng phc Floro (CF- >> CFe3+).

Trong dung dch A:

T cn bng (4) ta c:

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Gi s

Ta c:

Vy vn c kt ta Fe(OH)3

4.2.4. nh hng ca nhit .

V qu trnh ha tan thu hoc ta nhit, n nh hng khng c qui lut nn khng c nhng biu thc nh lng chung. S thay i tan mt cht theo nhit c lin quan ti hiu ng nhit khi ha tan. i vi nhng cht thu nhit khi ha tan th tan s tng theo nhit , th d PbI2 tan nhiu khi un nng. tan ca AgCl 1000C ln gp 25 ln tan ca n 100C. i vi nhng cht ta nhit th ngc li, tan gim khi nhit tng. Th d CaSO4.0,5H2O 600C c tan ln gp 3 ln tan ca n 1000C.

4.2.5. nh hng ca kch thc ht kt ta.

i vi cng mt lng cht kt ta c dng ht nh tan nhiu hn kt ta c dng ht ln v b mt ca cc tinh th nh c cc gc, cnh, cc ion v tr gc, cnh d tan hn nhng v tr khc v cc ion v tr bn trong b gi cht hn.

Ngoi cc yu t k trn, cn c yu t khc nh qu trnh sp xp tinh th t dng ny sang dng khc bn hn.

4.3. KT TA PHN ON.

Nu trong dung dch c cha hai hay nhiu con c kh nng to c kt ta cng vi mt con khc, nhng cc kt ta hnh thnh c tan khc nhau th khi thm cht lm kt ta vo dung dch cc kt ta ln lt c to thnh. l s kt ta phn on.

n gin cho vic tnh ton, cc v d sau ta b qua s l thp phn ca cc i lng tch s tan.

V d 1: Cc ion Cl-, I- u c kh nng to kt ta vi ion Ag+, nhng c tch s tan khc nhau.

Ta c:

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Khi thm AgNO3 vo dung dch hn hp gm Cl- v I-, v TAgI

59

Tc l t khi nng Cl- trong dung dch nh hn hng vn ln nng Cr)42-

Theo cc d liu ca bi ton khi Ag2CrO4 bt u kt ta th:

V % Cl- cn li trong dung dch l:

Cng c th coi khi Ag2CrO4 bt u kt ta th Cl- i ht vo kt ta AgCl.

4.4. KT TA KEO

C nhiu trng hp tch s con vt qu tch s tan nhng kt ta to thnh vn khng tch ra c m ch hnh thnh nhng ht rt nh l lng trong dung mi, dung dch gi l dung dch keo.

Nguyn nhn c kt ta trong trng thi keo l do cc ht kt ta hp ph cc ht mang in nn n mang in tch. Chng hn, thm Na vo dung dch AgNO3, khi trong dung dch hnh thnh keo AgCl.

Lc s c hai kh nng hp ph xy ra ty thuc vo lng d Ag+ hay Cl-

AgCl

Khi d Ag+ th v d Cl-

lm ng t keo ny ta phi thm vo dung dch cht in li mnh khc c

nng cao v c in tch ln.

Nhng loi keo a nc nh silic oxit (SIO2) thng c khuynh hng to nn dung dch keo rt bn v cc ht keo c bao bc bi lp dung mi. Mun cho keo ng t trong trng hp ny phi ph b c lp v dung mi th cht in li mi xm nhp c trung ha in tch cc ht keo. ph keo silic ta phi axt ha bng dung dch axit clohydric v phi un cn kh, i khi phi t nng ti nhit trn 1000C th lp nc bc ht keo mi b ph v.

60

Khi lc cc kt ta keo gp rt nhiu kh khn v cc ht keo thng nh nn phi dng loi giy lc mn, rt kh chy. Khi ra kt ta keo, cht in li dng lm ng t keo b loi bt mt phn, kt ta c th tr li trng thi keo hin tng ny gi l s ti keo hay pptin ha. V vy khi ra kt ta keo phi chn cht in li thch hp trnh s ti keo.

4.5. S HA TAN CC KT TA KH TAN TRONG NC.

Qu trnh ha tan xy ra ngc vi qu trnh to kt ta, mun c qu trnh ha tan xy ra cn lm gim tch s con ca cht t tan xung i lng b hn tch s tan ca hp cht bng cc bin php sau:

- Ha tan kt ta trong axt hoc kim.

- Chuyn kt ta sang phc cht bn t phn li.

- Chuyn ion ca cht t tan sang dng oxi ha kh.

- Chuyn hp cht t tan ny sang hp cht t tan khc c kh nng tan d trong thuc th thch hp.

4.5.1. Ha tan kt ta trong axt v kim

4.5.1.1. Ha tan cc hyroxit

a. Cc hyroxit kim loi tan trong cc axit to thnh mui tan v nc t phn li. tan ph thuc tch s tan ca hyrxit v hng s phn li ca axit.

V d 1: Mg(OH)2 (pTS = 10,9) tan d trong c cc axt rt yu (NH4+ c DKa = 9.24).

Nhng Fe(OH)3 (TS= 10-37) ch tan trong t mnh, khng tan trong cc axt yu

nh NH4+.

b. Cc hyroxit lng lnh (Al(OH)3; Cr(OH)3; Zn(OH)2; Pb(OH)2) tan trong

axt v trong kim d.

4.5.1.2. Ha tan cc mui

Cc mui ca cc axt yu tan trong axt mnh hn to thnh axt t phn li.

a. Cc mui cacbonat tan trong axit (k c axit axetic, H2CO3).

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V d 2: nh gi tan ca CaCO3 trong axt axetic.

Khi nng CO2 vt qu tan th CO2 bay ra khi dung dch. Lc :

Hng s cn bng ca cc qu trnh (1) v (2) tng i ln KI = 0,072; KII =

2,19 cho php d on kh nng tan d ca CaCO3 trong axt axetic nht l khi un nng (ui CO2). Mt khc CaCO3 cng d tan trong dung dch bo ha kh CO2.

V d 3: nh gi tan ca CaCO3 trong dung dch c bo ha lin tc bng kh CO2 P = 1 at.

Coi cn bng (5) l ch yu, ngha l nng Ca2+ to ra t cn bng (5) ln

hn rt nhiu nng Ca2+ do qu trnh (1) sinh ra. V nng HCO3- cng ln hn nng HC O3- sinh ra do (3). T c th tnh tan theo (5).

Nh vy c th coi nng [Ca2+] ch yu do (5) to ra.

Cng vy, t (2) v (3).

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[H+]

63

CdS 26, 1 9, 18 Tan d trong HCl > 1 M

CuS 35,2 -15,28 Kh tan trong HCl

HgS 51,8 -31,88 Kh tan trong HCl

Li dng tnh tan khc nhau trong cc axt m ta c th thit lp cc iu kin tch cc mui sunfua.

V d 5: Thit lp nng HCl tch CdS v CuS.

Phn ng ha tan CdS trong HCl.

ho tan hon ton v d 0,10 mal CdS cn thit lp nng HCl bng bao

nhiu?

Gii ra ta c C = 0,73M

Trong iu kin ny CuS khng tan. V d trong HCl 2M th:

Tng t trn ta c:

Nh vy tch CdS khi CuS ch cn thit lp nng axit HCl khong 1 -

2M

4.5.2. Ho tan kt ta trong cc thuc th to phc

Trong trng hp ion ca kt ta to c phc bn vi mt thuc th, th c th dng thuc th ny ho tan kt ta ca ion . tan ph thuc tch s tan ca kt ta, hng s bn ca phc to thnh, pH ca dung dch, nng thuc

64

th...

V d, mt s ion to c phc bn vi NH3 do c th dng NH3 ho tan kt ta hyroxit ca nhiu ion kim loi: Cu(OH)2, Zn(OH)2, Ag2O, Cd(OH)2. Mt s mui ca cc kim loi ny cng tan c trong NH3, v d AgCl tan d dng trong NH3:

Tuy vy AgI, Ag2S, CuS, Zns, CdS c tch s tan qu b nn khng th tan

trong NH3:

Cng vy, HgS khng tan trong HCl bi v tch s tan ca HgS qu b, mc

du Hg2+ to phc bn vi ion Cl-:

Vic nh gi gn ng theo cn bng trn cho thy tan ca HgS trong HCl

1 M vo c S 3,3.10-9M. tan ny rt b mc du ln gp nhiu ln so vi tan ca HgS trong nc (= 10-26M).

4.5.3. Ho tan kt ta trong thuc th oxi ho - kh

Nu ion ca kt ta c tnh oxy ho th c th s dng phn ng oxy ho kh ho tan kt ta bng mt thuc th thch hp. tan ph thuc tch s tan ca kt ta v bng hng s cn bng ca phn ng oxy ho kh. V d cc kt ta sunfua kim loi c kh nng tan trong HNO3:

65

Thay cc i lng ny vo biu thc tnh K ta c:

i vi CuS, Ts = 10-35,2; lgK = 37,4

HgS, Ts = 10-51,8; lgK = - 12,4

Do ta thy CuS tan d dng trong HNO3

Cn HgS tan rt kh trong HNO3

Ngoi HNO3 c th dng H2O2 (mi trng axit) (NH4)2S2O8, PbO2. ho tan cc kt ta c tnh kh. V d: Ho tan kt ta trong HCl + H2O2

Ho tan kt ta MnO(OH)2 trong PbO2 + HNO3:

C th dng mt s thuc th c tnh kh ho tan cc kt ta c tnh oxy

ho: ho tan MnO(OH)2; CO(OH)3 trong HCld, trong H2O2 (mi trng axit)

Trong mt s trng hp s ho tan kt ta l kt qu phn ng o xi ho kh v

s to phc. V d: c th ho tan HgS trong hn hp HNO3 + HCl, ion S2- b oxi ho bi HNO3 ng thi vi s to phc ca Hg2+ vi Cl-:

66

Phn ng xy ra rt nhanh v k rt ln.

4.5.4. Ho tan cc kt ta kh tan bng cch chuyn chng sang hp cht khc d tan trong thuc th thch hp

Mt s kt ta kh tan trong cc thuc th thng thng, V d: BaSO4, CaSO4, SrSO4 rt kh tan trong cc axit. C th chuyn chng sang mui Cacbont tng ng tan d trong axit.

Nu tch s tan ca mui to thnh b hn tch s tan ca hp cht cn chuyn ho th qu trnh chuyn ho xy ra d dng. Cn nu ngc li s chuyn ho xy ra kh khn hn.

V d 6: Tnh nng Na2CO3 phi ly 1 lt dung dch ny c th chuyn ho hon ton cc mui sau:

a) 0.010 mol CaSO4

b 0,0 10 mol BaSO4

chuyn ho mui sunft thnh cabonnat ngi ta cho kt ta sunfat tc dng vi mui cacbonat tan, V d: Na2CO3

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trng hp (1) ch cn nng Na2CO3 0,010M l . Cn trng hp (2)

th cn nng Na2CO3 ln gp 50 ln. Thc t chuyn ho c th phi dng Na2CO3 bo ho v chuyn nhiu ln, mi ln u phi un nng dung dch.

68

CU HI V BI TP CHNG

4.4. Trong dung dch bo ho ca mui AB2X3 c cn bng:

Nng ln X- trong dung dch bng 4,0.10-3 M

1. Tnh tch s tan ca kt ta AB2X3

2. Tnh tan ca AB2X3 trong dung dch BCl 5,0.10-6M

3. Tnh tan ca AB2X3 trong dung dch ACl 1,0.10-4M

4.2. Hy cho bit nh hng ca cc cht n tan S ca kt ta trong cc trng hp sau:

1. NH4Cl n SMgNH4PO4 4. NaOH n SPbSO4

2. NH3 trn SMg(OH)2 5. NH3 n SAg2CrO4

3. H+ n SAg2CrO4

Gii thch ti sao?

4.3. Trn 1,0ml K2CrO4 0,120M vi 2,0ml Ba(OH)2 0,009M, c kt ta BaCrO4 xut hin hay khng,

4.4. Cho H2S li qua trung dch CuCl2 0,010M, FeCl3 0,010M v MnCl2 0,010M cho n bo ho bng 0,10M. C hin tng g xy ra? Cho pTs ca cc cht: CuS(35,2), FeS (17,2); MnS (9,6); pKa ca (H2S): 7,02; 12,9 4.5. Trn 1,0 ml HCl 0,30M vi 1,0ml Pb(NO3)2 0,01M . C kt ta PbCl2 tch ra khng? Cho T = 1,6. 10-5.

4.6. Tnh tan ca AgI trong:

a) Dung dch KNO3 10-2M

b) Dung dch Ba(NO3)2M

c) Dung dch Al(NO3)3 10-2M

tan ca AgI trong cc dung dch tng bao nhiu ln so vi tan ca n trong nc tinh khit.

Tch s tan ca AgI 20oC l 8,3.10-17.

4.7. Tnh tan ca CaC2O4 trong dung dch (NH4)2C2O4 5.10-2M v so snh vi tan ca n trong nc. Tch s tan ca kt ta ny l 1,38.10-9M

69

4.8. Ngi ta dng 20ml dung dch NH4NO3 ra kt ta MgNH4PO4. Hy tnh nng dung dch NH4NO3 theo phn trm khi lng khi ra khng mt qu 0,01mgMgO.

4.9. Tnh nng ion H+ t nht cn phi c trong dung dch CaCl2 0,02M v K2C2O4 0,1 M CaC2O4 khng kt ta.

Nu dung dch gm CaCl2 2.10-2M, HCl 10-2M v K2C2O4 10-2M th CaC2O4 c kt ta c khng?

4.10. Tnh tan ca cc sunphua sau: Ag2S, Cu2S, PbS v HgS trong nc, c

k n tng tc ca ion ca kt ta vi cc ion ca H2O.

4.11. Mg(OH)2 c kt ta c khng khi thm 1ml dung dch MgO2 1 M vo

100ml dung dch NH3 + NH4Cl (1 M); Mg(OH)2 c T = 10-9,22 v NH3 c pKb = 4,75.

4.12. Nng cn bng ca NH3 trong 100ml dung dch cha 10-2M kt ta AgO phi l bao nhiu ho tan hon ton lng kt ta , TAgCl = 1,1.10-10, cc phc Ag(NH3)+, Ag(NH3)2+ c 1.1 = 103,32, 1.2 = 107,24.

4.13. Thm dung dch Ag+ 0,1 M vo dung dch hn hp Cl- 0,1 M, Br- 0,1 M th ion no kt ta trc v khi ion th hai bt u kt ta th con th nht c nng cn bng l bao nhiu ? AgCl v AgBr c tch s tan ln lt l 109,75 v 10-42,29.

4.14. Thm dung dch Ag+ vo dung dch hn hp Cl-(10-1m)v Cr42- (10-2M)

Tnh nng cn bng ca ion Cl- khi kt ta nu Ag2CrO4 bt u xut hin. TAgCl= 10-9,75, TAg2CrO4 = 10-11,95.

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CHNG 5 PHN NG OXY HO - KH

5.1. KHI NIM V PHN NG OXY HO KH

5.1.1. nh ngha

5.1.1.1. Phn ng oxy ho kh l phn ng trong c s thay i mc oxi ho, cht oxi ho nhn electron v mc oxi ho gim, cht kh nhng electron v mc oxi ho tng.

C th nh ngha theo nhiu cch khc.

5.1.1.2. Cht kh l cht cho electron s oxi ha tng, ho tr tng, cn cht oxi ho l cht nhn electron v s oxy ha gim.

5.1.1.3. Qu trnh kh l s nhn eletrn qu trnh oxi ho l s nhng electron.

V d: 2Fe + 3O2 = 2 FeCl3

- Cht kh: Fe

- Cht oxi ho: Cl2

- S kh Fe: Cl0 + 1 e = Cl-

- S oxi ho Cl-: Fe - 3e = Fe3+

5.1.2. Cn bng phn ng oxi ho kh (Theo phng php thng bng e).

5.1.2.1. Nguyn tc: Tng electron m cht kh nhng i bng tng electron m cht oxi ho thu vo.

5.1.2.2. Cc bc:

a. Xc nh cht oxi ho, cht kh.

b. Thnh lp phng trnh electron

c. Cn bng phng trnh electron (Tm BSCNN ca tng thu v tng nhng ri tm h scn bng).

d. Hon thin phng trnh phn ng.

V d:

71

Bc 1: Cht kh l As2S3; Cht oxy ha l HNO3

Bc 2 v 3: thnh lp phng trnh electron v cn bng:

Bc 4: hon thin phng trnh phn ng:

5.2. TH OXI HO KH - CHIU CA PHN NG OXI HO KH

5.2.1. Th oxi ho kh - Phng trnh Nernst

n gin ta xt cp oxi ho kh lin hp.

Theo nguyn l II nhit ng hc, nu G < 0 th phn ng xy ra theo chiu

thun, G > 0 phn ng xy ra theo chiu nghch v G = 0, phn ng t trng thi cn bng. Nu G cng m, phn ng cng xy ra theo chiu thun v kh nng oxy ho ca cht oxi ha cng ln khi th kh ca cp cng ln. Th ng nhit ng p ca phn ng c tnh theo cng thc:

(K l hng s cn bng ca phn ng (5.1))

trng thi cn bng, G = 0, ngha l:

G = G0 + RTlnK = 0

Mt khc: G0 = -A' (A' l cng hu ch chuyn n moi electron trong in trng c hiu s in th E, m A' = -nFE v n gin ta t E = E. V mt nhit ng hc E c trng cho TTCB ca phn ng kh Ox + ne = Kh nn n c gi l th kh. V mt ho hc c trng cho mnh yu ca cp Oxh/kh nn gi l th oxi ho kh. Cn v in ha hc n ch ra th in cc ca cp in cc to nn nguyn t ganvanic nn c gi l th in cc.

Chia c 2 v cho - nF, ta c:

72

iu kin tiu chun, T = 25oC + 273 = 298o, F: 96.500C. ng thi chuyn

lnK = 2,3.lgK; R = 8,314 J/mol.K

l phng trnh Nernst tnh th oxi ho ca cp oxi ho kh lin hp.

Eo c gi l th oxi ho kh tiu chun ca cp oxi ho kh lin hp c o iu kin chun v nng dng oxi ho bng nng dng kh = 1mol/l

p dng (5.5) cho 3 trng hp sau:

5.2.2. Chiu ca phn ng oxi ho kh

xt chiu ca phn ng oxi ho kh, ta da vo th tiu chun Eo ca cc cp. Cp no c Eo ln th dng oxi ho ca cp s oxi ho dng kh ca cp c th tiu chun nh hn.

C nhiu yu t nh hng n th oxi ho kh v chiu ca phn ng oxi ho

kh.

73

5.2.2.1. nh hng ca nng

Xt chiu phn ng

a) Khi

b) Khi

Cho

T (1) ta c 2 bn phn ng:

a) Khi

Ta c:

Nh vy Hg22+ oxy ho Ag0 Ag+, phn ng xy ra theo chiu nghch.

b) Khi

Lc ny Ag+ oxi ho Hg22+ Hg0, phn ng xy ra theo chiu thun.

5.2.2.2. nh hng ca s kt ta

Xt chiu phn ng:

T (2) ta cng c hai bn phn ng:

74

Do c kt ta CuI to thnh m TCuI = 10-12 rt b nn nng Cu+ trong dung

dch gim mnh, lm th E, tng ln:

y chnh l th tiu chun iu kin khi (Eo) khi c s kt ta CuI. Th ny

ln hn th nn Cu2+ oxi ho c I- thnh I2 theo chiu thun.

5.2.2.3. nh hng ca pH

Xt phn ng

Xt chiu phn ng pH = 0 v pH =8 cho

T phn ng (3) ta cng tch ra c hai bn phn ng:

Do c nh hng ca [H+] nn lm nh hng n th E1

* Ti pH = 0, [H+] = 1mol/1

V E1 > E2 nn AsO43- oxy ha c I- thnh I2 theo chiu thun.

* Ti pH = 8, [H+] 10-8 mol/l

V E1 < 0,54V nn I2 oxi ha AsO33- thnh AsO43- (theo chiu nghch).

75

5.2.2.4. nh hng ca cht to phc

Tnh th oxi ho kh tiu chun iu kin ca cp Co3+/ Co2+ khi c mt NH3.

Cho hng s bn tng cng ca phc Co(NH3)63+ l hng s bn tng cng ca phc CO(NH3)62+ l

Trong dung dch c cn bng:

Phn ng oxi ho kh khi c mt NH3

Khi khng c mt NH3:

Khi c mt NH3 th:

Ta c cc biu thc tnh bng s bn

T (2) ta c:

76

Vy IIII

oCoCo

oCoCo EE

6.1

6.1/

'/ lg0591,02323

+= ++++

oCoCo

oCoCo EVE

'/21,35

39,4'

/ 2323 (05,01010lg0591,084,1 ++++ =+= l th tiu chun iu kin)

Nh vy hot tnh oxi ha cu Co3+ gim, hot tnh kh ca Co2+ tng ln.

5.3. HNG S CN BNG CA PHN NG OXI HO KH.

Xt hai bn phn ng sau:

Gi s OX1 l cht oxi ho mnh v Kh2 l cht kh mnh th:

trng thi cn bng G = 0, tc l

Nh vy lc cn bng ta c:

Nhn c t v mu ca v tri vi n1, a vo biu thc logant, nhn c t v

mu v phi vi n2 a vo biu thc logarit, bin i ta c:

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Thay cc gi tr: F = 96.500C, lnK = 2,3lgK, R = 8,314J/mol.K,T = 298K

Ta c:

Khi n1 = n2 = n th:

V d: Ngi ta thm d thu ngn lng vo dung dch Fe3+.10-3M. Lc cn

bng ch cn 4,6% Fe dng Fe3+ ( 298K). Bit th kh tiu chun ca cp Fe3+/Fe2+ = 077V Tnh th kh ca cp Hg22+ /Hg nu phn ng xy ra duy nht trong h l:

Bi gii:

HSCB ca phn ng trn l K:

Theo (5.7):

78


5.1. Thit lp s ph thuc th - pH i vi cc h oxi ho - kh sau:

5.2. nh gi kh nng oxi ho ca Fe3+ i vi I khi c mt ca F-. Gi s Fe3+

ch to phc vi F-:

v cc qu trnh trao i electron:

5.3. nh gi kh nng oxi ho I- bi Fe(CN)63- cho bit hng s khng bn

tng hp ca FeF63- l 10-42 v ca FeF64- l 10-35 v cc hng s CB ca cc sn phn ng:

K1, K2 tng ng c

5.4. nh gi kh nng oxi ho - kh ca cp Ag+/Ag khi c mt ca ion

5.5. Tnh th oxy ho kh tiu chun iu kin ca cp Fe(III)/Fe(H) trong

dung dch c d F- to phc FeF63+c 1.6 = 106,1, bit 0 23 ++FeFeE =0,77V.

5.6. Th oxi ho kh tiu chun ca cp Cu2+/Cu l 0,337V. Th oxi ho kh tiu chun iu kin khi c d NH3 to phc Cu(NH3)42+ l - 0,07V. Tnh hng s bn tng cng ca phc .

5.7. Th oxy ho kh tiu chun ca cp Cu2+/cu+ l E0 = + 0,153V. Tnh th oxi ho kh tiu chun iu kin ca cp khi c thiocyanat SCN- to kt ta CuSCN c tnh s tan ca TCuSCN = 10-14.32.

5.8. Gii thch ti sao bc kim loi khng tc dng vi HCl m tc dng vi HI gii phng ra Hidro (H2) cho 0 0AgAgE + = 0,80V, TAgCl= 10

-10, TAgI= 10-16

5.9. Tnh hng s cn bng cc phn ng oxi ho kh:

79

trong dung dch nng ln H+ l 1M.

Cho VE CeCe 55,1

0/ 34

=++ , VE CrOCr 36,10 2/ 3272 =+

VE MnMnO 51,10

/ 24=+ , VE FeFe 77,00 / 23 =++

VEl

I 54,00

22

=

,

5.10. Tnh HSCB ca phn ng gia ASO33- v I2

a) Trong mi trng axit c pH = 2

b Trong mi trng NaHCO3 0,1 M c pH = 8.

80

PHN TH HAI CC PHNG PHP PHN TCH HA HC

Nh nu trn ho hc phn tch gm phn tch nh tnh v phn tch nh lng. Phn tch nh lng gm cc phng php phn tch ho hc v cc phng php phn tch ho l. Cc phng php phn tch ho hc gm c phn tch khi lng v phn tch th tch. Cc phng php ho l gm phn tch o mu, phn tch sc k, phn tch in ho.

CHNG 1 PHN TCH KHI LNG

1.1. NGUYN TC CHUNG CA PHNG PHP PHN TCH KHI LNG

Phn tch khi lng l phng php nh lng ha hc trong ngi ta o chnh xc bng cch cn khi lng ca cht cn xc nh hoc nhng hp phn ca n c tch ra trng thi tinh khit ha hc hoc l di dng hp cht c thnh phn bit trc.

Th d, nh lng vng trong hp kim, ngi ta ly mt mu i din cho hp kim em ha tan mu ny trong mt lng thch hp nc cng toan 3HCl + 1HNO3 c chuyn hon ton mu thnh dung dch. em ch ha dung dch bng nhng thuc th thch hp, ri kh chn lc v nh lng vng (III) thnh vng kim loi (Au). em lc, ra kt ta Au ri sy v nung n khi lng khng i. Cui cng cn lng Au trn cn phn tch xc nh khi lng ca n. T khi lng ny, xc nh hm lng vng trong mu hp kim.

xc nh Mg, ngi ta tin hnh nh sau: ha tan mu phn tch trong dung mi thch hp chuyn ton b lng Mg vo dung dch di dng in Mg2+. Ch ha dung dch bng cc thuc th thch hp kt ta hon ton v chn lc in Mg2+ di dng hp cht kh tan MgNH4PO4. Lc, ra kt ta v sy n nhit thch hp chuyn hon ton thnh hp cht Mg2P2O7. Cui cng cn xc nh khi lng ca n. Da vo cng thc ca kt ta v khi lng va cn c s tnh c hm lng ca Mg trong mu phn tch. Trong th d ny hp cht MgNH4PO4 c kt ta tch nh lng Mg c gi l dng kt ta, cn

81

Mg2P2O7 hp cht c to thnh sau khi nung dng kt ta v em cn xc nh hm lng ca Mg c gi l dng cn. Phng php phn tch khi lng Mg nh trn c gi l phng php kt ta. Phng php kt ta l phng php c s dng ph bin nht trong phn tch khi lng.

xc nh CO2 trong qung ccbnt ngi ta phn hy lng mu CaCO3 bng axit trong mt dng c ring:

Ton b lng kh CO2 gii phng ra c hp th ht vo hn hn Cao +

NaOH ng trong mt bnh ring. Lng CO2 c xc nh theo tng khi lng ca bnh ng hn hp hp th, phng php xc nh hm lng CO2 nh trn gi l phng php ct.

xc nh SO42- ngi ta kt ta n di dng BaSO4 (dng kt ta), lc ra, sy, nung, cn kt ta (dng cn), ta tnh ra c hm lng SO42- trong dung dch no .

1.2. YU CU CA DNG KT TA V DNG CN

phng php phn tch khi lng t c chnh xc cao, dng kt ta phi tha mn nhng yu cu sau:

- Kt ta cn phi thc t khng tan. Mun vy khi tin hnh kt ta ngi ta phi chn nhng iu kin thch hp nh pa ti u, nng thuc th, nhit thch hp kt ta hnh thnh thc t khng tan, hoc nh ngi ta ni cht phn tch c kt ta mt cch nh lng, th d kt ta ti 99,99%.

- Kt ta thu c cn phi tinh khit, khng hp ph cng kt v ni hp cc tp cht. Ch c nh vy th dng cn mi c thnh phn xc nh ng ng vi cng thc ha hc ca n.

- Kt ta cn thu c di dng d lc ra c th tch ra khi dung dch mt cch nhanh chng v thun li nht.

Yu cu quan trng nht ca phn tch khi lng l cht rn thu c cui cng phi c cng thc xc nh t khi lng ca n tnh ra c chnh xc hm lng nguyn t hoc tn cn nh phn. i vi nhng kt ta loi BaSO4 c cng thc xc nh, bn vng nhit cao, nn sau khi ra sch v sy kh th t khi lng ca n s c th tnh c lng ln Ba2+ hoc SO42- c trong dung dch phn tch. Nh vy, trong trng hp ny ang kt ta v dng cn l mt hp cht. Nhng khng t kt ta, chng hn Fe(OH)3 v Al(OH)3 thng khng c cng thc xc nh nn khng th chn l dng cn m phi nung chng nhit

82

cao ti khi c khi lng khng i chuyn chng thnh Fe2O3 hoc Al2O3 l nhng dng cn phi tha mn nhng yu cu sau:

- Phi c cng thc xc nh, c thnh phn khng i t khi sy hoc nung xong n khi cn n trn cn phn tch. Th d, dng cn khng c ht m, khng hp th kh CO2 c trong khng kh, khng b phn hy bi nh sng... tha mn yu cu ny cn phi tin hnh phn tch theo nhng k thut nht nh.

H s chuyn (cn gi l h s phn tch) cng nh th cng tt. Th d, c th xc nh Cr3+ di dng cn l BaCrO4 hoc Cr2O3. Gi s, sai s tuyt i khi cn BaCrO4 v Cr2O3 u l 1 mg, nhng khi cn dng Cr2O3 th sai s l 52 x 2/152 = 0,7mg Cr, cn khi cn dng BaCrO4 th sai s l 52x1/253,3 = 0, 20mg Cr, nh hn trng hp trc 0,7/0,2=3,5 ln.

1.3. CCH TNH KT QU TRONG PHN TCH KHI LUNG

a/ H s chuyn cn gi l h s phn tch

Thng thng dng cn khng phi l dng cn xc nh hm lng, v vy, t khi lng ca dng cn tnh khi lng ca dng cn xc nh hm lng. Do , tin cho vic tnh kt qu phn tch, ngi ta a ra khi nim h s chuyn. l i lng m ta cn phi nhn khi lng ca dng cn vi n c khi lng ca dng cn xc nh, thng thng h s chuyn l t s ca khi lng ca mt, hai hoc nhiu nguyn t hoc phn t hoc tn ca dng cn xc nh v khi lng phn t ca dng cn. Ni cch khc h s chuyn ch ra c bao nhiu gam nguyn t (cht) cn nh phn trong 1 gam dng cn. Trong trng hp cn xc nh Si th h s chuyn t SiO2 (dng cn) thnh Si dng cn xc nh l:

Th d khc: nu dng cn l Mg2P2O4 v dng cn xc nh hm lng l Mg;

MgO hay MgCO3 th h s chuyn ln lt l

b) Cch tnh kt qu phn tch

Thng thng ngi ta tnh kt qu theo % khi lng ca cht cn nh phn trong mu.

83

Gi s lng cn mu l p (g). Khi lng dng cn l q (g). K l h s chuyn.

Hm lng tnh theo % khi lng x.

Nu p (g) mu cha trong v (ml) dung dch m ly ra v (ml) phn tch th:

Trong trng hp phn tch kh v hi th cch tnh kh n gin. V d, xc

nh m ca mu, ta ly p (g) mu, Sau khi sy kh th cn li p (g). Vy m ca mu l:

84

CHNG 2 PHN TCH TH TCH

2.1. NHNG KHI NIM C BN CA PHN TCH TH TCH

2.1.1. Nguyn tc ca phng php

2.1.1.1. nh ngha: Phng php phn tch th tch l phng php phn tch da trn vic o chnh xc th tch dung dch thuc th (l dung dch chun) phn ng va vi dung dch phn tch. T th tch v nng ca dung dch chun chng ta tnh ra hm lng cht cn phn tch.

o th tch chnh xc ca dung dch chun ta dng dng c l Bu rt (cn gi l ng chun ) cn bnh ng dung dch phn tch gi l bnh nn. Bu rt c chia n 0,1ml, thng dng l 25, 50, 100ml. Cn bnh nn thng dng c dung tch 50,100, 250ml.

Qu trnh thm t t dung dch thuc th B t trn Buret xung cht nh phn A gi l s chun .

im m A phn ng va vi B gi l im tng ng. xc nh im tng ng ngi ta cho ch th vo bnh nn. Ti im tng ng ch th b mt mu hoc xut hin mu hoc i mu do bn thn n phn ng vi lng d ca thuc th (ch cn 1,2 git) l xut hin hay i mu.

V d: Khi chun HCl bng NaOH

Ta cho phenolphtalein vo bnh nn cha HCl, dung dch khng mu. Nhng

khi lng HCl ht th 1 git NaOH d xung s lm dung dch chuyn sang mu hng do phenolphtalein tc dng vi OH-.

2.1.1.2. Cc yu cu ca phn ng dng trong phn tch th tch

Do qu trnh chun din ra nhanh li i hi nhn bit i mu tc khc kt thc chun ng lc nn phn ng trong phn tch i hi cc yu cu sau:

a) Phn ng phi hon ton: C ngha l phn cht cn li sau khi kt thc nh phn nh hn sai s cho php.

V d: Sai s 0,1% l cho php c. iu c ngha l tc phn ng phi ln, qua tnh ton v qua thc t tc phn ng phi ln hn 106.

Nu phn ng din ra khng hon ton th phi c bin php thc y phn ng

85

bng cch to phc hoc kt ta vi sn phm, v d:

phn ng hon ton phi cho thm KCNS vo va to phc bn Cu2(CNS)2 va trnh kt ta c, va d nhn bit s chuyn mu.

b) Phn ng phi chn lc: Ngha l ch cho mt loi sn phm duy nht khng km theo phn ng ph to ra cc sn phm ph va kh xc nh im tng ng va gy sai s ln v thuc th (dung dch chun) hoc cht nh phn tiu tn mt lng no vi cht l m ta khng tnh c loi tr. khc phc tnh trng ny ta phi dng cht cht bng cch thm cht to phc vo n s ngn cn ion l khng phn ng vi thuc th hoc cht nh phn cng c trng hp ch cn iu chnh pa ca mi trng s ngn cn c phn ng ph. v d khi chun Cl- bng AgNO3, ta phi thc hin trong mi trng trung tnh hoc kim yu v nu mi trng kim mnh th:

hoc axit mnh s phn ng vi CrO42- (cht ch th) theo phn ng:

lm mt tc dng cht ch th:

c) Tc phn ng phi ln: Trong phn tch th tch im tng ng c xc nh bng s i mu ca ch th, nu chm s d rt nhiu dung dch chun nu phn ng chm th phi thm vo h phn ng cht xc tc no .

d) Phi xc nh c im tng ng bng ch th: Trong phng php trung ho ta dng ch th pH, trong phng php oxy ho kh v trong phng php kt ta v to phc dng ch th l cc cht v c hoc hu c c kh nng to vi thuc th d mt sn phm c mu c trng.

e) ng lng gam ca thuc th cng ln cng tt: khi pha dung dch tiu chun sai s cn l nh nht.

2.1.2. Phn loi cc phng php chun .

2.1.2.1. Phn loi theo bn cht v c ch phn ng: Theo cch ny c bn cch chun :

a) Chun trung ho (cn gi l phng php trung ho):

l php xc nh nng mt axit hay kim bng dung dch chun kim hay axit, ch th ca php chun ny l ch th pH, khong pH i mu ca ch

86

th nm trong bc nhy pH ca php chun , v d Phenolphtalein, metyldacam.

b) Chun oxy ho kh (cn gi l phng php oxy ho kh): Phng php ny c da trn vic xc nh nng cc cht oxy ho (hoc cht kh) bng cc cht kh (hoc oxy ho) ch th ca php chun l cht ch th oxy ho kh c khong th i mu nm gn hoc gn gn trong bc nhy th.

V d chun Fe2+ bng dung dch chun Cr2O7.

Sau khi lng Fe2+ (di bnh nn) ht th Cr2O72- d s phn ng vi iphenyl

quan, lm cho n bin i t khng mu sang mu tm xanh. Khi ta kt thc nh phn.

c) Chun kt ta (phng php kt ta): Sn phm to thnh l kt ta, song vn nhn bit c s i mu ch th no . V d vic xc nh Cl- bng AgNO3, khi d AgNO3 n s phn ng vi K2CrO4 (l Cht ch th di bnh nn):

Mu ca dung dch chuyn t c trng (mu ca AgCl) sang hng nht bo

hiu s kt thc chun .

d) Chun to phc (phng php to phc): Sn phm to thnh phi l nhng phc cht bn v tan trong nc. Dung dch chun cng l nhng cht to phc, chng to vi ion kim loi thnh phc bn hn phc ca ch th vi ion kim loi. Sau tng ng cng c s i mu r rt ca ch th:

2.1.2.2. Phn loi theo trnh t v thao tc th nghim.

a) Chun trc tip: Cho dung dch chun tc dng trc tip vi dung dch nh phn.

87

b) Chun gin tip: l vic xc nh nng cht nh phn thng qua

sn phm to thnh gia A (dung dch nh phn) v B (dung dch chun).

V d:

Thc t l vic xc nh A khng thng qua B m phi c C (sn phm ca A

v B) tc dng vi E ri t E suy ngc li gin tip tnh ra A.

c) Chun ngc: l vic xc nh A..phi qua hai dung dch chun (B v C)

B d + A = E + D

B cn + C = G + H

Tnh A theo cng thc sau:

2.1.3. Tnh ton trong phn tch th tch.

tnh ton ngi ta phi da vo loi nng c bn l nng ng lng v chun theo cht c xc nh v phi dng nh lut ng lng.

2.1.3.1. Nng ng lng.

a) Nng ng lng (cn gi l nng chun hay l nguyn chun).

l s ng lng gam cht tan c trong mt lt dung dch.

Trong : a l s gam cht tan, D l ng lng gam cht tan, V l s lt dung

dch.

xc nh ng lng gam (D) ta da vo biu thc:

(P l phn t gam ca cht tan n l i lng c trng cho tng loi phn

ng).

- Trong phn ng trung ho, n: S mol H + (hoc OH-) ca axit(hay baz) trao i.

88

- Trong phn ng oxy ho kh, n = s electron m cht oxy ho (hoc cht

kh) trao i.

- Trong phn ng kt ta, n = s in tch trao i,

b) chun theo cht c xc nh (TA/B)

chun ca mt thuc th (A) theo cht cn xc nh (B) l s gam cht cn xc nh (B) tc dng va vi 1 ml dung dch chun A.

tc l 1 ml do AgNO3 tc dng va vi 0,00355g Cl-.

chun

Trong a l l s gam cht A, DA' DB l ng lng gam ca A, B.

2.1.3.2. nh lut ng lng:

Cc cht phn ng vi nhau theo nhng ng lng bng nhau:

Gi VA, NA l th tch v nng ca cht nh phn A.

Gi VB, NB l th tch v nng ca dung dch chun B. Theo nh lut ng lng ta c:

y l dng c bn ca nh lut ng lng tnh ton trong phn tch th

tch.

89

V d 1: C bao nhiu gam Ba(OH)2 ho tan trong 250ml dung dch nu chun 20ml dung dch ny ht 22,4ml dung dch HCl 0,09884N? Ta c phn ng chun :

p dng nh lut ng lng ta c:

V d 2: Tnh % H2C2O4 trong mu, nu cn 0,200g mu v ho tan vo 50ml

dung dch, chun dung dch ny trong mi trng axit ht 30,5ml KMnO4 o,lon.

Gii: Ta c phn ng chun .

Dng cng thc:

Ta c:

Suy ra

2.2. CC PHNG PHP PHN TCH TH TCH

2.2.1. Phng php chun axit - baz (phng php trung ha)

Phng php chun axit - baz da trn phn ng trao i proion gia dung dch chun v dung dch nh phn xc nh nng ca cc axit v baz. Cc phn ng dng trong phng php ny u tha mn cc yu cu ca phn ng dng trong phn tch th tch. Cc dung dch chun dng trong phng php ny thng l cc dung dch axit mnh hoc baz mnh. Trong qu trnh chun nng ca cc in H+ v OH- thay i, tc l pH ca dung dch ny thay i. V vy, xc nh im tng ng ngi ta thng dng nhng cht m mu sc

90

ca chng thay i theo pH ca dung dch. Nhng cht ny c gi l cht ch th axit - baz hoc cht ch th pH. ng biu din s ph thuc ca pH trong qu trnh chun vo th tch dung dch chun thm vo hoc lng cht nh phn c chun c gi l ng nh phn. Ngi ta thng da vo ng nh phn chn cht ch th thch hp nht.

2.2.1.1. Cht ch th axit-baz.

Cc cht ch th axit - baz phn ln l cc phm nhum hu c. Chng l cc axit hoc baz hu c yu trong dng axit v baz lin hp c mu khc nhau. v vy mu ca chng ph thuc vo pa ca dung dch. Ta k hiu cht ch th l axit HInd v baz l IndOH. y rong nc cc cht ch th phn ly nh sau:

Trong cc dng lin hp tng ng ca mi cht c mu khc nhau.

S thay i mu ca cht ch th ph thuc vo pH ca dung dch c gii thch bng thuyt ion - mang mu.

Khi pH thay i th cc cn bng (a) hoc (b) s chuyn dch v pha phi hoc pha tri, cht ch th s tn ti ch yu di mt trong hai dng lin hp, nn c mu ca dng no tn ti ch yu trong dung dch. V d: nu cht ch th l HInd th khi thm axit vo dung dch ca n th cn bng (a) s chuyn v pha tri nn dung dch c mu ca dng axit HInd. Ngc li nu gim axit ca dung dch (chng hn thm kim vo) th cn bng (a) s chuyn sang pha phi nn dung dch c mu ca dng Ind- (dng baz).

Khi pH ca dung dch cha cht ch th thay i th cu trc ca phn t trong c nhng nhm mang mu thay i dn n s thay i mu ca cht ch th.

V d: Phenolphtalein c th tn ti trong dung dch cc dng cu trc phn t sau y:

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V vy trong mi trng axit v trung tnh phenolphtalein tn ti ch yu di

dng lacion khng mu, trong mi trng baz tn ti ch yu di qui non mu hng, nhng trong mi trng kim mnh li tn ti dng khng mu.

Metyl da cam c th tn ti trong dung dch di cc dng sau:

Mi cht ch th axit - baz thng c mt khong pH i mu. Ta hy xt cht

ch th l axit hu c yu HInd. Trong dung dch nc cha lng nh ca cht ch th :

T s quyt nh mu ca dung dch.

Mt ta thng ch phn bit c s i mu khi t l trong khong

1/10 - 10. Ni cch khc khi nng ca dng axit chnh lch vi nng dng baz khong 10 ln th ta ch thy c mu ca dng c nng ln. Thay i t s nng :

vo biu thc (2.7), ta c:

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Nh vy, khong i mu m mt ta nhn thy c l 2 n v pH. Tuy nhin

i vi mt s cht ch th mt ta c th nhn c s i mu nh hn 2 n v, ngha l khi t s nng ca cc dng nh hn 10 ln. i vi cht ch th loi IndOH, ta cng lp lun tng t xc nh khong pH i mu. Trong bng sau y l mt s cht ch th axit - baz quan trng nht.

Bng 2. Mt s cht ch th axt-baz quan trng Tn thng dng Khong i mu pKa Mu ca dng axit-baz

Thimol xanh Metyl vng Metyl da cam Bromcresol xanh Metyl Bromcresol ta Bromthimol xanh Phenol Cresol ta Phenolphtalein Thimolphtalein Alizann vng GG

1,2 - 2,8 2,9 - 4,0 3,1 - 4,4 3,8 - 5,4 4,2 - 6,3 5,2 - 6,8 6,2 - 7,6 6,8 - 8,4 7,6 - 9,2 8,3 - 10 9,3 - 10,5 10 - 12

1,65 3,55 3,46 4,66 5,00 6,12 7,10 7,81 8,40 9,15 9,90 11,00

vng - vng da cam Vng - xanh - vng Vng - ta Vng - xanh Vng - Vng - ta K0 - hng K0 - xanh K0 - vng

i vi loi cht th th c mu tc l ch mt trong hai dng axit - baz lin hp c mu, chng hn cht ch th loi HInd m dng HInd khng mu th mu ca dung dch s do nng dng c mu Ind- quyt nh. Nu C l gi tr m nng Ind- cn t ti ta nhn ra mu ca n v Co l nng ban u ca cht ch th th pH ca dung dch ti mu ca Ind bt u xut hin l:

Nh vy, pH lm i mu ph thuc vo nng cht ch th. V d, trong

dung dch phenolphtalein bo ha, mu hng xut hin khi pH = 8 cn trong dung dch long hn 10 ln th pH = 9 mi xut hin mu.

Ngoi khong i mu ca cht ch th, ngi ta cn dng khi nim ch s pT ca cht ch th axit - baz ch pa m ti cht ch th i mu r nht. Ta thng thy gi tr pT trng vi gi tr pKaHInd ca cht ch th.

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2.2.1.2. ng nh phn

ng nh phn trong chun axit - baz l ng biu din s lin h gia nng cn bng ca ion H+ v lng axit hoc baz chun . Thit lp c phng trnh nh phn ta c th v c ng thy c s bin thin pH ca dung dch trong qu trnh chun , chn cht ch th thch hp cho vic chun v khi c phng trnh ng phn nh ta d dng tnh sai s ch th.

a. ng nh phn khi chun axit mnh bng baz mnh: Gi s chun V0 ml axit mnh HA nng Co (mol/l) bng dung dch chun baz mnh NaOH hoc KOH nng C (mol/1).

Phng trnh phn ng chun l:

Hoc vit di dng ion:

Gi F l phn axit c chun , tc l:

Phng trnh bo ton proton ca dung dch trong qu trnh chun :

a F vo phng trnh (2.12) bng cch nhn 2 v ca phng trnh vi

Phng trnh (2.14) l phng trnh tng qut ca ng nh phn, n cho ta

bit mi lin h gia pH ca dung dch v phn axit c chun . Nu bit c gi tr ca V, ta bit c F v da vo phng trnh trn ta tnh c [H+], tc l pH ca dung dch.

Khi mi chun cn tng i xa im tng ng, trong dung dch cn nhiu [H+], nn [H+] >> [OH-] nn t (2.14) suy ra:

94

Phng trnh trn cho ta thy nng [H+] bng nng axit cha c chun

b pha long bng V ml dung dch chun NaOH c thm vo. Tuy nhin, iu ny ch ng khi tng i xa im tng ng. Cng gn n im tng ng ta phi k n s phn ly ca nc. Ti im tng ng khi F = 1, t (2.14) ta c:

Sau v xa im tng ng v d tng i nhiu NaOH nn [OH-] >> [H+]

v t phng trnh (2.14) suy ra:

St trc v sau im tng ng [H+] [OH-] nn phi gii phng trnh

(2.14) tnh pH. Cn ch rng, c th coi CV = CoVo, tc l:

V d 1: v ng nh phn khi chun 100ml dung dch HCl 0,1M bng

dung dch NaOH 0,1M.

p dng cng thc (2.15), (2.16) kt hp vi (2.17) ta tnh c pH khi cho nhng th tch NaOH khc nhau vo dung dch HCl chun HCl. Cc kt qu c tp hp trong bng sau:

V(ml) 0 10 50 90 99 99,9 100 100,1 101 110

F 0 0,1 0,5 0,9 0,99 0,999 1 1,001 1,01 1,1

pH 1 1,1 1,48 2,28 3,30 4,30 7,0 9,70 10,7 11,68

Nhn xt v ng nh phn axit mnh bng baz mnh:

- Trc v sau im tng ng khi F nh hn 0,99 v ln hn 1,01 th pH ca dung dch bin i rt t. Tri li vng gn im tng ng pH thay i rt nhiu to nn bc nhy pH st im tng ng.

- on pH ng vi thi im cn 0,1 % lng axit cha c chun v 0,1 % lng kim d so vi axit, tc l khong pH ng vi F = 0,999 v 1,001 thng

95

c quy c l bc nhy ca ng nh phn. Trong trng hp ny bc nhy ca ng nh phn l 5,4 n v pH (9,70 - 4,30)

- Da vo bc nhy ca ng nh phn ta c th d dng chn cht ch th. Trong trng hp ny ta chn bt k cht ch th pa no c pT nm trong bc nhy. Tc l trong khong pH t 4,3 - 9,7 ta u nhn ra im tng ng vi sai s khng vt qu 0,1 %.

Hnh 2. ng nh phn chun HCl 0,1M bng NaOH 0,1M

- Nng axit cng ln th bc nhy cng di.

Ta hy Tnh pH ca bc nhy.

Khi chun thiu 0,1 % tc l:

Gi s C v Co gn im tng ng nn V gn Vo, ngoi ra c th b qua

OH- cnh H+, nn phng trnh trn c n gin thnh:

Khi chun tha 0,1 % tc l:

V sau im tng ng nn ta b qua H+ cnh OH- v c th coi V = Vo nn

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phng trnh trn c n gin ha thnh:

Bc nhy bng hiu s 2 gi tr pH trn pH

V d 2: Tnh pH ca bc nhy trong cc trng hp sau:

Chun dung dch HCl c nng bng: a) 1 M; b) 0,1 M; c) 0,01 M v d) 0,005M bng dung dch NaOH cng nng .

Sau khi p dng cng thc (c) trn v tp hp cc kt qu trong mt bng ta c:

Nng 1 0,1 0,01 0,005 0,001

Bc nhy 7, 4 v pH 5,4 v pH 3,4 v pH 2,8 v pH 1,4 v pH

Ta thy ngay rng, bc nhy pa ti im tng ng khi chun axit mnh bng baz mnh cng nng cng ln khi nng cng ln.

Sai s ch th:

Trong phn tch th tch ni chung v phng php chun axit - baz ni ring, ngoi sai s th tch do s dng dng c (buret, pipet, bnh nh mc...) gy nn, cn hai loi sai s quan trng khc l sai s do pa ca cht ch th khng trng vi pH im tng ng gi tt l sai s ch th v sai s do xc nh sai pT ca cht ch th, tc l xc nh pH im cui c gi l sai s im cui.

Sai s ch th l sai s do im cui c nhn ra bng cht ch th khng trng vi im tng ng. Ta hy thit lp phng trnh tnh sai s di dng sai s tng i S. Theo nh ngha:

hoc di dng phn trm: S% = (F 1).100

Khi F nh hn 1 th S l phn axit cha c chun v c du m. Khi F ln hn 1, S l phn axit tng ng vi lng baz d v c du dng. Da vo phng trnh ng nh phn, ta lp c phng trnh sai s ch th theo pH

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im cui tc l theo pT ca cht ch th. i vi trng hp chun axit mnh bng baz mnh t phng trnh (2.14) ta c:

Trong : c l ch vit tt ca ch cui. Nu kt thc chun gn im

tng ng th:

Trc im tng ng, nu [H+] >> [OH-] th:

Sau im tng ng, nu [OH-] >> [H+], th:

Nu st im tng ng th dng phng trnh (2.19).

Th d 1: Tnh sai s chun dung dch HCl 0,1M bng dung dch NaOH 0,1M nu kt thc chun a) pH = 5,0 v b) pH = 10,0.

Trng hp a, ta p dng (2. 14):

Trng hp b, ta p dng (2.21):

Th d 2: Nu chun dung dch HCl 0,1M bng dung dch NaOH 0,1 M th

cn kt thc trong khong pH no sai s ch th khng qu 0,1 %. i vi sai s - 0,1 % tc l S = - 0,001: ngha l lng NaOH thm vo cha . p dng cng thc (2.20), ta c:

i vi sai s + 0,01%; tc l S = +0,001; ngha l lng NaOH thm vo d:

Ta dng (2.2 1):

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Nh vy, mun sai s ch th khng vt qu 0,1 % ta phi kt thc chun

trong khong pH t 4,3 v 9,7 hay ni cc khc l dng cc cht ch th c pH nm trong khong 4,3 - 9,7.

b. Chun n t yu bng baz mnh

Phn ng chun :

Phng trnh bo ton proton

Ta cng t F = v a F vo phng trnh trn, ta c:

Phng trnh trn l phng trnh bc 3 theo [H+] nhng trong cc giai on

ca qu trnh chun n c n gin ha.

Khi cha chun F = 0, V = 0 trong dung dch Axt nng C, ta b qua nng ln OH-, phng trnh (2.22) c bin i thnh:

V nu [H+] >> Ka th [H+]2 = Ka.Ca (2.24)

Ta c cng thc quen thuc tnh pH ca dung dch axit yu v t cng thc d dng suy ra: pH = 0,5pKa - 0,5lgCo (2.25)

- Sau khi thm v ml NaOH vo nhng cn xa im tng ng, th [H+] v [OH-] thng nh hn [Na+] v [A-] nhiu nn c th b qua s hng do phng trnh (2.20) tr thnh:

T (2.26) suy ra c:

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Trong phng trnh (2.27), 1 - F ng vi phn axt HA cn li cha c chun

, cn F ng vi phn axit c chun sinh ra baz A- lin hp vi n. v vy, phng trnh chnh l phng trnh tnh pH ca dung dch m gm axit HA v baz lin hp A-.

Ti im tng ng F = 1, ta c dung dch A-, nn [OH-] >> [H+] v phng trnh (2.22) tr thnh:

Nu axit khng qu yu th Ka >> [H+], nn ta c th b qua [H+] cnh Ka v

phng trnh trn ch n gin thnh:

y chnh l cng thc tnh pH ca dung dch A- c th k n s pha long.

Sau v tng i xa im tng ng F > 1 ta c dung dch bm yu A- v baz mnh NaOH d, nn [OH-] >> [H+] v nu:

tc l [HA]

100

V(ml) 0 20 50 90 99 99,9 100 100,1 101 110 150

F 0 0,2 0,5 0,9 0,99 0,999 1,0 1,001 1,01 1,1 1,5

pH 2,88 3,4 4,75 5,8 6,8 7,8 8,74 9,7 10,7 11,6 12,5

Hnh 2. ng nh phn CH3COOH 0,1M bng NaOH 0,1M

Nhn xt v ng cong chun trong trng hp chun axit yu bng baz mnh:

- Cng ging nh chun n axt mnh bng baz mnh, trc v sau im tng ng khi chun n axit yu bng baz mnh pH t bin i theo F nhng st im tng ng pH tng t ngt theo F to nn bc nhy ca ng nh phn. Nu cng qui c bc nhy l pa ng vi s chun tha v thiu 0,1 % lng axit th trong trng hp ny bc nhy ngn hn trng hp chun axit mnh bng baz mnh (1,9 n v pH so vi 5,4 n v pH).

- Khc vi trng hp chun axit mnh khi chun axit yu pH ti im tng ng khng bng 7,0 m ln hn 7 v sn phm ca pa phn ng chun l baz yu CH3COONa.

- Bc nhy ca ng nh phn ph thuc vo cng axit. Axit cng yu th bc nhy pH vng im tng ng cng ngn. Ta hy gi s chun cc dung dch axit HA nng 0,1 M bng dung dch chun NaOH 01M. Cc axit c pha ln lt l 3, 4, 5, 7 v 9. Trong trng hp ny ta s tnh pH ti im tng ng v bc nhy l gi tr pH khi chun thiu v tha 0,2%. Ta c bng sau:

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Cng axit (pKa) pH tng ng Bc nhy pH pH

3,0

4,0

5,0

7,0

9,0

7,85

8,35

8,70

9,85

10,85

5,7 10,0

6,7 10,0

7,7 10,0

9,7 10,3

10,84 10,9 1

4,3 v

3,3 v

2,3 v

0,43 v

0,06 v

Qua cc kt qu trn ta thy, cc dung dch axt cng nng nhng c pKa khc nhau th vi axit cng yu, tc c pKa cng ln th pH ti im tng ng cng ln hn 7 nhiu v bc nhy pa ti im ng ng cng ngn. Khi pKa bng 9 th thc t khng c bc nhy. i vi axit c pKa bng 7 bc nhy pH cha t c 0,5 n v pH; nn kh chn c cht ch th c th phn bit c s i mu bng mt thng. i vi axit c pKa bng 8 hoc 9 th khng th chun c v khng th chn c cht ch th.

- Bc nhy ca ng nh phn cn ph thuc vo nng ca axit cn chun v nng ca dung dch chun baz mnh.

Ta hy tnh nc nhy ng nh phn khi chun cc dung dch CH3COOH pKa = 4,75 c cc nng l 1,0; 0,1; 0,01; 0,001M bng cc dung dch chun NaOH cng nng . Trong trng hp ny gi s bc nhy ng vi F - 1 = 0,002; tc l chun thiu v tha 0,2% lng axit cn chun . Sau khi gii phng trnh (a), ta c cc kt qu sau:

Nng CH3COOH v NaOH pH bc nhy pH

100 M 7,45 - 11,0 3,55 v pH

0,10 M 7,45 - 10,0 2,55 v pH

0,01 M 7,44 - 9,0 1,50 v pH

0,001 M 7,37 - 7,8 0,43 v pH

Qua cc v d trn ta thy bc nhy cng ngn nu nng axit cng nh. Cc dung dch nng nh hn 0,001M gn nh khng c bc nhy, nn nu chun s mc sai s ch th rt ln hoc khng th chn c cht ch th.

Phng trnh tnh sai s ch th khi chun axt yu bng baz mnh: T phng trnh ng nh phn ta rt ra phng trnh tnh sai s cht ch th:

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Nn im cui gn im tng ng th:

Ty theo pT ca cht ch th, ta c th n gin ha c cc phng trnh trn

hn na.

V d 1: Tnh sai s ch th, khi chun dung dch CH3COOH 0,1 M bng dung dch NaOH 0,1M dng cc cht ch th c pT bng a) 4,0; b) 8,0; c) 9,0. Trc ht tnh pH ti im tng ng theo (2.28)

a) Trng hp pH = 4:

V [H+] = 10-4 v [OH-] = 10-10 nn c th b qua s hng th hai trong phng trnh (2-31) tnh sai s:

Ngha l mc sai s m rt ln: 85% lng axit cha c chun .

b) Trng hp pH = 8:

Ta kt thc chun trc im tng ng nn vn b qua s hng th 2 trong phng trnh (2.31) tnh sai s:

hoc -0,056%. Sai s kh nh v ch cn 0,056% lng axit chua b chun .

c) Trng hp pH = 9:

Ta kt thc chun sau im tng ng khi d lng nh kim ca dung dch chun. Trong trng hp ny ta b qua s hng th nht trong phng trnh (2.31) tnh S:

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Sai s rt nh v ch ch chun tha 0,02% lng axit.

V d 2: tnh sai s chun d

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