Chapter 24. Coulometry

1) Electrogravimetry: sample soln.+ Pt(wire, gauze)

, weighing

( , )

2) Coulometry : sample soln. + (100%)

1. Electrogravimetry

1-1. : Pt wire Pt gauze

Cathode : Cu2++2e- Cu Eo = 0.34 V

Anode : 1/2O2+2H++2e- = H2O Eo = 1.23 V

Deposit physical characteristic :

Current density Temp. Complexity agent

Electrolysis : Eapp=EC-Ea+c+a-IR

Fig. 24-1 Changes in (a)current and (b) potentials

during the electrolytic deposition of Cu(II) ion.

Current changes during an electrolysis at constant applied potential;

It = Ioe-kt , k = 25.8DA / V

Typically, D = 10-5 cm2/s, = 2 x 10-3 cm

1-2. Constant current electrolysis :

By maintaining the current : .

Fig. 24-2

Eapp = (Ea+a)-(Ec+c)+IR

Ec = EoCu2+,Cu + 0.059/2log[Cu2+]

ex) 0.01M Cu2+ 99.9% voltage

EC = 0.34 + 0.059/2*log[10-2]=0.28 V

EC = 0.34+0.059/2*log[10-5] = 0.19 V ()

a ,c H+ accumulation depolarizer

Cu2++H2O = Cu+1/2O2(g)+H+

NO3-+10H++8e- = NH4++3H2O

Electrode reaction (Pt )

IR

1-3. Controlled potential electrolysis (limited cathode potential electrolysis)

: ( working elec. reference

)

Reference

(working elec.)

Fig. 24-3

: ,

()

Controlled potential electrolysis

I t

I = Ie-kt . logI = -2.303kt+logI0

Ct/C0 = it/io = e-(DA/V)t = 10-0.43(DA/vg)t

v : volume

:

Ct/C0 = e-ktk =mA/V

m : mass transfer coefficient = D/

k = 25.8DA/(V)

1-4. Mercury cathode electrolysis

: ->

Mn++ne+Hg M(Hg)

ex) [Ag+] = 10-6M, [Cu2+] = 0.1M

EC = 0.08+(-0.35) =0.45V

EC-C = 0.45-0.07 = 0.38(V)

EC =0.31V 0.38V volt Ag+

1-5. Internal electrolysis

:

:

2-3. Coulometers

H2, O2 coulometer

Ag coulometer

H2, N2 coulometer

Cu coulometer

H2 coulometer

coulometer

1. H2-O2 coulometer

K2SO4 Pt H2-O2 (16.80ml/1mg)

2. Ag coulometer

Cathode : Pt crueible containing AgNO3 solon

Anode : Ag

Pt+ Ag Q

3. N2-H2 coulometer

Hydrazine sulfate , gas

4.

: KBr+K2SO4

: Pt

2Ag+2Br-+2H2O 2AgBr+H2+2OH-

OH- 0.01 NHCl .

Fig. 24-6

Cl3CCOO- + H+ + 2e- Cl2HCCOO- + Cl2

24D Coulometric titration

Titraton error

Variation in the current source of error

Departure of the process from 100% currnet efficiency

Error in the measurement of current

Error in the measurement of time

Titration error due to the difference between the equivalence point

& end point

: 0.01%

:

1) Pt gauze nitric acid rinsed-dried weighing

2) Deposition :

Eapp=(Ea+a)-(EC+C)+iR

EC=Ecu2++0.059/2log[Cu2+]

@ 0.01M-Cu2+ 99.9% Voltage

EC=0.34+0.059/2log[10-2]=0.28V

a : electrode reaction Pt

e : H+ accumulation (depolarizer) (stirring)

Cu2++H2O Cu+1/2O2(g)=2H+

* IR .

H+ (depolarizer) H2 gas(spongy )

(Cu2+ )

Cathode : Cu2++2e-=Cu

Anode : 1/2O2+2H+=2e- H2O

NO3-+10H+=8e- NH4++3H2O

Cu H2

3-1. Electrolysis at controlled potential

Copper, ion

Working elec. reference

3-2. Electrolytic separations

Easily reduced metals from small currents of less easily reduced material

Fig. 24-7