Anal.chem21st Lec

8/10/2019 Anal.chem21st Lec

1/13

Analytical Chemistry 2

1 st lec


2/13

CHEMICAL ANALYSIS

Chemicalanalysisprovides informationaboutthecompositionofasample:

Qualitativei.e.what ispresent inasample.Quantitativei.e.determinehowmuchanalyteispresent inasample

QUANTITATIVE CHEMICAL ANALYSIS

Therearetwobroadcategoriesof quantitative chemicalanalysis:

Classical

methods

of

analysis.

These

are

the

oldermethods

of

analyses

and

are

based

on

e.g.

volumetricanalysis

Instrumental methodsofanalysis.Tendto berelativelymodernandinvolve instrumentation


3/13

IS THERE A DIFFERENCE BETWEEN CLASSICAL AND INSTRUMENTAL

METHODS OF ANALYSIS

There isnorealdifference!

Allformsofchemicalanalysesresult inananalyticalsignal.

Classicalanalysis

tends

to

use

indicators

to

signal

the

presence

of

ananalyteordetectanendpoint.Theanalyticalsignal isusuallyachange incolour

ortheappearnceofprecipitate.

Examplesofclassicalmethodsofanalysisyouhavecarriedout:

Quantitative:Acid/basetitrationusinganindicatortodeterminetheend.

Qualitative:Reactionofmetalionswithselectivereagents.


4/13

Instrumentalanalysis involvesan instrumentthatconvertsasignalthat isnotusuallydirectlydetectable intoaformthat is.Inan instrumentaltechnique,somephysico-chemicalpropertyoftheanalyte ismeasuredelectronically,e.g.theabsorptionof light(iftheanalyte iscoloured).

o electricpotential

o Emissionabsorptionordiffractionofradiationo mass


5/13

Components of an instrument

You will come across a number of different instruments

throughout this module. You will use some expensive and

sophisticated instruments in the practical sessions. However,

most instruments contain 4 fundamental components:

Signal Generator:These produce the analytical signal from the

components of the sample. This may simply be the sample itself

or by a device which produces a signal which the analyte

interacts with. It is the change in this signal that is measured.


6/13

Input Transducer or Detector: This is a device that converts one type of signal

to

Signal processor: The signal processor modifies the signal

into a form that is more convenient for the readout device.

Usually this is simply amplification but can be others e.g. for

voltage AC DC.

Readout Device:This can range from a simple meter or

chart recorder or mostly likely with modern instruments a PC

based data system


7/13

.


8/13

In some cases components

may be merged together

e.g. detector and signal

processoroftenoccur

together e.g. photomultiplier.


9/13

WhydowehaveAnalysisofsignal&Separationofsignal?

Somemethodsofdetectingasignalmeasureapropertythat isnotspecifictoananalyte.

Forexample,takeasolutioncontainingNa+andK

+ ions.

IfwemeasuretheconductivityofthesolutionwecannotdifferentiatebetweenNa+.andK

+.

i.e.conductivity isnotspecific.

However, ifweaspiratethesolution intoaflame,the

elementswillemitradiationataspecific(different)wavelength.WecanmeasurethisanddifferentiatebetweenNa

+andK

+ ions(signalseparatedand

analysed).

Wecouldcarryoutchromatographyandseparatethe

ionsthenmeasuree.g.conductivity.(analytesseparatedand individualanalytesanalysed).


10/13

ComparisonofAnalyticalMethods

Approximateworkingranges

Gravimetry ___________________

Titrimetry __________________________

Potentiometry ____________________________________

Electrogravimetry ____________________________

Voltammetry ________________________________

Photometry _____________________

Fluorimetry _____________________

AtomicSpectroscopy ___________________________________

Chromatography___________________________________

g/L 10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10


11/13

CalibrationHowdowerelatethesignalfromthe

instrumenttotheconcentrationofa

solution?

Thevariousmethodsofdeterminingananalytecanbeclassifiedaseitherabsoluteor

relative.Absolutemethodsrelyonaccuratelyknownfundamentalconstantsforcalculatingtheamountofanalyte,e.g.theatomicmassofelementsingravimetricanalysis.However,most instrumentalmethodsare relative.Usually (hopefully!) the responseof the instrument isproportionaltotheanalyteconcentration,soweget:

ResponseConcentration

or

Response=ConstantConcentration

andwecan findtheanalyteconcentration from the instrumentoutput ifweknow thevalueofthe

proportionality

constant.

We

can

find

the

proportionality

constant

by

calibrating

theinstrument.Thisisnormallyachievedbyusingaseriesofstandardsolutions.Theinstrumentresponseismeasuredfortheseknownconcentrationsandagraphofresponseagainst

concentrationofanalytecanbeconstructed.


12/13


13/13

Anal.chem21st Lec

Documents

Transcript of Anal.chem21st Lec