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Precipitation Titrimetry

Compiled by

Dr. L. dlC. Coo

Precipitation titrimetry

Based upon reactions that yield ioniccompounds of limited solubility

Slow reaction limits its application

 AgNO3 : the most important precipitating

reagent for the detmn of

1. halides

2. halide-like anions (SCN-, CN-, CNO-)

3. mercaptans

4. fatty acids

5. divalent inorganic anions

= argentometric titration

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Titration Reaction:

Cl-(aq) + Ag+(aq)   ↔  AgCl(s)

#mmol Cl- = #mmol Ag+

Ksp

 –AgI 8.3 x 10-17

 –AgIO3 3.0 x 10-8

 –AgBr 5.2 x 10-13

 –AgBrO3 5.7 x 10-5

 –AgCl 1.8 x 10-10

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Classification of argentometric

titration by chemical indicators

1. Chemical reagents which react withexcess titrant to yield a colored ppt

2. Chemical species which react with excesstitrant to form a soluble colored reactionproduct, often complex of the titrant andthe indicator 

3. Chemical species which are adsorbed onthe surface of the ppt formed during thetitration

#mmol analyte =

#mmol Ag+DCF- /Ag+

red cpd adsorbed

on the surface of

the ppt

 Ag+ + Cl- AgCl(s)

Pre-equiv pt:

DCF- / Cl- no rxn

Fajan

#mmol analyte =

#mmol Ag+inxs-

#mmol Ag+unrcted

SCN- + Fe3+

Fe(SCN)2+

red complex soln

 Ag+inxs + Cl-

 AgCl(s)SCN- + Ag+unreacted AgSCN(s)

Volhard

SCN-/

Fe3+

#mmol analyte =

#mmol Ag+2Ag+ + CrO4

2-

 Ag2CrO4(s) red ppt

 Ag+ + Cl- AgCl(s)Mohr 

 Ag+/

CrO42-

StoichiometryEndpoint rxnTitrant rxnTitrant/in

dicator 

1. To determine the purity of a bottle of a

reagent grade KCl, Fajan’s method ofprecipitation titration method was

employed. 1.8640 g KCl sample was

dissolved in 250 mL water. A 25.00 mL

aliqout of the KCl soln required 24.93 mLof 0.1000M AgNO3. What is the purity ofthe KCl sample?

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