Precipitation Titrations

1

• Precipitation Titrations
• Most precipitation titrations involve formation
  of an insoluble precipitate with Ag
• Cl-, Br-, I-, CN-, SCN-, CNO-, IO3-, S2-, AsO43-,
  CrO42-, C2O42-, PO43-, by direct titration with
  Ag
• BH4- titrate excess Ag BH4- 8Ag 8OH-
  8Ag(s) H2BO3- 5H2O
• ? add known excess of f4?- to precipitate
  Kf4?(s), add excess Ag to form Agf4?(s) and
  back titrate excess Ag
• Mercaptans, fatty acids, V(OH)4,
  electroanalytical method by direct ditration
  with Ag
• Zn2 precipitate as ZnHg(SCN)4, filtration,
  dissolution in acid, addition of excess Ag,
  back titration of excess Ag
• F- precipitation as PbClF, filtration,
  dissolution in acid, addition of excess Ag,
  back titration of excess Ag
• Some analyses can be done with Hg2 Hg2 2Cl-
  HgCl2(aq)

2

• Precipitation Titrations
• For the reaction between Ag and A- AgA Ag
  A- KspAgA-
• Classify regions of titration curve when Ag is
  titrant
• Initial point VAg 0 mL, no Ag, A- CA-
• Add Ag, prior to equivalence point common ion
  problem with A- in excess
• Equivalence point solubility of salt in water
• Beyond equivalence point common ion problem with
  Ag in excess
• Example Titrate 50.00 mL of 0.1000 M Cl- with
  0.1000 M Ag
• Initial point 0.00 mL added Ag solution
• Cl- CCl- 0.1000 M pCl 1.00
• Ag 0 pAg ?
• Add 1.00 mL Ag solution
• AgCl Ag Cl- Ksp AgCl- 1.82
  x 10-10

3

• Precipitation Titrations
• Example Titrate 50.00 mL of 0.1000 M Cl- with
  0.1000 M Ag
• Add 49.90 mL 0.1000 M Ag
• Equivalence point Ag Cl-
• Add 50.01 mL 0.1000 M Ag or 0.10 mL beyond
  equivalence point
• AgCl Ag Cl-

4

• Precipitation Titrations
• Effect of Concentration of Reagents See Figure
  13-1, FAC7, p. 267
• Effect of size of Ksp see Figure 13-2, FAC7, p.
  268
• Indicators
• For AgA Ag A-
• InA In A-
• color 1 color 2
• see color 1 if
• see color 2 if

5

• Precipitation Titrations
• Mohr Method makes use of a second, colored
  precipitate for indicator action
• CrO42- is used as indicator Ag2CrO4 is brick red
• Ag2CrO4 is more soluble than AgCl
• As Ag is added to a solution of Cl- and CrO42-,
  AgCl forms first until Cl- is low enough for
  CrO42- to begin reacting with Ag
• If Ag2CrO4 begins precipitating when Cl- is at
  the equivalence point concentration
• AgCl Ag Cl-
• Ag2CrO4 2Ag CrO42-
• 6 x 10-3 CrO42- is too colored to see the brick
  red Ag2CrO4
• Thus 2 x 10-3 M CrO42- is used the titration
  will over consume Ag
• Correct the over consumption with an indicator
  blank
• The analytical Solution must be buffered
• 2CrO42- 2H 2HCrO42- Cr2O72-
  H2O
• 2Ag 2OH- 2AgOH Ag2O
  H2O

Use borax
6

• Precipitation Titrations
• Volhard Method form a colored coordination
  compound with Fe3
• Ag is titrated with SCN-
• AgSCN Ag SCN-
• Fe3 SCN- FeSCN2
• blood red
• If Fe3 ? 0.01 M, negligible error is
  introduced Kf(FeSCN2) 103
• Analytical solution must be acidic to prevent
  formation of Fe(OH)3
• Usual application - indirect determination of
  hilides by adding a known excess of Ag and
  back titrating the excess with a standard
  solution of SCN-
• AgCl is more soluble than AgSCN AgCl SCN-
  AgSCN Cl-
• This results in an fading endpoint and over
  consumption of SCN-
• Isolate the AgCl from the solution either by
  filtration or coating AgCl(s) with nitrobenzene

7
Precipitation Titrations Fajans Method makes use
of an adsorption indicator, such as
dichlorofluorescein or tetrabromofluorescein
Fl- not high enough to precipitate AgFl In the
titration of Cl- with Ag, prior to the
equivalence point, Cl- ions are in the primary
adsorption layer, and Fl- is found in bulk
solution colored yellow-green Beyond the
equivalence point, Fl- is in the counter ion
layer and shows a pink color

• HFl H Fl-
• yellow-green
• Fl- Ag AgFl(s)
• pink

8

• Precipitation Titrations
• Fajans Method makes use of an adsorption
  indicator
• Additional requirements
• Make sure colloidal AgCl is formed - add dextrin
• The precipitate must adsorb its own ions
• The pH must not be too acidic - must produce Fl-
  ions
• The pH must not be to basic - avoid precipitation
  of AgOH and formation of Ag2O