Page 84 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

The chromate concentration required to initiate precipitation of

silver chromate under these conditions can be also calculated from its

solubility product:

Ksp = [Ag+]2 [CrO4--]

[CrO4−− ] = KSP = 1.1x10−12 = 10−2
[ Ag + ]2 (1.05x10−5 )2

This means that the concentration of chromate necessary to give the
brick red of silver chromate at equivalence point is 0.01M.

Interferences and limitations of Mohr method

1- The Mohr titration is applicable only in neutral or slightly alkaline

solution with pH values from about 6 to 10. In acid solution, the CrO 2-
4

concentration is greatly decreased according to the following equilibrium:
2 H+ + 2 CrO42- ↔ 2 HCrO4- ↔ Cr2O72- + H2O

Silver dichromate is considerably more soluble than silver chromate.

This decrease in [CrO42-] makes it necessary to add a large excess of Ag+

to bring about precipitation of AgCrO4 and thus leads to large errors.

If, on the other hand, the medium is alkaline, silver will precipitate as

its oxide

2 Ag+ + 2 OH- ↔ 2 AgOH ↔ Ag2O + H2O

2- If ammonium salts are present, the pH of the solution must not
exceed pH 8 otherwise free ammonia will be produced and dissolves
silver chloride precipitate. Therefore, the halide solution should be
neutralized before titration if necessary, by adding NaHCO3 or dilute
HNO3.

3- Cations which give insoluble chromate e.g. barium ions must be
absent or removed before the titration.

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