Page 84 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202

4- Titration of iodide and of thiocyanate is not successful because
silver iodide and silver thiocyanate adsorb chromate ions so strongly that
a false and somewhat indistinct end point is obtained.

B- Formation of a soluble colored compound (Volhard method)
This procedure is exemplified by the method of Volhard for the

titration of silver ion in the presence of free nitric acid with standard
potassium or ammonium thiocyanate solution. The indicator is a solution
of ferric nitrate or ferric ammonium alum. The addition of the thiocyanate
solution produces first a precipitate of silver thiocyanate (Ksp = 7.1×10-13).

Ag+ + SCN- ↔ AgSCN
When this reaction is complete, the slightest excess of thiocyanate
produces a reddish-brown coloration, due to the formation of the complex
ferri-thiocyanate ion:

Fe3+ + SCN- ↔ [Fe(SCN)]2+
This method may be applied to the indirect determination of
chlorides, bromides, and iodides in acid solution. Excess of standard
silver nitrate solution is added, and the excess is back-titrated with
standard thiocyanate solution. In the analysis of chloride ion, an error can
occur if AgCl precipitate is allowed to react with SCN- because AgSCN
(Ksp = 1.16×10-12). is more insoluble than AgCl (Ksp = 1.1×10-10).

AgCl + SCN- ↔ AgSCN

It is therefore absolutely necessary to prevent the reaction between
the thiocyanate and silver chloride by filtering the latter before back
titration or, by adding a small quantity of an immiscible liquid such as
nitrobenzene to "coat" the silver chloride particles and thereby protect
them from the interaction with the thiocyanate. Bromide and iodide can
be determined by indirect Volhard method without filtering off AgX

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