Feature 13-3 Calculating the Concentration of Indicator Solutions

The indicator concentration is not critical in the Volhard titration. In fact, calculations similar to those shown in Feature 13-3 demonstrate that a titration error of one part in a thousand or less is possible if the iron(III) concentration is held between 0.002 M and 1.6 M. In practice, an indicator concentration greater than 0.2 M imparts sufficient color to the solution to make detection of the thiocyanate complex difficult because of the yellow color of Fe-. Therefore, lower concentrations (usually about 0.01 M) of iron(IIl) ion are employed. 

The most important application of the Volhard method is the indirect determination of halide ions. A measured excess of standard silver nitrate solution is added to the sample, and the excess silver is determined by back-titration with a standard thiocyanate solution. The strong acidic environment required for the Volhard procedure represents a distinct advantage over other titrimetric methods of halide analysis because such ions as carbonate, oxalate, and arsenate (which form slightly soluble silver salts in neutral media but not in acidic media) do not interfere. 

Silver chloride is more soluble than silver thiocyanate. As a consequence, in chloride determinations by the Volhard method, the reaction 

Both silver nitrate and potassium thiocyanate are obtainable in primary-standard quality. The latter is, however, somewhat hygroscopic, and thiocyanate solutions are ordinarily standardized against silver nitrate. Both silver nitrate and potassium thiocyanate solutions are stable indefinitely. 

Epoxide Volhard Titration of excess Cr following hydrohalogenation 

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