Influence of instability of thiocyanate in argentimetry and mercurimetry

Abstract

The silver - thiocyanate and mercury(II)- thiocyanate reactions have been examined by precise mass titrimetry for possible errors arising from decomposition of thiocyanate in nitric acid media. For silver, potentiometric titration in either direction in 0.15 mol kg^–1 nitric acid is free from error provided steps are taken to overcome adsorption, but thiocyanate cannot be stored at this acid concentration. Using iron (III) as the indicator, visual titration in the Volhard direction (thiocyanate titrant) is unaffected by time delay, acid concentration and indicator concentration; in the reverse direction an error of the order of 0.5% is incurred and increases as time, acid and indicator concentration increase. For mercury(II), potentiometric titration is unsatisfactory in both directions. Standard mercury (II) ion solutions hydrolyse in nitric acid less than 4.0 mol l^–1 while thiocyanate is unstable in solutions containing more than 0.05 mol l^–1 of nitric acid. Neutral thiocyanate solution and mercury(II) solution in 4.0 mol l^–1 nitric acid can be titrated in either direction using iron(III) as the indicator, but the precision is poor and time delays are not tolerated. Higher indicator concentrations favour better precision through increased ease in locating the end-point.