Improvement of Determination of Tungsten by Thiocyanate Colorimetry

Thiocyanate colorimetric method used in determination of tungsten is based on the fact that W⁵⁺ reacts with thiocyanate and forms a yellow complex. In order to reduce tungsten to W⁵⁺ completely, titanium trichloride or stannous chloride were generally used as reducing agents. In application, when adding more reducing reagents, Ti³⁺ purple color masks yellow color from the complex, and when adding less reducing reagents, red ferric rhodanide affects determination. Through plenty of experiments, titanium trichloride and stannous chloride were choosed as dual reducing agents. The stable complex can be obtained by controlling acidity, temperature and reaction time and the stability of the analytical results can be remarkably improved. The method has been tested by the determination of tungsten in national standard reference materials and the results are in good agreement with certified values. The improved method is suitable for routine analysis of trace tungsten in geological samples.