Determination of hexacyanoferrates III

Excellent results are obtained by the following method, which is of wider applicability. When excess of standard sodium arsenite solution is treated with hydrogen sulphide solution and then acidified with hydrochloric acid, arsenic(III) sulphide is precipitated  

The excess of arsenic(III) oxide is determined with 0.05M iodine and starch. 

The procedure is illustrated by determination of the concentration of hydrogen sulphide water. 

If certain sulphides are treated with hydrochloric acid, hydrogen sulphide is evolved and can be absorbed in an ammoniacal cadmium chloride solution upon acidification hydrogen sulphide is released. 

Discussion. The reaction between hexacyanoferrates(III) (ferricyanides) and soluble iodides is a reversible one  

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