Determination of ultramicro amounts of sulphate as methylene blue—II The reduction

Sulphate may be determined spectrophotometrically as methylene blue after reduction to sulphide, which is allowed to react with p-aminodimethylaniline and ferric iron to form the dye. A previous paper¹ dealt with the colour reaction; this paper deals with the reduction.

A method for the reduction of sulphate to sulphide by hydriodic and hypophosphorous acids in acetic acid solution is proposed. It is founded on an investigation of the following variables: composition of the reducing reagent, composition of the carrier gas and gas-washing liquid, temperature at reduction, and time. The yield of the reduction is found to be 100% within the experimental errors. The standard deviation of a single determination of 100 μg S or 2 μg S is found to be 0.14 μg S or 0.05 μg S, respectively. Some interferences in connection with the application of the method for certain purposes are discussed.