EXTRACTION AND COLORIMETRIC DETERMINATION OF THIOSULFATE AND TETRATHIONATE IN SOILS

Thiosulfate and tetrathionate are two intermediate forms of S produced during oxidation of elemental S to sulfate in soils, but no simple and specific method is available for determination of microgram quantities of these S forms in soil extracts. A simple and precise colorimetric method of determining thiosulfate and tetrathionate in soil is described. It involves extraction of these S forms in soils with 0.1 M LiCl or 0.15 percent CaCl2 solution and alkaline cyanolysis of thiosulfate S in the presence of Cu2+ or alkaline cyanolysis of tetrathionate S in the absence of Cu2+ and colorimetric determination of the thiocyanate produced as ferric thiocyanate complex. The method is rapid, sensitive, and accurate, and it permits determination of microgram quantities of thiosulfate or tetrathionate, or both, in the presence of large amounts of sulfate. Application of the method developed for determination of thiosulfate and tetrathionate on extracts obtained from soils amended with elemental S and incubated at 30°C for various times indicated that thiosulfate was produced within the first few days of incubation and that tetrathionate was accumulated in some soils.