Microcolorimetric Determination of Sulfate by Reduction to Hydrogen Sulfide with Tin(II)—Strong Phosphoric Acid

A new reagent, “Tin(II)-strong phosphoric acid” has been prepared to permit a rapid and simple determination of sulfate sulfur by reduction to hydrogen sulfide. The procedure for the determination of semi-micro quantities of sulfate sulfur, previously devised by the authors, has been extended to the microgram scale by adopting the sensitive methylene blue colorimetric method. The rapidity and simplicity of the original method are still retained in this new colorimetric method, by which 1 to 35 micrograms of sulfate sulfur can be estimated. The apparatus is very simple, a test-tube of fused silica being employed as the reaction vessel. The standard calibration curve for colori-metric determination of sulfate sulfur by this method coincides well with that drawn for sodium sulfide by direct coloration with p-amino dimethyl aniline and ferric chloride. Interference of other anions can be kept out of consideration, because the sulfate ion is easily separated by addition of barium chloride solution and coprecipit-ation with ferric hydroxide. Large amounts of ferric hydroxide do not affect the method. Some examples of the determination of sulfate sulfur in ferric chloride reagent and Japanese acid clay are described.