A New Color Reaction for Soluble Organic Sulfur Compounds* by Irvine W. Grote

In view of the increasing importance of reduced sulfur compounds in biological chemistry, a study of the nitroprusside reaction was recently made in this laboratory in an attempt to render the reaction more specific for compounds of the type C-S-H, since sodium nitroprusside in alkaline solution not only gives coloration with compounds of this type but also with ketones, aldehydes, creatinine, thio ethers, and many alkaloids. Such interference was found to be practically eliminated by working at pH 8 to 9 with saturated sodium bicarbonate as the alkaline agent in place of the more generally used carbonate or hydroxide. In bicarbonate solution, the immediate coloration from ketones, aldehydes, etc. is so slight that the test for C S-H may be carried out in aqueous acetone solution without interference. With potassium cyanide being used as a reducing agent, the test can also be applied to compounds of the disulfide type C-S-S-C. During the course of this work the observation was made that sodium nitroprusside solution previously exposed to light for some time gives an intense blue color with compounds of the thiourea type, in addition to the normal purple-red color with C-S-H. A search of the literature revealed that while Hofmann’ noted a blue color as an intermediate in the formation of the red compound, NazFe(CN)sNOSCNHNH2, from thiourea and ordinary nitroprusside, we were unable to find further mention of a blue coloration. Sodium nitroprusside solution after several days exposure to indirect light, or a few hours to direct sunlight, changes from a