Mechanism of reduction of cadmium by aminoiminomethanesulfinic acid in alkaline media

Abstract

Amlndiminomethanesulflnic acid, formamidinesulfinlc acid, or thiourea dioxidé Is the compound made by the reaction of thiburea arid hydrogen peroxide. This compound Is rémárkable In that it readily reduces cadmium to the metal from aqueous solutions. Zinc metal does not precipitate, Dithionite, S2042-, occurs in alkaline solutions of aminoiminomethanesulfinic acid. Soliitions of dithionite do not reduce cadmium to the metallic state and finding carbon in the precipitated cadmium Indicated that the specieé responsible for the strong reducing properties of the compound is a radical ion formed by a homolytic cleavage of the carbon-sulfur bond. A scheme to explain the above observations is presented.

Recent Developments in the Chemistry of Thiourea Oxides

“…N,N′ ‐Dimethylthiourea and N ‐methylthiourea dioxides produce dithionite under aerobic conditions as well 50. These observations show that, unlike previously assumed, the sulfur dioxide anion radical, SO 2 − , is not a primary product of thiourea dioxide decomposition 52…”

“…A very strong reducing ability of thiourea dioxide in alkaline solutions is manifested in the reduction of Cd 2+ to the metal 52. In contrast to TDO, sodium dithionite does not reduce cadmium to the metallic state52.…”

“…[50] These observations show that, unlike previously assumed, the sulfur dioxide anion radical, SO 2 À , is not a primary product of thiourea dioxide decomposition. [52] Interestingly, dioxygen influences also the yield of ZnS during the reaction between zinc salts and thiourea dioxide in slightly acidic solutions. [4a] Davies and co-workers [4a] reported lower yields of ZnS from precipitation under anaerobic conditions compared to aerobic ones, presumably due to the formation of dithionite in presence of dioxygen.…”

Recent trends in the chemistry of sulfur-containing reducing agents

“…57 The reaction of (NH 2 ) 2 CSO 3 with methyl anthranylate affords 5H-quinazolino[3,2-a]quinazoline-5,12(6H)dione. 102 Numerous studies have been devoted to the use of reduction reactions involving HMS and TUDO to produce metals Ð silver, 103 cadmium, 104 technetium, 105 nickel, 106 and metallic coatings on fibres. 107…”

“…Presumably, 126 decomposition of TUDO gives sulfoxylate as the primary product [however, it is not indicated what process is responsible for the formation of sulfite needed for the formation of dithionite by reaction (16)]. Yet another mechanism of TUDO decomposition has also been proposed; 104 according to this mechanism, the strong reducing properties of TUDO in alkaline media are due to the homolytic decomposition of the H 2 N( = NH)CSO À 2 anion to give reactive species, SO À 2 and (NH 2 ) 2 C OH. It has also been suggested 127 that the SO À 2 radical anion (in the aqueous alkali ± ethanol system) is formed as the primary product of TUDO decomposition.…”