Simple Thiourea Hydrolysis or Intermediate Complex Mechanism? Taking up the Formation of Metal Sulfides from Metal–Thiourea Alkaline Solutions

By means of a combination of a brief but complete review and a simple experimental design, this work aims to elucidate the most probable reaction mechanism taking place during the chemical synthesis of a metal sulfide from thiourea and a metal ion in alkaline aqueous solution under typical conditions. The presented review covers all of the available information on this matter, starting from the first use of thiourea in the nineteenth century, and such information is thoroughly discussed in the context of each experimental observation. The experimental section provides a simple analysis by optically monitoring at 236 nm the disappearance of thiourea during the reaction for the synthesis of metal sulfide; in this case, cadmium sulfide as example, at 70°C. With this experiment, it was found that the desulfuration of thiourea only occurs in the presence of both the metal ion and the hydroxide ion, and it is proportional to [Cd(OH)2]. An excess in the relation 1:1:2 of thiourea:‌metal:‌hydroxide does not result in a further desulfuration of thiourea. Thus, thiourea does not desulfurate if only metal or hydroxide is present. This result fits with the intermediate complex mechanism, for which a clear reaction scheme is included.