Sodium monothiophosphate
Sodium monothiophosphate, or sodium phosphorothioate, is an inorganic compound with the chemical formula. It is a sodium salt of monothiophosphoric acid. Sodium monothiophosphate forms hydrates. The anhydrous form and all hydrates are white solids. The anhydrous salt decomposes without melting at 120-125 °C. More common is the dodecahydrate. A nonahydrate is also known.
Related salts are the sodium dithiophosphate undecahydrate, sodium trithiophosphate undecahydrate, and sodium tetrathiophosphate octahydrate.
Preparation
Sodium monothiophosphate is prepared by the base hydrolysis of thiophosphoryl chloride using aqueous sodium hydroxide:This reaction affords the dodecahydrate, which is easily dehydrated.
Partial dehydration over 6.5 M [Sulfuric acid|] gives the nonahydrate. Under flowing [Nitrogen|], the anhydrous salt is formed.
Sodium monothiophosphate decomposes at neutral pH. Silicone grease catalyses the hydrolysis of the monothiophosphate ion, so it is recommended that it is not used in the glass joints.
In the anhydrous salt, the P-S bond is 211 pm and the three equivalent P-O bonds are short at 151 pm. These disparate values suggest that the P-S bond is single.