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 (x = 0) () decomposes without melting at 120-125 ðC. More common is the dodecahydrate (). A nonahydrate is also known ().
Related salts are the sodium dithiophosphate undecahydrate , sodium trithiophosphate undecahydrate , and sodium tetrathiophosphate octahydrate .
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 gives the nonahydrate. Under flowing , 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.