Sodium monothiophosphate, or sodium phosphorothioate, is an inorganic compound with the chemical formula Na3PO3S. It is a sodium salt of monothiophosphoric acid (H3PO3S). Sodium monothiophosphate forms hydrates Na3PO3SxH2O. The anhydrous form and all hydrates are white solids. The anhydrous salt (x = 0) (Na3PO3S) decomposes without melting at 120-125 °C. More common is the dodecahydrate (Na3PO3S12H2O). A nonahydrate is also known (Na3PO3S9H2O). Related salts are the sodium dithiophosphate undecahydrate Na3PO2S211H2O, sodium trithiophosphate undecahydrate Na3POS311H2O, and sodium tetrathiophosphate octahydrate Na3PS48H2O.

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 H2SO4 gives the nonahydrate. Under flowing N2, the anhydrous salt is formed. Sodium monothiophosphate decomposes at neutral pH. Silicone grease catalyses the hydrolysis of the monothiophosphate ion PO3S(3−), 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.