Sodium monothiophosphate

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 Na3PO3S*xH2O. 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 (Na3PO3S*12H2O). A nonahydrate is also known (Na3PO3S*9H2O).

Related salts are the sodium dithiophosphate undecahydrate Na3PO2S2*11H2O, sodium trithiophosphate undecahydrate Na3POS3*11H2O, and sodium tetrathiophosphate octahydrate Na3PS4*8H2O.

Preparation
Sodium monothiophosphate is prepared by the base hydrolysis of thiophosphoryl chloride using aqueous sodium hydroxide:
 * PSCl3 + 6 NaOH + 9 H2O → Na3PO3S*12H2O + 3 NaCl

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.