Photolysis of sodium nitroprusside and nitroprussic acid

Abstract

The pH of neutral solutions of sodium nitroprusside decreases on irradiation with ultraviolet light. On cutting off the light, the pH slowly rises and tends to reach its original value. If irradiation is far too prolonged, secondary reactions set in, and the pH change is no longer reversible. Titration with a base shows that irradiation under photo-reversible conditions produces a weak acid whose amount increases as irradiation proceeds, up to a maximum of two equivalents per mole of nitroprusside. The same weak acid is also produced when nitroprussic acid which, ordinarily, titrates as a strong acid, is irradiated. A mechanism of the observed changes is suggested, based on the assumption that the primary (photochemical) reaction is one of a photolysis of the nitroprusside ion, giving NO⁺, which then hydrolyses to give H⁺ ions.