Acid character of octacyanotungstic(IV) acid and isolation of a red product of its photodecomposition

Abstract

Octacyanotungstic(IV) acid was prepared from aqueous potassium octacyanotungstate(IV) by ion exchange with Amberlite 1R 120 reson (H + form). pH- and conductometric titration of the acid gave four strong H + ions per W(IV). Photolysis of an ice-cold solution of the acid gave an intensely red product. Titration of the photolysed solution showed that it now contained besides strong H + ions, HCN, produced by the liberation of CN − ions by the action of the UV light. The presence of HCN was further confirmed polarographically. The red species was found to be reducible at the D.M.E., though the original octacyanotungstate(IV) ion was not. The red photoproduct was the anion [W(CN) 7H 2O] 3− of the acid H 3[W(CN) 7H 2O]. When the latter was titrated with KOH, three strong H + ions were first neutralized and then one more equivalent of the alkali was used up to give a yellow anion [W(CN) 7OH] 4−. The red photoproduct has been isolated as a silver salt.