Quantum yields of the primary photoaquation reactions of octacyanomolybdic-(IV) acid and octacyanotungstic(IV) acid

The quantum yield of the primary photoaquation reaction of octacyano- molybdio(1V) acid and octacyanotungstic(IV) acid has been determined by direct estimation of the two products of the reaction, viz., a CN- ion and a red heptacyano-aquo complex. The CN- ion was estimated polarographically, the red photoproduct spectrophotometrically. Both the methods gave the same value of the quantum yield, which was about 0.88 for the molybdenum and 0.78 for the tungsten system. Photolysis was done at O�C. This and the fact that acidic conditions were maintained during the photolysis sufficiently stabilized the red species to reveal, if the time of irradiation was not too long, an isosbestic point where only two species, the red complex and the M(CN)84- ion, were in equilibrium. The quantum yield was determined subject to the condition that the absorbance curve of the photolysed solution passed through the isosbestic point and secondary reactions had not yet set in.