ENHANCED DECOMPOSITIONS OF SOME CYANO COMPLEXES DURING DIODE-ARRAY SPECTROPHOTOMETRIC MEASUREMENTS

Acidic solutions of the Fe(III), Mo(V), W(V) and Mn(IV) cyano complexe s are known to decompose photochemically. These reactions do not take place to an appreciable extent when the cyano complexes are subjected to spectrophotometric measurements by means of conventional spectropho tometers, but they do take place when diode array spectrophotometers a re used. The reactions are autoaccelerated, at least in the cases of M o(V), W(V) and Mn(IV), and consist of photoinduced reductions of the m etal ion. This effect should be taken into account in all cases where the measured signal has to be used for quantitative purposes, but its significance must be emphasized mainly in the field of kinetic analysi s, where it can lead to large errors. The extent of these reactions, a nd therefore the size of the errors, can be lowered by using UV cut-of f filters and by taking discontinuous measurements with suitable integ ration times. All these possibilities, which are of general applicatio n, are discussed.