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 complexes 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 spectrophotometers, but they do take place when diode array spectrophotometers are used. The reactions are autoaccelerated, at least in the cases of Mo(V), W(V) and Mn(IV), and consist of photoinduced reductions of the metal 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 analysis, where it can lead to large errors. The extent of these reactions, and therefore the size of the errors, can be lowered by using UV cut-off filters and by taking discontinuous measurements with suitable integration times. All these possibilities, which are of general application, are discussed.