Stopped-flow devices have been constructed and are described. Using this technique reactions of iron(lll) and chromium(VI) with organic substrates in aqueous perchlorate media have been investigated. Iron(IIl) has been found to react in the form of Fe3+ (aq) and FeOH2+ (aq) with both salicylaldehyde and salicylamide. The results obtained in this study not only confirm that Fe(H20)50H2+ reacts by an 1d mechanism but also indicate that a similar mechanism operates in the case of Fe(H2O)63+. This latter finding is of some particular interest in view of the current controversy regarding the mode of anation of this latter ion. HCrO4- has been found to oxidise L-cysteine via the formation of intermediate sulphur-bonded esters which have been identified spectrophotometrically. The rate constant for the acid-catalysed formation pathway has been found to be markedly smaller than has been measured previously for other ligands with this metal-ion. The significance of this phenomenon is discussed, as also is the interpretation of the second-order substrate dependence of the redox rate. Chromate(VI) esters have also been shown to form in the course of the reduction of HCrO4- by both malic and thiomalic acids. Absorbance-time profiles have been simulated in order to aid the analysis of the kinetics of this latter reaction. As has been found for other metal-ions, the most significant common factor in the reactions of these two substrates with chromium(VI) is the participation of intermediate complexes. The influence of the disimilar chemical natures of oxygen and sulphur is discussed with particular reference to the relative kinetic complexities of these systems
