The rate of the second-order reaction between persulphate and ferrocyanide ions can be accurately measured in dilute solution by spectrophotometric determination of the ferricyanide ions produced. The kinetic salt effect of added salts of various valency types has been investigated. The specific effects of added cations on the rate of reaction are large. Analysis of the results suggests that a cation is a necessary member of the activated complex, and a set of catalytic constants is reported. The significance of the results in relation to those for other reactions is discussed.We believe that at sufficiently low ionic concentrations the primary kinetic salt effect of added "inert" salts on the rate constant k of a reaction involving ions is correctly described by the equation obtained by substitution of the Debye-Huckel equation for ionic activity coefficients into the Brarnsted 1 equation, which for a second-order reaction between ions A and €3 isand f A , f~ and f A B are activity coefficients. Deviations from standard behaviour at higher ionic concentrations can sometimes be adequately accounted for 2 by extending the Debye-Hiickel equation to take account of specific interactions between oppositely charged ions. However, in other cases the specific effects are large and the existence of additional equilibria involving reactant ions, e.g., ion-pair formation, and/or additional rection pathways is postulated.3The rate of the second-order reaction between persulphate and iodide ions 4 is specifically affected by a variety of cations even at ionic strengths below 0.1 mole 1.-1, and there are also pronounced specific anion effects. Another reaction involving the persulphate ion which proceeds at a convenient rate for kinetic study is that with the ferrocyanide ion, and a comparison of the specific kinetic salt effects with those observed in the persulphate +iodide reaction is of interest. Previously 5 the kinetics of the persulphate-ferrocyanide reaction had been investigated by a volumetric method. At suitable time intervals excess potassium permanganate, which reacts rapidly with ferrocyanide, was added to samples ; the unreacted permanganate was then determined by back titration against standard ferrocyanide solution. For accurate measurements in dilute solution it is better to follow the reaction spectrophotometrically. The ferricyanide ion has an absorption peak at 4200 A, whereas the optical absorption of the ferrocyanide ion is negligible at this wavelength. Chlebek and Lister 6 have also used the spectrophotometric technique to study the effect of potassium ions on the same reaction. EXPERIMENTAL MATERIALS A N D S T O C K SOLUTIONSAnhydrous potassium ferrocyanide was prepared by heating once recrystallized A.R. trihydrate to constant weight at 100-1 10°C and cooling the product over anhydrous calcium chloride. The purity of the product was checked by titration against standard ceric sulphate.