The kinetics of some reactions involving ferricyanide

Abstract

Fast electron transfer reactions have been shown to follow a correlation based on magnetism, Bor reactions of ferricyanide ion with iodide, sulphite, thiosulphate and cyanide ions, a step of the type Fe(CN)6

3- + X- [equilibrium]Fe(CN)6

4- + X

O should be fast on this basis. Failure to explain the observedslow reactions of these ions was due to the uncertainty regarding their mechanism. A study of the two former reactions has therefore been a major subject of the present work. The rate of the reaction between ferricyanide and iodide has been measured both titrimetrically using the Harcourt-Esson method and also spectrophotometrically by observing the optical density at 420 mu where absorption is virtually due to ferricyanide only, if iodine is removed by excess thiosulphate.The effects of many variations in reaction conditions are studied. One of the most important of these concerns the effect of cations on the reaction rate. A mechanism is proposed which accounts for all the important observations. The main reaction is shown to be first order in ferricyanide, second order in iodide and first order in cation concentration. Other minor reaction paths identified consist of a direct reaction between the main reactants and an anion assisted process. The rate of reaction between ferricyanide and sulphite ions has been measured in the pH range (7.6 - 10), using a spectrophotometric method. The reaction shows a positive neutral salt effect, the rate is different in the presence of different cations and anions, the order of effectiveness as catalysts are CS+> K+> NH4+> Na+> Li+> and Br-> F-> SO4

2-1> Cl-> NO3->H2PO4- respectively. The apparent order of reaction with respect to each main reactant is unity. The stoichiometry of the reaction in the pH range (3.52 - 11.4) has been measured and found to be 2:1 for ferricyanide:sulphite ions, and the sulphur product is predominently sulphate. Finally, a mechanism for the reaction is suggested on the basis of these observations.