The influence of ionic effects upon the rates of some redox reactions

Abstract

Electron-transfer reactions have been observed, in many cases, to proceed too fast to be followed by normal kinetic methods. Many new techniques have been devised to follow the rates of such reactions. The rate of reaction between hexaeyanoferrate (II) and chromium (Vl) ions have been found too fast to follow using conventional kinetic methods. Accordingly a fast mixing-quenching technique was adopted. The unreacted chromium (VI) was then estimated spectrophotometrically. The stoichiometry of the reaction in the pH range 3.5 - 4.5 has been shown to be 6 : 1 for hexacyanoferrate(II) : dichromate. The reaction is found to be of the third order. The order with respect to each reactant, i.e. chromium (VI), hexacyanoferrate (II) and hydrogen ions, is unity. The rate has also been found to be greatly dependent on the type and concentration of cation present. Thus K+ is a more effective catalyst than Na. Hexacyanoferrate(III) retardation of reaction rate was confirmed. A value of the activation energy was determined, and a mechanism compatible with the main features of the reaction has been proposed. The rate of oxidation of hexacyanoferrate (II) by peroxydisulphate has been followed spectrophotometrically at 420mu. The order of this reaction has been confirmed to be unity with respect to each reactant. The rate of this reaction also varies with the type and concentration of a cation present. The order of effectiveness of cations in assisting the reaction is Cs >K > NH4>(CH3)4N>H, and for bivalent cations Ca2>Mg. The rate of the reaction was also measured over a wide range of H ion concentration. The effect of temperature on reaction rate in presence of K+, NH4,Na and H ion has been determined and an activation energy for each cation assisted process evaluated. Possible reaction paths have been proposed and on the basis of some quantitative treatment the more favourable ones pointed out.