The kinetics of some reactions in solution involving the ferricyanide ion

Abstract

The reaction between potassium iodide and potassium ferricyanide has been studied by several workers, using the Harcourt - Esson method to follow its progress but the results obtained have not furnished unambiguous evidence regarding its mechanism. Some preliminary measurements confirmed the previous experimental evidence but suggested that a side reaction between the ferricyanide and thiosulphate ions may be a possible cause of the complications arising. A study of the latter reaction has therefore been a major subject of the present work. The reaction has been followed by titration of the sodium thiosulphate remaining after various time intervals with a standard solution of iodine, using starch as indicator. The rate curve for this change in unbuffered solutions shows an initial period of rapid reaction, followed by a period of retardation and a further one of slight acceleration before the normal steady decrease in rate with time takes place. These stages are accompanied by marked changes in the pH of the solution. Some, but not all of these features are removed in buffered solutions. The reaction is apparently of first order in acid solutions and of second order under neutral or alkaline conditions. It is accelerated by the presence of tetrathionate ions and retarded by ferrocyanide ions and it shows a pronounced positive neutral salt effect. A mechanism for the reaction is suggested on the basis of these observations which accounts satisfactorily for the behaviour in neutral and alkaline solutions.