Kinetics of the reduction of hexacyanoferrate (III)‎ with metabisulfite as function of pH

الملخص EN

Kinetics of the reduction of potassium hexacyanoferrate (III) with sodium met bisulfite was investigated as a function of pH and at four temperatures, viz.

15o, 20˚, 25˚and 30˚C.

The ionic strength was maintained at 1.0 M KCl.

The pH range used varied between 2.6 to4.7 using McIlvaine phosphate /citric acid buffer system.

The second order observed rate constant was found to follow an inverse H+ dependence of the form kobs= k1 + k2/[H+].

At 298.15 K, the second order rate constant k1= (1.02 ± 0.20) X 10-2 s-1 and -1mol-1L the first order rate constant k2= The activation parameters for k1 are: ΔH‡= 73.64 kJ mol-1, ΔS‡= -36.63 JK-1 mol-1and ΔG‡= 84.56 kJ mol-1 and for k2 are 41.71 KJ mol-1, -205.03 JK-1 mol-1 and 102.84 KJ mol-1 respectively.

The kinetic data were very well explained by invoking radical mechanism in which the HSO3- and SO32- formed HSO3* and SO3-* radicals, alternatively the redox process could take place by a mechanism in which a fast reversible equilibrium is established between the hydrogen ions and hexacyanoferrate (III), followed by a slow reaction of the protonated species with the sulfite ion leading to the formation of sulfite free radical.