Kinetic study of iron(III)‎ salicyl hydroxamate complexes

Abstract EN

The formation of Salicylhydroxamic acid iron (III) complexes were studied at different pH.

The reaction at pH 8 and 6 between iron nitrate and salicylhydroxamic acid is very fast and reddish brown colour with iron at 425 nm appears within seconds i.e.

within mixing time.

The concentration of salicylhydroxamic acid was 20–80 times higher than the concentration of iron (III) solution in order to fulfils pseudo first order conditions.

The reddish brown colour appears within mixing time and further change in colour was very slow and observed at 425 nm wavelength.

The rate constant at pH 8 is 0.1886 sec and at pH 6 is 1.472 sec.

The sharp appearance of colour is due to the formation of 1: 1 and 1 : 2 complexes while the observed slow change in colour may be due to the rearrangement of salicylhydroxamic acid from bidentate to tridentate or it may be due to the formation of 1 : 3 complex.

In the next set of reactions the 1 : 1 complex of salicylhydroxamic acid iron (III) was prepared by mixing Iron (III) and salicylhydroxamic acid in 1 : 1 mole ratio and then the formation of 1 : 2 complex was observed at pH 5, 4.5 and 4.

The concentration of salicylhydroxamic acid solution was 2-10 times higher than the 1 : 1 complex of salicylhydroxamic acid iron (III) complex.

The observed reactions were very fast and were not truly a first order reaction.

The rate constant is 24.85 sec at pH 5, 10.52 sec at pH 4.5 and 16.98 sec at pH 4.

The reaction of 1 : 1 complex with salicylhydroxamic acid at pH 3 was very fast.

The λ max of iron complex is 500 nm and of final mixture is 476 nm.

The reaction was assumed to be reversible.

The absorbance of both species at a particular wavelength is additive.

Using this property the equilibrium constant was calculated which was not constant at different ratios of 1 : 1 complex and salicylhydroxamic acid, which further indicate the possibility of a rearrangement reaction.