Kinetics of VO 2+ extraction by D2EHPA

Abstract

The kinetics of the forward extraction of VO 2+ from sulphate–acetate medium by di-2-ethylhexylphosphoric acid (D2EHPA) in toluene has been investigated with Hahn and Lewis cell techniques using both (rate/area) or, pseudo-rate constant ( q) and flux ( F) methods for data treatment. It has been found for both Lewis and Hahn cell experiments that the product of the rate constant obtained from the ( q) method and the volume of each phase taken in m 3 almost equals the rate constant obtained from the ( F) method. It is concluded that k f F values are absolute, whereas k f q is organic phase volume dependent. The k f value obtained from the Hahn cell measurements is 60 times lower than the Lewis cell k f values. The reason for this variation has been discussed. Analyses of the rate expressions suggest the following reaction step occurring in the aqueous film of the interface: VO 2++A −→VOA + as rate-determining. The activation energy in the higher temperature region, it is for a diffusion control reaction, whereas in the lower temperature region, it is for an intermediate control reaction. The entropy of activation, ΔS ± suggests that the rate-determining step proceeds through S N1 mechanism.