Kinetics of VO2+ extraction by D2EHPA

The kinetics of the forward extraction of VO2+ 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 m3 almost equals the rate constant obtained from the (F) method. It is concluded that kfF values are absolute, whereas kfq is organic phase volume dependent. The kf value obtained from the Hahn cell measurements is 60 times lower than the Lewis cell kf 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: VO2++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 SN1 mechanism.