DERIVATIVES OF IMIDOPYROPHOSPHORIC ACIDS AS EXTRACTANTS .5. THE DETERMINATION OF THE FUNDAMENTAL CONSTANTS OF N,N-N',N'-OCTAALKYLIMIDODIPHOSPHOTETRAMIDES BY POTENTIOMETRY

The interaction of nitric acid in two-phase systems in the pH range with the extractant, N,N-N',N'-octabutylimidodiphosphotetramide (OBIPA) in toluene has been investigated using two-phase potentiometric titrations. The data thus obtained were analysed by the ESTA library of programs which was designed for modelling solution equilibria in aqueous phases. The use of such a program in the analysis of two-phase potentiometric data is novel, and the feasibility and corrections needed to be applied are discussed. Gratifying results were obtained indicating that a program such as ESTA may indeed be used for two-phase systems. The extraction equilibrium constant for the formation of the (2HA.HNO3)O species was found to be 7.54 x 10(4). Constants could also be calculated for the dimerisation and the partition of the extractant between the two-phases. These were found to be 18.5 M-1 and 1.92 x 10(2) respectively. A water soluble, methyl analogue of OBIPA was used to calculate the acid dissociation constant (pk(a)) which was found to be 9.043.