Acid-base and organic-water distribution equilibria for symmetrically-substituted P,P′-dialkyl alkylenebisphosphonic acids

P,P′-dialkyl alkylenebisphosphonic acids are powerful metal extraction reagents. The acid dissociation constants for a homologous series of aqueous-insoluble P,P′-di-3-(trimethylsilyl)-propyl and aqueous-soluble P,P′-diethyl methylene-, ethylene- and propylene- bisphosphonic acids were determined in a 70:30 w/w methanol-water solvent by potentiometric titration and ³¹P NMR spectrometry. The values obtained for the diethyl-substituted acids were compared with those determined in water and used to assess the effect of the medium on the aqueous acid dissociation constants of the lipophilic series of P,P′-di-3-(trimethylsilyl)-propyl alkylenebisphosphonic acids. The dependence of the organic/aqueous distribution equilibrium on the aqueous acid concentration was also investigated using electrospray ionization mass spectrometry. The acid dissociation and organic/aqueous distribution properties of the substituted alkylenebisphosphonic acids are discussed in terms of their influence on metal ion extraction.