UPTAKE OF METAL-IONS BY A NEW CHELATING ION-EXCHANGE RESIN .3. PROTONATION CONSTANTS VIA POTENTIOMETRIC TITRATION AND SOLID-STATE 31P NMR-SPECTROSCOPY

A new chelating ion exchange resin which incorporates methylenediphosphonate, carboxylate, and sulfonate functional groups in a polystyrene-divinylbenzene matrix has been prepared. This resin exhibits exceptionally high affinity for polyvalent cations even from moderately acidic aqueous media. Metal ion coordination occurs primarily at the diphosphonate group with the secondary binding sites contributing to charge neutralization when necessary and possible, and to increasing hydrophilicity of the resin pores. In the present investigation, the protonation equilibria of the phosphonate groups in the resin are investigated via potentiometric titration and solid-state P-31 NMR spectroscopy of the resin. Intrinsic equilibrium constants for the first two diphosphonate protonation reactions are pK(4) = 10.47 and pK(3) = 7.24. The last two protons added to the diphosphonate group are acidic having pK(a) values less than 2.5. These protonation constants are consistent with those reported previously for monomer analog 1,1-diphosphonic acids. This result implies that thermodynamic data available in the literature can be used to predict the relative affinity of the resin for polyvalent cations.