Calcium ion-selective electrode studies: covalent bonding of organic phosphates and phosphonates to polymer matrices

Abstract

The covalent linking of organophosphates and phosphonates to the copolymer VAGH (a partially hydrolysed copolymer of vinyl chloride and vinyl acetate) is described along with attempts to phosphonate polystyrene by Friedel - Crafts and free-radical processes. The various products obtained have been evaluated for use in calcium ion-selectiv electrode membranes. The best membranes were all based on VAGH, a product from each type being selected for further study for their calcium ion-sensing qualities, namely, VAGH PI, VAGH PII and VAGH PIII.

VAGH PI is based on the covalent bonding of monodecylphosphate to VAGH, while VAGH PII resulted from the binding of mono-[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate. Viable electrode membranes required additional poly(vinyl chloride) support and dioctyl phenylphosphonate solvent mediator. VAGH PIII was obtained by a synthesis amounting to covalently binding monooctyl phenylphosphonate to VAGH and was used with some additional dioctyl phenylphosphonate solvent mediator and calcium bis{di-[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate} sensor in electrode membranes.

The VAGH PI electrode matched those based on equivalent ungrafted didecylphosphate sensors, except that calcium ion selectivity was poor in a high sodium ion background. The VAGH PII electrode, on the other hand, did not fulfil the performance of electrodes based on free calcium bis{di-[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate} as ion sensor, and there was less selectivity for calcium over sodium and magnesium. Some loss of selectivity was also characteristic of the VAGH PIII electrode, but its slightly lengthened lifetime was insufficient to warrant synthesis of the phosphonated polymer matrix.