Calcium ion-selective electrodes with covalently-bound sensor groups.

A review of the development of ion selective electrodes (ISE), with particular emphasis on calcium ISE, is presented. The utility, in terms of an increased electrode operational lifetime and opportunities for the enhancement of selectivity and sensitivity of a calcium ISE membrane with a covalently-bound ion-exchange sensor group is indicated. The production of such a covalently-bound sensor (CBS) membrane by the crosslinking of styrene-b-butadiene-b-styrene triblock elastomer with triallyl phosphate (TAP) using a free radical initialed copolymerisation is described. Various methods of forming such ion-exchange membranes were investigated. The resulting.membranes were hydrolysed in methanolic KOH to yield covalently-bound dialkylphosphate exchange sites. These membranes were then used to fabricate a robust calcium ISE. The response range, selectivity and general analytical utility of such electrodes are evaluated and the electrodes used in the determination of water hardness and of calcium in coking plant effluent. The modification of the ion-exchange site by using either trioleyl phosphate, tri(10-undecenyl) phosphate or diallyl phenylphosphonate as cross-linking agent is described and the subsequent effects on electrode performance are discussed. A method enabling the elimination of the hydrolysis step during membrane production resulting in a second pathway to the production of a covalently-bound dialkylphosphate sensor is also described. The use of covalently-bound mediator materials in the electrode membrane is investigated and their effect .on membrane performance discussed. The incorporation of an entangled solvent-mediator in conjunction with a dialkylphosphate sensor, which yielded a calcium ISE. with good selectivity fpr Ca[2+] in the presence of Mg[2+] and Na[+] ions, is described. Results from various mechanistic studies including radiotracer and electron microprobe techniques are presented. Possible mechanisms for the response and selectivity of CBS electrodes are discussed on the basis of these studies.