Liquid ion-exchanger calcium electrodes

The fact that phosphate and polyphosphate ions formed relatively stable complexes with the Group II, but not Group I, cations led to the first commercial calcium ISE (2). This was based on a liquid ion-exchanger comprising at least calcium bis(didecylphosphate) (1) and di-octylphenyl-phosphonate (2). 

In this Orion model the water-immiscible exchanger solution was essentially supported by a thin Millipore membrane and constantly refurbished from the reservoir of the exchanger until, of course, the supply was exhausted. Apart 

The drawbacks of the original Orion liquid membrane electrode model were successfully overcome by simply incorporating the neat exchanger within a thin matrix of PVC (9). Besides having more favourable mechanical and physical characteristics, these novel PVC electrodes had prolonged functional lifetimes and a performance, in terms of Nernstian calibration, response times and selectivity coefficients, on a par with the original Orion 92-20 model. In fact, the mean content of calcium determined with the respective electrodes in tap water on 10 consecutive days was 31.55 and 31.57 ppm (9). 

The utility of the PVC model electrode for calcium has been long since reflected in the reports of many other, diverse sensing systems similarly incorporating appropriate cocktails in PVC matrices (10). Thin-layer chromatography (TLC) of the Orion 92-20-02 exchanger (11) revealed at least three compounds in addition to the expected calcium salt and mediator (Table 3.1). 

Differences between five batches of the cocktail have also been inferred from diffusion studies with Ca tracer through PVC membranes (12). 

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