Calcium ion-selective electrode measurements in the presence of complexing ligands

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Abstract
Calcium ion-selective electrodes based on calcium bis[di(4-octylphenyl)-phosphate] sensor and dioctyl phenylphosphonate solvent mediator have been used for monitoring free calcium-ion levels (from below 10–7 to above 10–3 M) in the presence of citrate, malate, malonate, oxalate, EDTA, NTA, sulphate, orthophosphate, tripolyphosphate and pyrophosphate anion ligand systems under conditions of constant ionic strength maintained by sodium chloride. Log β data fall in the range of those previously measured for the various equilibria by alternative methods, thus demonstrating that calcium ion-selective electrodes of the type used here can be employed for free calcium-ion measurements to below the detection limits characteristic of calibrations with serial dilution standards and without disturbing the equilibria of complexation. Equilibria existing in the tripolyphosphate and pyrophosphate systems were discerned by application of the MINIQUAD program for computing formation constants and species distribution. Predictions concerning the existence of [CaP3O10]3– and [Ca(P3O10)2]8– in tripolyphosphate and of [CaP2O7]2– and [Ca(P2O7)2]6– in pyrophosphate systems are briefly discussed. These calcium ion-selective electrodes are not affected by added phosphate except insofar as free calcium-ion levels are lowered by complexation.

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