A PHOTO-CURED COATED-WIRE CALCIUM-ION SELECTIVE ELECTRODE FOR USE IN FLOW-INJECTION POTENTIOMETRY

A multi-sensor cell containing a new photo-cured calcium ion selective electrode sensor is reported. Four membranes containing different com ponents are prepared to determine the one with optimum selectivity and sensitivity. This is shown to consist of the N,N,N',N'-tetracyclohexy l-3-oxapentanediamide ligand (ETH 129) as the ionophore, 2-nitrophenyl octyl ether as the plasticiser and tetradodecyl ammonium tetrakis(4-c hlorophenyl) berate as the lipophilic additive. The photo-curing proce ss is applied after coating a thin membrane on a silver wire as substr ate transducer to produce the calcium sensor. The curing process is de monstrated to be faster (1 min) than previous methods and does not req uire a nitrogen atmosphere for reproducible production of membrane res ponse characteristics. Four sensors constructed with the identical opt imum membrane are shown to function reproducibly in a multi-sensor flo w-through cell using the steady-state mode of flow measurement, and an average calibration slope of 28.5 +/- 0.4 mV change per activity deca de is observed over a log-linear concentration range between 0.01 and 10 mM. The sensor is also shown to respond to changes in pH. Hence, in the flow injection potentiometric mode, a constant carrier buffer com position of pH 8.3 is required for accurate potentiometric calcium det erminations. The sensor is used to determine calcium in water samples by flow injection potentiometry. The accuracy of the electrode determi nation relative to atomic absorption spectroscopy was in the range 5-9 % for three different water samples. (C) 1997 Elsevier Science B.V.