OPTIMIZATION OF A BIPOTENTIOMETRIC FLOW-INJECTION ANALYSIS SYSTEM FORSIMULTANEOUS DETERMINATION OF CALCIUM AND CHLORIDE-IONS IN NATURAL-WATERS

The experimental optimization by the modified simplex method of a bipo tentiometric flow injection analysis (FIA) system, constituted by two different ion selective electrodes (ISEs) placed in series, for the si multaneous determination of calcium and chloride in waters is reported . The response function used was a weighted linear combination of thre e variables, chosen to achieve maximization of the calcium electrode s ensitivity, minimization of the interference of the calcium signal on the chloride response and maximization of the sampling rate. The optim ized system is able to determine calcium in the range 5 x 10(-5)-5 x 1 0(-3) mol l(-1) with a sensitivity of ca, 27 mV decade(-1) and chlorid e in the range 2 x 10(-4)-2 x 10(-2) mol l(-1), the maximum value test ed, with a sensitivity of ca, 55 mV decade(-1). The repeatability foun d was 1.9 and 0.6%, respectively, at 2 x 10(-4) and 2.2 x 10(-3) mol l (-1) (8 and 90 mg l(-1)), for calcium determinations, and 1.8 and 1.2% , respectively, at 3.4 x 10(-4) and 8.3 x 10(-4) mol l(-1) (12 and 29 mg l(-1)), for chloride determinations. A sampling rate of 40 h(-1) wa s obtained, The influence of the ionic strength on the results is disc ussed. The relative accuracy of the bipotentiometric: FIA system was d emonstrated by determination of the contents of both ions in natural w aters in parallel with ASTM certified methods.