Optimization of a bipotentiometric flow injection analysis system for simultaneous determination of calcium and chloride ions in natural waters

Abstract

The experimental optimization by the modified simplex method of a bipotentiometric flow injection analysis (FIA) system, constituted by two different ion selective electrodes (ISEs) placed in series, for the simultaneous determination of calcium and chloride in waters is reported. The response function used was a weighted linear combination of three variables, chosen to achieve maximization of the calcium electrode sensitivity, minimization of the interference of the calcium signal on the chloride response and maximization of the sampling rate. The optimized system is able to determine calcium in the range 5 × 10^–5–5 × 10^–3 mol l^–1 with a sensitivity of ca. 27 mV decade^–1 and chloride in the range 2 × 10^–4–2 × 10^–2 mol l^–1, the maximum value tested, with a sensitivity of ca. 55 mV decade^–1. The repeatability found was 1.9 and 0.6%, respectively, at 2 × 10^–4 and 2.2 × 10^–3 mol l^–1 (8 and 90 mg l^–1), for calcium determinations, and 1.8 and 1.2%, respectively, at 3.4 × 10^–4 and 8.3 × 10^–4 mol l^–1 (12 and 29 mg l^–1), for chloride determinations. A sampling rate of 40 h^–1 was obtained. The influence of the ionic strength on the results is discussed. The relative accuracy of the bipotentiometric FIA system was demonstrated by determination of the contents of both ions in natural waters in parallel with ASTM certified methods.