Determination of sulphate in water by flow-injection analysis with electrode-separated piezoelectric quartz crystal sensor

A new flow-injection analysis (FIA) system with detection by an electrode-separated piezoelectric quartz crystal (ESPQC) was developed for sulphate determination in water. The method was based on the decrease in conductivity upon precipitation of barium sulphate due to the mixing between reagent and sample with change indicated by the frequency shift in the ESPQC sensor. The best performance was achieved using a three-channel FI system consisted of a carrier stream of water, a buffer stream containing 0.05% KCl, 10 mM acetic acid and l mM sodium acetate, and a reagent stream of 0.1% BaCl2. Under the optimized conditions, the linear range for sulphate determination was 0.5–50 ppm with a correlation coefficient of 0.9991, detection limit 0.3 ppm (S/N = 2) and sample throughput 50 samples/h. The effect of co-existing cations and anions were investigated and found not to cause interference for most samples except those with very high salt content. The reliability of the method was established by parallel determination using standard turbidimetric method for sulphate in drinking and pond water samples with results within statistical variation. The method developed was shown to provide a simple, sensitive, rapid and reproducible FIA detection method for ionic compounds in the liquid medium.