Indirect electrochemical determination of sulfates in mineral water by a flow-through system

The paper describes the indirect determination of sulfates by in-electrode coulometric indication method. The principle of the determination of sulfates consists in the on-line coupling of the exchange column filled with barium chromate and the flow-through coulometric analyser. In the column BaCrO4(s) converts to BaSO4(s) according to the equation: BaCrO4(s) + SO42- (aq) -> BaSO4(s) + CrO42-(aq). This conversion is caused by the lower solubility of BaSO4(s) compared to the solubility of BaCrO4(s). Subsequently, the concentration of the chromate in the eluate solution was determined by the in-electrode coulometric indication method in the electrode made of reticulated vitreous carbon. The solution flowing out of the column was mixed in the flowthrough analyser with 0.4 mot L-1 hydrochloric acid and the concentration of chromates was determined by direct reduction of Cr(VI) to Cr(III) with a constant current. The determination was evaluated by the calibration curve method. The linear range of the calibration curve was 3.9-100 mg L-1. The detection limit and the accuracy were 1.3 mg L-1 and 3.2%, respectively. The interference of the bicarbonate was eliminated through a conversion to insoluble carbonate by boiling the sample. The procedure was validated by analyses of certified reference material and bottled mineral water samples. A good agreement with the ion chromatography was achieved for all samples.