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)+SO₄^2-(aq)→BaSO₄(s)+CrO₄^2-(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 flow-through analyser with 0.4 mol 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.