Analysing sulphate and chloride in mineral drinking water by flow injection analysis with a single acoustic wave sensor

摘要

Sulphate is a very hydrophilic anion, and, therefore, difficult to be selectively determined in aqueous solution with a coated sensor. Zinc(II) 2,3,9,10,16,17,23,24-Octatosylaminophthalocyanine was used for the first time as a sensitive coating. Selectivity to sulphate regarding chloride was not enough to ignore its contribution in mineral waters, and, therefore, an analytical protocol was carefully designed to allow the determination of both anions with this sensor. Results displayed on the label of eight commercial bottles of mineral waters are within the confidence interval of the values obtained with the FIA-sensor system, both for chloride and sulphate. However, results for chloride obtained by titrimetry are, in half of the cases, statistically different from the ones obtained by the sensor, and in 7 out of 8 of them more precise. There is an evidence of a systematic error in the chloride titrimetric analysis, consistent with a small overtaking of the equivalence point. Precision of the results obtained by the titrimetric analysis of sulphate were in 6 out of 8 of the analysis less precise than with the sensor, probably due to losses of the barium sulphate precipitate, which is consistent with the occurrence of lower values.