Direct spectrophotometric determination of sulphate ion based on the formation of a blue molybdosulphate complex

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Abstract
In an aqueous medium containing 50%V/V acetone, sulphate ion reacts with a mixed reagent consisting of MoV-MoVI-HCl to give a blue molybdosulphate complex, the amount formed being proportional to the sulphate ion present. Using this colour forming reaction, sulphate ion could be determined spectrophotometrically, and the limit of detection (signal to noise ratio = 3) was 3.1 × 10–6 M. When combined with an extraction-concentration of the blue complex, the proposed method gave a lower limit of detection, viz., 1.0 × 10–6 M. Phosphate, arsenate and germanate ions interfered owing to the formation of similar blue complexes at concentrations 1.0 × 10–5 M, but the presence of 1.0 × 10–4 M silicate ion and 1.0 × 10–2 M chloride ion caused no significant interference. Sulphate in natural waters, particularly saline waters, was determined and the results showed good agreement with those obtained by ion chromatography or inductively coupled plasma atomic emission spectrometry.

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