Improved extraction method for the spectrophotometric determination of trace amounts of boron in river water with 1,8-dihydroxynaphthalene-4-sulphonic acid and removal of the excess of reagent

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Abstract
A simple method for removing the excess of co-extracted reagent in the ion-association extraction of metal complex anions with quaternary ammonium salts has been applied successfully to the spectrophotometric determination of boron at the parts per 109 level in river water with 1,8-dihydroxynaphthalene 4-sulphonic acid (DHNS) and tetradecyldimethylbenzylammonium chloride (zephiramine). The procedure using DHNS described here greatly improves the previous method using chromotropic acid. Acetate buffer (pH 3.8), EDTA and DHNS are added to the sample solution (less than 50 ml) and the pH of the resulting solution is adjusted to 10.2. Then sodium chloride is added and the mixture is shaken with 5 ml of a 2 × 10–3 M solution of zephiramine in 1,2-dichloroethane (DCE). The organic phase is washed once with 10 ml of a back-washing solution (1.0 M in sodium chloride, pH 10.2) and the absorbance of the organic phase is measured in a quartz cell. The boron complex with DHNS is extracted quantitatively into DCE and its apparent molar absorptivity in DCE is 2.4 × 104 l mol–1 cm–1 at 341 nm. The detection limit and precision achieved with the method are 1 µg l–1 and 5%, respectively. EDTA allows most interferences caused by metals to be suppressed, and other sources of bias due to the effect of co-extractable anions are almost eliminated by adding relatively large amounts of sodium chloride to the extraction system. Parts per 109 amounts of boron present as boric acid in river water are determined spectrophotometrically, and the results obtained are successfully compared with those obtained by the methylene blue method.

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