Application of a Lithium-ion Selective Metallacrown to Extraction-Spectrophotometric Determination of Lithium in Saline Water

The solvent-extraction behavior of Li+ and Na+ with a Li+ selective metallacrown, [{Ru(η6-3,5-dimethylanisole) (2,3- pyridinediolate)}3], was investigated in the presence of organic dye anions, 3′,3″,5′,5″-tetrabromophenolphthalein ethyl ester ([TBPE]–), 2,6-dichloroindophenolate, and picrate ([pic]–). Each alkali metal ion was extracted as a 1:1:1 ternary complex of the metal ion, metallacrown, and anion. The Li+/Na+ extraction selectivity is anion dependent and highest with [pic]–. Therefore, we devised an extraction–spectrophotometric determination method for Li+ in saline water based on the extraction of Li+ using the metallacrown and [pic]– for high selectivity and subsequent replacement of [pic]– in the extracted species with [TBPE]– for high sensitivity. When applying this to artificial seawater samples containing known concentrations of Li+, a linear relationship was observed between the absorbance at 571 nm of the organic phase and the Li+ concentration in the samples. By this method, the determination of Li+ at the sub-ppm level in natural seawater is possible.