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

Abstract

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.