Extraction spectrophotometry using a lithium-ion selective metallacrown: temperature effect on extraction reaction and application to determination of lithium in serum and seawater

Abstract

A metallacrown-type ionophore, 2,3-pyridinediolate-bridged (3,5-dimethylanisole)ruthenium trinuclear complex, has a high extraction selectivity for Li+, but the extraction reaction is very slow. To solve this problem, the effect of temperature on the rapidity and equilibrium of the extraction of Li+ and Na+ as picrates from water to toluene with the metallacrown was investigated in this study. While the extraction of Li+ requires 6 h of shaking for equilibration at 25 °C, the distribution ratio becomes nearly constant after 4 h and 2 h of shaking at 37 °C and 50 °C, respectively. The extraction equilibrium constants (Kex) and associated thermodynamic parameters determined for Li+ and Na+ indicate that the extraction reactions are exothermic and enthalpy-driven: ΔH° = − 53 kJ/mol, ΔS° = − 0.03 kJ/(mol K) for Li+; ΔH° = − 28 kJ/mol, ΔS° = − 0.03 kJ/(mol K) for Na+. Although the extraction ability for Li+ and selectivity for Li+/Na+ decrease with increasing temperature, the values of Kex and Kex(Li+)/Kex(Na+) are 1.0 × 107 and 1.3 × 104, respectively, even at 50 °C, indicating that both are sufficiently high. In the determination of Li+ by extraction spectrophotometry using this metallacrown, extraction at 50 °C for 2 h was employed to speed up the analysis. The method was applied to seawater and serum samples containing a large amount of coexisting ions such as Na+ and Mg2+, and trace amounts (10−6–10−5 mol/L order) of Li+ in microvolume samples (sub-mL order) could be successfully determined. Graphical abstract