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

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 (K) 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 K and K(Li)/K(Na) are 1.0 × 10 and 1.3 × 10, 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 Mg, and trace amounts (10-10 mol/L order) of Li in microvolume samples (sub-mL order) could be successfully determined.