A Sodium Ion Selective Electrode Based on a Highly Lipophilic Monensin Derivative and Its Application to the Measurement of Sodium Ion Concentrations in Serum

Five kinds of monensin derivatives were synthesized by chemical modifications of the natural carboxylic polyether antibiotic monensin. Their usefulness as active compounds (Na⁺-ionophores) for Na⁺-selective poly(vinyl chloride)(PVC)-matrix membrane electrodes was examined. The electrodes based on monensin ester derivatives exhibited high Na⁺-selectivity. Among them, the electrode based on the highly lipophilic monensin dodecyl ester (log K° (partition constant)=8.7) had enough Na⁺-selectivity to determine the Na⁺-concentration in serum (log K^pot_{Na K}=-1.1, log K^pot_{Na Mg}==-4.0 and log K^pot_{Na Ca}=-3.6); the electrode gave a Nernstian response (59.0 mV/pNa) ranging from 5×10^-6 to 1 M Na⁺. The response potential of this electrode to a 0.1 MNa⁺ solution drifted within ±0.01 mV/h. The values of the Na+-concentration in artificial and real serum samples were successfully determined with the monensin dodecyl ester-based electrode using diisodecylphthalate as the electrode membrane solvent.