Alkali metal ion-selective electrodes based on relevant alkali metal ion doped manganese oxides

Abstract

Potentiometric properties of manganese oxides doped with alkali metal ions (Na+, K+, Rb+ and Cs+), which were prepared by heating mixed solutions (starting solution) of each alkali metal and Mn2+ ions, were examined. Electrodes based on mixed phases of Nao44MnO2/Mn2O3 and hollandite KMn8O16/M2O3 found by X-ray powder diffraction (XRD) exhibited Na+- and K+-selective responses with a near-Nernstian slope, respectively, when the molar ratio of alkali metal ion to Mn2+ ion in the starting solution was 0.1. When no alkali metal ions were added in the manganese oxide films, no significant potentiometric response was observed to any alkali metal ions. The selectivity coefficients of these electrodes were\(K_{Na^ + , Li^ + }^{Pot} \) = 6.7 × 10−2,\(K_{Na^ + , K^ + }^{Pot} \) = 7.1 × 10−3,\(K_{Na^ + , Rb^ + }^{Pot} \) < 9 × 10−4 and\(K_{Na^ + , Cs^ + }^{Pot} \) < 9x 10−4 for the Na0.44MnO2/Mn2O3, and\(K_{K^ + , Li^ + }^{Pot} \) <4 × 10−4\(K_{K^ + , Na^ + }^{Pot} \) <4x 10−4,\(K_{K^ + , Rb^ + }^{Pot} \) =60 × 10−2 ×10−4,\(K_{K^ + , Cs^ + }^{Pot} \) < 4 × 10−4, for the KMn8O16/Mn2O3, respectively. Electrodes based on manganese oxides made from mixed solutions of Rb+/Mn2+ and Cs+/Mn2+ also responded to the respective primary ions, that is, Rb+ and Cs+ ions, although XRD patterns for the manganese oxides thus made did not show any peaks except for Mn2O3 (bixbyite); it was concluded in these cases that some amorphous type manganese oxides were formed in the Rb+/Mn2+ and Cs+/Mn2+ systems and they responded to the respective ions. Conditioning of these electrodes in an aerated indifferent electrolyte solution, 0.1M tetramethylammonium nitrate (TMA-NO3), for relatively long time, typically more than 2 hours, was found to be a prerequisite for near-Nernstian response to the respective alkali metal ions. During this electrode conditioning, vacant sites (template) suitable in size for selective uptake of primary ions seemed to be formed by releasing the doped alkali metal ions from the solid phase into the adjacent electrolyte solution accompanying oxidation of the manganese oxide film.