Hydrogen phosphate (HPO₄^2-) and hydrogen carbonate (HCO₃^-) ions are major in environmental water and blood, and the quantification is normally carried out by molybdenum blue method for HPO₄^2- and by titration method or the gas analysis of blood for HCO₃^-, while no quantitative method by electrochemical sensing has been developed. We have recently reported that these ions can be detected and quantified by electrochemical oxidation with the novel catalyst, amorphous RuO₂-Ta₂O₅ mixed oxide, prepared by thermal decomposition [1,2]. For practical uses of the amorphous oxide catalyst for the detecting electrode, it is important to know the effects of the oxide composition of the catalyst to the sensitivity and selectivity to HCO₃^- and HPO₄^2-. In this paper, we studied the sensing properties to these ions with RuO₂-Ta₂O₅ catalysts prepared at different compositions.

RuO₂-Ta₂O₅ catalysts were prepared by thermal decomposition of a precursor solution containing Ru (III) and Ta (V) painted on a titanium disk. The Ru:Ta mole ratio was 20:80, 30:70, 50:50, or 80:20, and thermal decomposition was carried out at 260 ^oC. The characterization of the oxide coating was done with XRD, SEM, and EDX. Electrochemical measurements were performed with a conventional three-electrode cell with KCl-saturated Ag/AgCl reference electrode and 50 mmol/L KCl solutions with or without various concentrations of KHCO₃ or Na₂HPO₄. Cyclic voltammetry was carried out to analyze the oxidation reactions of these ions, and chronoamperometry was also used to measure the oxidation current at various concentrations of HCO₃^- or HPO₄^2-. All measurements were done at 30 ^oC.

XRD and SEM measurements revealed that the obtained RuO₂-Ta₂O₅ catalysts were amorphous, irrespective of the oxide composition. The cyclic voltammograms presented an oxidation wave for each of HCO₃^- and HPO₄^2-, and the oxidation current density increased with increasing concentration of each ion, indicating that HCO₃^- and HPO₄^2- are oxidized on the catalyst prior to oxygen evolution. The onset potential and peak potential of the oxidation of HCO₃^- or HPO₄^2- shifted negatively with increasing Ru mole ratio of the catalyst, in which the onset potential of HCO₃^- was 0.1 V nobler than that of HPO₄^2-. The oxidation current at different ion concentrations measured by chronoamperometry revealed that the concentration of HCO₃^- or HPO₄^2- was linear to the oxidation current density for each ion at all oxide compositions from Ru = 20 mol% to 80 mol%. Among them, the sensitivity for HCO₃^- or HPO₄^2- was highest at Ru = 50 mol%. More detailed results will be shown in this paper.

This work was supported by “Kyoto Area Super Cluster Program” of Japan Science and Technology Agency (JST).

References

[1] T. Tsukuma and M. Morimitsu, The 66th Annual Meeting of International Society of Electrochemistry, Abs# s01-014, Taipei (2015).

[2] A. Honda and M. Morimitsu, PRiME2016, Abs# 3785, Hawaii (2016).