A Novel Electrochemical Sensing of HCO3
- with Amorphous RuO2-Ta2O5 Catalyst
Authors: Honda Ai , Morimitsu Masatsugu 
Published: February 2020

We have recently reported that hydrogen phosphate (HPO4
2-) can be detected and quantified by electrochemical oxidation with the novel catalyst, amorphous RuO2-Ta2O5, prepared by thermal decomposition and that the oxidation current is linear to the concentration in the wide range from 10-6 mol/L to 10-2 mol/L [1]. The finding motivated us to explore other sensing targets possible with the amorphous mixed oxide and one of them was hydrogen carbonate (HCO3
-); this anion is a major one in river water and blood and the quantification is normally carried out by titration method or the gas analysis of blood, while no quantitative method of HCO3
- by electrochemical sensing has been developed. In this paper, we studied the oxidation reaction of HCO3
- on RuO2-Ta2O5 catalysts and the relationship between the oxidation current and the concentration to examine a possibility of a novel electrochemical sensing of HCO3
-.
RuO2-Ta2O5 catalysts were prepared by thermal decomposition of a precursor solution containing Ru (III) and Ta (V) as a thin coating on a titanium disk. The Ru ratio was 50 mol% and thermal decomposition temperature was 260 oC. The characterization of the coating was done with XRD and SEM. Electrochemical measurements were performed using a conventional three-electrode cell and 50 mmol/L KCl solutions with or without various concentrations of KHCO3. Cyclic voltammetry and normal pulse voltammetry were carried out to analyze the oxidation reaction, and chronoamperometry was also used to measure the oxidation current at various concentrations of HCO3
-. All measurements were done at 30 oC.
XRD and SEM measurements revealed that the obtained RuO2-Ta2O5 catalyst was an amorphous mixed oxide, as similar as previously reported [1]. The cyclic voltammograms presented a new oxidation wave at about 0.85 V when KHCO3 was added to the KCl solution, and the current density increased with increasing concentration, indicating that HCO3
- is oxidized on the catalyst prior to oxygen evolution. The normal pulse voltammograms also showed the diffusion-limited current of HCO3
- oxidation, and the voltammograms were analyzed in the same way reported in the literature [2] to obtain the stoichiometric ratio of the oxidation reaction. From the results, the stoichiometric ratio of the reduced species and the oxidized species was 1:2, so that the following reaction mechanism was proposed considering some possible oxidized species implied in Pourbaix diagram of carbon.
HCO3
- + CO2 + 3H2O → 2HCO4
- + 5H+ + 4e-

The oxidation current at different concentrations of HCO3
- obtained by chronoamperometry further revealed that the oxidation current density is linear to the concentration in the range from 10-3 mmol/L to 1.0 mmol/L. Therefore, HCO3
- can be electrochemically detected and quantified with the novel catalyst, amorphous RuO2-Ta2O5, and 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, Taiwan (2015).

[2] R. Marassi, J. Q. Chambers, and G. Mamantov, J. Electroanal. Chem., 69, 345 (1970).