Effects of Temperature on Electrochemical Sensing of HPO4
2- with Amorphous RuO2-Ta2O5 Catalyst
Authors: Yoshida Yuuki , Morimitsu Masatsugu 
Published: February 2020

Our research group has recently revealed that a novel sensing catalyst, amorphous RuO2-Ta2O5 mixed oxide, can detect and quantify hydrogen phosphate (HPO4
2-) by electrochemical oxidation, of which the current density is linear to the concentration in the wide range from 10-6 mol/L to 10-2 mol/L [1], while no other catalyst has been found for electrochemical sensing of HPO4
2-. This new catalyst and sensing method can be applied to measure the concentrations of hydrogen phosphate or phosphorus in environmental water, blood, foods, medicines, and agricultural products. For practical uses of the amorphous oxide catalyst for the detecting electrode, it is important to know the effects of the temperature of the test solution on the sensitivity to HPO4
2-, especially for continuous monitoring of phosphorus in environmental water. In this paper, we report the effects of temperature on the onset potential, the oxidation current density, and the sensitivity of the oxidation of HPO4
2- examined by cyclic voltammetry and chronoamperometry using rotating disk electrode (RDE).
The amorphous RuO2-Ta2O5 catalyst was prepared on a titanium disk for RDE by calcination of the precursor solution containing Ru (III) and Ta (V), in which the Ru mole ratio was 50 mol% and thermal decomposition was carried out at 260 oC. Electrochemical measurements were performed using a conventional three-electrode cell and 50 mmol/L KCl solutions with or without various concentrations of Na2HPO4 at a temperature from 3 oC to 50 oC.
The cyclic voltammograms showed the oxidation wave or plateau of HPO4
2-, and the onset potential shifted negatively and the maximum current density increased with increasing temperature of the test solutions. From the results, the potential to measure the oxidation current by chronoamperometry was determined at each temperature, so that the applied potential was more positive than that at which the maximum oxidation current in the cyclic voltammograms was observed and was less than the onset potential of oxygen evolution. The relationship between the oxidation current density and the concentrations of HPO4
2- obtained at 3 oC, 10 oC, 20 oC, 30 oC, 40 oC, or 50 oC revealed that the linear relationship was observed at all the temperatures and the slope increased with increasing temperature, suggesting that the sensitivity is enhanced. However, the linear region was seen in the concentration range from 0.4 mmol/L to 10 mmol/L, which was independent of temperature. These results are reasonable, because chronoamperometry measured the diffusion-limited current of HPO4
2- oxidation with RDE and the diffusion co-efficient becomes large at higher temperature. It was also found that the sensitivity was proportional to the temperature, which implies that it is easy to calibrate the sensitivity in practical uses. 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).

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