Development of Ag2O-TeO2-based glass and glass/stainless steel reference electrodes for pH sensors

Glass pH sensors are unsuitable for in vivo biomedical, clinical, or food applications because of the brittleness of glass, the difficulty in measuring small volumes and complicated matrix, and the necessity of calibration. Enamel structures such as glass/stainless steel (SUS) may substitute glass-based pH electrodes. This study developed new reference electrodes for pH sensors using ternary Ag2O-TeO2-MaOb (M=B, Ge, and P) glass and glass/SUS. The effect of the partial replacement of TeO2 in Ag2O-TeO2 glasses with B2O3, GeO2, and P2O5 on the pH sensitivity was investigated. (1) The effect of the partial replacement on the pH 4–9 sensitivity, which is defined as 100% for the ideal working electrode and 0% for the ideal reference electrode estimated from pH potential, became prominent in the order of GeO2, B2O3, and P2O5 in glass electrodes. Ag2O-TeO2-B2O3 glasses showed the lowest pH 7–9 sensitivity of 1% among the present electrodes. Ag2O-TeO2-P2O5 glasses showed the lowest pH 4–9 sensitivities of 24-33% among the present electrodes. (2) 25Ag2O･55TeO2･20P2O5/SUS showed a very low pH 4–9 sensitivity of 5% in glass/SUS electrodes. In this case, AgP2 may be responsible for the low pH 4–9 sensitivity because only AgP2 was detected from XRD measurements, and a small particle on the surface was observed in AFM observations. On the other hand, the crystallization of the glass matrix of 25Ag2O･55TeO2･20P2O5/SUS was suppressed by adding P2O5. Thus, 25Ag2O･55TeO2･20P2O5/SUS is a candidate for a robust KCl-leakless reference electrode for pH sensors.