Effect of Ag–Te alloys on pH sensitivity of Ag2O–TeO2 glass/stainless steel enamel

Abstract

Glass pH sensors are unsuitable for in vivo biomedical, clinical, or food applications because of brittleness of glass, the difficulty in measurement of small volumes and complex matrices, and the need for calibration. Herein, a novel enamel reference electrode for pH sensors is developed using a binary xAg₂O·(100–x)TeO₂ glass/stainless steel (SUS) electrode. Subsequently, the effects of Ag₂O content on the precipitated crystalline phases and pH sensitivity of Ag₂O–TeO₂ glass/SUS electrodes are examined. The results indicate that the xAg₂O·(100–x)TeO₂ glass/SUS samples exhibit relatively low pH sensitivity. The pH sensitivities of the samples can be classified into two distinct groups. The first group, comprising samples with an Ag₂O content of 25 mol% or more, exhibits a pH sensitivity higher than 10 %. The second group, comprising samples with an Ag₂O content of 22 mol% or less, exhibits a pH sensitivity of less than 10 %. The pH sensitivity tends to decrease with increase in stutzite fraction in Ag–Te alloys. Results of X-ray diffraction measurements and transmission electron microscopy observations indicate that the hessite and stutzite Ag–Te alloys, which are minor and conductive, are the primary contributors to the low pH sensitivity observed. Moreover, the pH sensitivity of xAg₂O·(100–x)TeO₂/SUS is observed to be between those of the Ag–Te alloy and the Ag₂O–TeO₂ glass matrix. We propose occurrence of an electrical shunt should occur between the Ag–Te alloy and the Ag⁺-ion conductive Ag₂O–TeO₂ glass matrix. Regarding xAg₂O·(100–x)TeO₂ glass/SUS, the 20Ag₂O·80TeO₂ glass/SUS to 21Ag₂O·79TeO₂ glass/SUS compositions represent promising candidates as for KCl-leakage-free reference electrodes for pH sensors.