Monitoring pH in corrosion engineering by means of thermally-produced iridium oxide electrodes

A pH sensor to be used in highly alkaline media under continuous long-term immersion conditions is crucial in various engineering applications. This work develops the production protocol and posterior conditioning of thermally oxidized iridium (IrOx) electrodes to be used as potentiometric pH sensors embedded in highly alkaline environments such as concrete or cathodically protected steel in soil. The main investigated aspects for the desired applications are the potential-pH response, its reproducibility, accuracy, and oxygen dependency. The stability during long-term immersion is also studied in detail. The studied IrOx electrodes responded to pH changes with slopes between -50 and -68 mV/pH unit, even after continuous immersion in alkaline solutions for almost 2 years. Additionally, the electrodes response did not show oxygen dependency. Our results highlight the importance of sufficient conditioning in alkaline media prior to use. When properly produced, conditioned, and pre-calibrated the electrodes reproducibly permit measuring the pH with a maximum error of 0.5 pH units over a range of at least pH 9 to 13.5. Preliminary results show that the studied electrodes are promising sensors for monitoring pH changes in concrete.