A Calibration-Free pH Sensor Using an In-Situ Modified Ir Electrode for Bespoke Application in Seawater

A bespoke calibration-free pH sensor using an in situ modified Ir electrode for applications in seawater is reported. The electrochemical behaviour of an iridium wire in air-saturated synthetic seawater was studied and the formation of pH-sensitive surface layers was observed that featured three pH-sensitive redox couples, Ir(III/IV), <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>IrO</mi></mrow><mi mathvariant="normal">x</mi></msub><msup><mi mathvariant="normal">O</mi><mrow><mi mathvariant="normal">I</mi><mo>−</mo></mrow></msup><mo>/</mo><msub><mrow><mi>IrO</mi></mrow><mi mathvariant="normal">x</mi></msub><msup><mi mathvariant="normal">O</mi><mrow><mi>II</mi><mo>−</mo></mrow></msup><mi mathvariant="normal">H</mi><mo>,</mo></mrow></semantics></math></inline-formula> and H_upd/H⁺, where H_upd is adsorbed hydrogen deposited at underpotential conditions. The amperometric properties of the electrochemically activated Ir wire were investigated using linear sweep voltammetry first, followed, second, by square wave voltammetry with the formation conditions in seawater for the optimal pH sensitivity of the redox couples identified. The sensor was designed to be calibration-free by measuring the “super-Nernstian” response, in excess of ca 60 mV per pH unit, of Ir(III/IV) relative to the less sensitive upd H oxidation signal with the pH reported on the total pH scale. The pH dependency of the optimised sensor was 70.1 ± 1.4 mV per pH unit at 25 °C, showing a super-Nernstian response of high sensitivity.