The working principle of an electrochemical pH sensor based on gold oxide reduction operates by measuring changes in potential as a result of pH variations. Gold oxide is reduced at the sensor surface in response to changes in pH, altering the potential. This potential shift is quantified by the Nernst equation, relating the electrode potential to the pH of the solution. As pH changes, the concentration of hydrogen ions alters, affecting the potential across the electrode interface, which is then measured to determine the pH of the solution. The Nernst equation is fundamental in electrochemistry and is used to relate the concentration of ions in a solution to the measured electrode potential . For a pH sensor based on gold oxide reduction, the Nernst equation can be expressed as (Lu and Compton, 2014):