pH dependent voltammetry at partially oxidized palladium and its potential for voltammetry-based pH sensing

We studied the effects of pH on voltammetry at partially oxidized palladium. Below pH 7.2, a cathodic process is seen in cyclic voltammetry whose peak potential as well as the potential of the inflection point preceding the peak are pH dependent with super-Nernstian pH response in both phosphate buffer (-90 mV/pH) and phosphate-buffered saline (-160 mV/pH). Above pH 7.2, a pH-independent secondary electrochemical process, hysteresis and/or a drop in sensitivity were observed. The mechanism of pH-dependent voltammetry is linked to the reduction of Pd(II) oxide and is modulated by adsorbed and partially protonated phosphate anions. The potential for pH sensing via voltammetry was evaluated in dynamic measurements, and even without a reference electrode when the microelectrode surface was partially covered with silver whose sharp anodic peak is not pH-dependent and therefore, can be used as internal reference. Practical pH measurement with voltammetry requires further research and optimization of material and stabilization protocols in the future.