Measurement of potential of zero charge at a renewable pencil electrode

• Potential of zero charge (PZC) measured by a renewable pencil electrode. • Accurate PZC values for various electrolyte solutions. • Relationship between specific adsorption and hydrophobicity of ions. Potential of zero charge (PZC) was studied by using a renewable pencil lead electrode. A cutting device for the pencil electrode enabled clear observation of the direction of charging current, providing accurate values of PZC. The PZCs of the pencil electrode in aqueous solutions with nine different electrolytes at various concentrations were measured. The positive and negative shifts of the PZC upon the increase of electrolyte concentrations were observed for hydrophobic cations and anions, respectively, suggesting the presence of specific adsorption of the ions onto the electrode surface. An approximately linear relationship was found between the degree of the PZC shift (the Esin–Markov coefficient) and the hydrophobicity parameter of ion (the standard Gibbs energy of transfer of ion at nitrobenzene | water interface), whereas Cs + ion gave practically no shift of PZC in spite of the moderate hydrophobicity.