A surface charge is at its point of zero charge when the @S06159@ is zero. It is a value of the negative logarithm of the activity in the bulk of the charge-determining ions.
Source: PAC, 1991, 63, 895. (Nomenclature, symbols, definitions and measurements for electrified interfaces in aqueous dispersions of solids (Recommendations 1991)) on page 902 [Terms] [Paper]
Citation: 'point of zero charge' in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.P04704 RISBibTexEndNote