Acidity Function p(a_(H)γ_(Cl)) as a step to pH assessment

摘要

The conventional assignment of pH reference buffer standards, pH(S), is attained through a series of procedures that follow from the measurement of Harned cell potentials where the electrolyte solution is the buffer solution under concern. An intermediate step is the assessment of the acidity function p(a_(H)γ_(Cl))~(0), the extrapolated value of a linear representation of p(a_(H)γ_(Cl)) vs. m_(Cl) for at least three different molalities m_(Cl) of added alkali chloride (0.005; 0.010; 0.015 mol kg~(-1) KCl). This experimental value can be compared with a theoretically expected one assessed from the dissociation constants of the pertaining buffer species. While for this, a negative slope is always the case for diprotic and triprotic acids and a zero slope for monoprotic acids, experimental values can be found with slopes ranging from negative to positive, of well fitted straight lines for buffer solutions of ionic strengths from 0.0025 to 0.144 mol kg~(-1). Such disagreement between theoretically and experimentally assessed values are prone to introduce an extra source of uncertainty in the establishment of pH(S) and on its traceability chain. In this work examples are presented and discussed where discrepancy between expected and experimental values leads to different intercept p(a_(H)γ_(Cl))~(0).