Acidity function p( a H γ Cl ) as a step to pH assessment

Abstract

The conventional assignment of pH reference buffer standards, pH(S), is achieved by means of a series of procedures that follow from measurement of Harned cell potentials for an electrolyte solution which is the buffer solution of interest. An intermediate step is assessment of the acidity function p( a(H) gamma(Cl))(0), the extrapolated value of a linear representation of the dependence of p( a(H) gamma(Cl)) on 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 value calculated from the dissociation constants of the buffer species. Whereas these calculations always give negative slopes for diprotic and triprotic acids and zero slope for monoprotic acids, experimental values with negative or positive slopes can be obtained for well fitting straight lines obtained for buffer solutions with ionic strengths from 0.0025 to 0.144 mol kg(-1). Such disagreement between theoretically and experimentally obtained values 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 for which the discrepancy between expected and experimental values leads to different intercept p( a(H) gamma(Cl))(0).

Major applications of electrochemical techniques at national metrology institutes

“…Each primary buffer is a dilute aqueous solution between pH 3 and pH 10 and is certified in a temperature range from 5 • C to 50 • C. Criteria for the selection of primary buffers are given in [8]. The reagents used to produce the primary buffers must also be free from contaminant halides, to ensure linearity [32] in the extrapolation of pa to obtain pa 0 . An alkali chloride of certified high purity and low Br − impurity [33] is used for the addition of Cl − to the buffer.…”

Role of the activity coefficient in the dissemination of pH: comparison of primary (Harned cell) and secondary (glass electrode) measurements on phosphate buffer considering activity and concentration scales

“…By extrapolating the data to zero chloride concentration ( Fig. 1), the term p(a H c Cl )°is obtained, according to a procedure based on the specific ion interaction theory [13,14]. Finally, to evaluate the paH of the buffer, the following calculation is used:…”