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

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Electrochemistry

Background:

• Conventional pH reference buffer standards (pH(S)) rely on Harned cell potential measurements.
• An intermediate step involves assessing the acidity function p(a(H)γ(Cl))(0) through extrapolation.
• This value is compared to theoretical calculations based on buffer species dissociation constants.

Purpose of the Study:

• To investigate the discrepancy between theoretically expected and experimentally obtained acidity function values.
• To analyze how these discrepancies affect the assignment of pH reference buffer standards.
• To highlight an additional source of uncertainty in pH(S) traceability.

Main Methods:

• Measurement of Harned cell potentials for buffer solutions.
• Addition of alkali chloride (KCl) at varying molalities (0.005, 0.010, 0.015 mol kg⁻¹).
• Extrapolation of p(a(H)γ(Cl)) versus m(Cl) to determine p(a(H)γ(Cl))(0).

Main Results:

• Experimental slopes for the acidity function can be positive or negative, unlike theoretical predictions for monoprotic, diprotic, and triprotic acids.
• Disagreements were observed for buffer solutions with ionic strengths ranging from 0.0025 to 0.144 mol kg⁻¹.
• These discrepancies lead to variations in the intercept value, p(a(H)γ(Cl))(0).

Conclusions:

• The difference between theoretical and experimental acidity function values introduces uncertainty in pH(S) assignment.
• This impacts the traceability chain of pH measurements.
• Understanding these discrepancies is crucial for accurate pH standardization.