Free acidity of aqueous solutions was initially defined in 1909 by Søren Peter Lauritz Sørensen as pH = −lgcH₊ (c/mol·dm⁻³ or m/mol·kg⁻¹ of the free hydrogen ions in solution, H⁺) soon (1910) was changed to pH = paH₊ = −lgaH₊, integrating the new concepts of activity, aᵢand activity coefficient γᵢ, for the ionic species i under concern, H⁺ in this case; it is aᵢ = −lg(mᵢγᵢ). Since individual ions do not exist alone in solution, primary pH values cannot be assigned solely by experimental measurements, requiring extra thermodynamic model assumptions for the activity coefficient, γH₊, which has put pH in a unique situation of not being fully traceable to the International System of Units (SI). Also the concept of activity is often not felt to be as perceptible as that of concentration which may present difficulties, namely with the interpretation of data. pH measurements on unknown samples rely on calibration of the measuring setup with adequate reference pH buffers. In this work, the assignment of pH values to buffers closely matching the samples, e.g., seawater, is revisited. An approach is presented to assess the quantity pmH₊ = −lgmH₊ profiting from the fact that, contrary to single ion activity coefficients, mean activity coefficients, can be assessed based on experimentally assessed quantities alone, γExp ±, thus ensuring traceability to the mole, the SI base unit for amount of substance. Compatibility between γExp ± and mean activity coefficient calculated by means of Pitzer model equations, γPtz ±, validates the model for its intended use.
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