Activity Coefficients of HCl in Solutions Related to “Tris” Buffers in Artificial Seawater. III. Tris Buffer + NaCl + H ₂ O, from 0.2 to 3.25 mol kg ^–1 Ionic Strength and from 5 to 45 °C

The substance Tris-(2-amino-2-hydroxymethyl-1,3-propanediol, CAS 77-86-1), and its protonated form TrisH⁺, are used in the preparation of "total" pH buffers in artificial seawater media. Here, as part of a series of studies, we present Harned cell measurements of potentials in solutions containing equimolal Tris and TrisHCl (hence TrisH⁺), and also NaCl which is the major constituent of artificial seawater. The methods of preparation of the hydrogen and chloride electrodes are described. The data contribute to the development of a chemical speciation model of the buffer solutions in which solute activity coefficients are calculated using the Pitzer equations. Such a model is required in order to quantify the effects of composition change, convert the total pH to other scales, and to address metrological requirements for traceability. The results are expressed in terms of an acidity function and are compared to previous measurements at 25 °C, including equivalent values for artificial seawater media, and also to calculations using a preliminary model. Agreement is good, and the small differences found between data and model predictions are likely due to offsets in the measured potentials, and uncertainties in some of the Pitzer model parameters and the TrisH⁺ dissociation constant.