Activity coefficients of HCl in solutions related to “Tris” buffers in artificial seawater. II. HCl + NaCl + TrisHCl + H₂O, and Tris buffer + NaCl + H₂O, to high ionic strength and from 5 to 40 °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. The development of a chemical speciation model of the buffer solutions, using the Pitzer equations to calculate solute activity coefficients, is desirable in order to quantify the effects of composition change, convert the total pH to other scales, and address metrological requirements for traceability to the International System of Units. Here, in the second of a series of studies, we present Harned cell measurements of potentials and mean activity coefficients of HCl in solutions containing HCl, NaCl, and TrisHCl for ionic strengths from 1.0 to 5.5 mol kg^–1and from 5 to 40 °C. The results at 25 °C are consistent with those of the literature studies of the two end-member solutions (aqueous HCl + NaCl, and HCl + TrisHCl). We also present results of measurements of buffer solutions containing equimolal Tris and TrisHCl (hence TrisH⁺), and NaCl, at ionic strengths of 0.2, 1.0, and 4.0 mol kg^–1at the same temperatures. These are compared with literature data for Tris buffers in an artificial seawater medium. Aspects of the development of a Pitzer model for these solutions are discussed.