Activity Coefficients of HCl in Solutions Related to “Tris” Buffers in Artificial Seawater. I. HCl + TrisHCl + H2O from 1.0 to 5.0 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 pH buffers in artificial seawater media for applications in marine chemistry. The development of a chemical speciation model of the buffer solutions has been proposed in order to quantify the effects of composition change on buffer pH and to address the metrological requirement for traceability of pH to SI base units. Such a model should be based upon data yielding solvent activities and mean activity coefficients (especially those of H+ with conjugate anions such as Cl- or SO4 2-) for aqueous solutions of single solutes (e.g., HCl, TrisHCl) and simple mixtures over a temperature range of about 0 to 40 °C. There are currently few data for solutions containing the ion TrisH+, and these are mostly restricted to 25 °C. Here, in the first of a series of studies, we present Harned cell measurements of potentials in solutions containing HCl and TrisHCl from 5 to 45 °C, yielding mean activity coefficients of HCl. The results at 25 °C are found to agree closely with those of literature studies. The Harned cell technique is described in detail, including the preparation of electrodes.