# Activity Coefficients of HCl in Solutions Related to “Tris” Buffers in Artificial Seawater. III. Tris Buffer + NaCl + H2O, from 0.2 to 3.25 mol kg–1 Ionic Strength and from 5 to 45 °C

## TL;DR

This study measures the activity coefficients of HCl in Tris buffer solutions to improve pH modeling in artificial seawater.

## Contribution

New Harned cell measurements of potentials in Tris buffer solutions with NaCl at varying ionic strengths and temperatures.

## Key findings

- Activity coefficients of HCl in Tris buffer solutions were measured from 0.2 to 3.25 mol kg–1 ionic strength.
- Results were compared to previous measurements and a preliminary model, showing good agreement with minor discrepancies.
- The data support the development of a chemical speciation model for buffer solutions in artificial seawater.

## Abstract

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.