The substance Tris (or THAM, 2-amino-2-hydroxymethyl-1,3-propanediol) is used in the preparation of pH buffer solutions for applications in natural water chemistry, including seawater. The development of a chemical speciation model of buffer solutions containing Tris, TrisH+, and the major ions of seawater is desirable, so that the effects of changes in the composition and concentration of the medium on pH can be calculated. The Pitzer activity coefficient equations, commonly used in such speciation models, describe the thermodynamic properties of solutions in terms of interactions between dissolved ions and uncharged solute species. To determine some of these interactions, we have measured solubilities of Tris(s) in water and aqueous solutions of NaCl, TrisHCl, and (TrisH)2SO4 and the solubility of NaCl(s) in aqueous Tris(aq), from 5 to 45 °C. We report measurements of the water activities of Tris solutions at 293.5 K to high supersaturation with respect to the solid. Using the Pitzer equations, we compare our results to literature data yielding stoichiometric dissociation constants of TrisH+ in aqueous NaCl, and to electromotive forces of cells containing dissolved Tris, TrisHCl, and NaCl. Values of parameters for the interactions of Tris with the ions TrisH+, Na+, and SO42– at 25 °C are determined.
Bibliographical noteFunding Information:
The work of P.L. and E.P.A. was supported by the Helmholtz Association. The work of S.L.C. was supported by the Natural Environment Council of the UK, award number NE/P012361/1 under the joint NERC/NSF:GEO scheme. Authors F.K.A.G. and J.P.R. were supported by the Engineering and Physical Sciences Research Council under the grant code EP/N025245/1. This publication is a contribution of SCOR Working Group 145 (SCOR is the Scientific Committee on Oceanic Research) and of the Joint Committee on Seawater, which is sponsored by SCOR, the International Association for the Properties of Water and Steam, and the International Association for the Physical Sciences of the Oceans. The work of WG 145 presented in this article results, in part, from funding provided by national committees of SCOR and from a grant to SCOR from the U.S. National Science Foundation (OCE-1840868). We thank Florian Radloff, Lucía González Ruiz, and Lesly Ayala Cabana for their assistance with the solubility experiments.
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