Determining the Pitzer interaction coefficients of TRIS in aqueous solutions of NaCl, TRISHCl and (TRISH)2SO4 by solubility measurements. A new experimental contribution towards the development of a traceable chemical speciation model of pH buffers used for applications involving seawater and other natural waters

Measurements of pH in seawater rely on the accuracy of pH values assigned for the TRIS-seawater buffers currently used for calibration. These buffer pH are derived from Harned cell measurements of solutions of TRIS in artificial seawater, with the results fitted to empirical functions of salinity and temperature. This largely measurement-based approach is limited to natural waters of seawater stoichiometry. Chemical speciation models, such as those based upon the Pitzer equations, can potentially calculate buffer pH, and other chemical equilibria, directly as functions of the total concentrations of the individual ions present. This offers the possibility of application to a much wider range of natural waters, including those of ionic compositions different from that of seawater. However, even at 25°C the necessary Pitzer interaction coefficients are not sufficiently well characterised to reproduce the measured Harned cell potentials for TRIS-artificial seawater solutions with adequate accuracy. In this work we quantify the interactions of TRIS with some of the major ions present in the TRIS buffer, by measuring the solubility of neutral TRIS in salt solutions, and of the same salts in aqueous solutions of TRIS. We have covered a wide range of concentrations, and temperatures between 5 and 45°C. The experiments carried out thus far have yielded Pitzer interaction coefficients for neutral TRIS with ions Na⁺, TRISH⁺, Cl^- and SO₄^2-, including both binary and triplet interactions. These results enable us to more accurately calculate the TRIS-TRISH⁺ equilibrium in solutions containing these ions, such as the aqueous buffers used to calibrate instruments for measuring the pH of seawater and potentially other natural waters. The improvements in modeled pH will be demonstrated.