pHT measurements of TRIS buffer solutions in an artificial seawater matrix in the salinity range 5–40 and temperature range 5–40 °C. Part 1: Measurements and data fitting

Abstract

Spectrophotometric measurements of seawater total pH ($p{H}_T$) contribute to the knowledge of the oceanic carbonate system, and hence to the assessment of the ocean carbon cycle and the monitoring of ocean acidification. Spectrophotometric measurements rely on the characterization of the indicator dye's second dissociation constant, obtained from measurements on solutions of known $p{H}_T$. Therefore, Harned cell potentiometric measurements were performed on equimolal TRIS buffers (i.e. buffers containing equal molalities of 2-amino-2-(hydroxymethyl)-1,3-propanediol (TRIS, CAS reference number 77–86-1) and TRIS hydrochloride (TRIS.HCl)), made in an artificial seawater (ASW) matrix. Buffer solutions were prepared in an ASW matrix of nominal practical salinities from 5 to 40, and Harned cell measurements were performed on these solutions at temperatures from 5 °C to 40 °C. This provides, for the first time, $p{H}_T$ values of ASW/TRIS buffers that are consistent for the entire ranges of salinity relevant for oceanographic measurements, and for a wide range of temperature. The work presented includes a comparison of results from three National Metrology Institutes to assess reproducibility, and provides a function allowing the calculation of equimolal ASW/TRIS buffers $p{H}_T$ as a function of salinity, temperature and TRIS buffer molality. This function can be used to derive $p{H}_T$ values for zero TRIS molality, thus representing a pure ASW (i.e. a chemical environment unaffected by the presence of TRIS and TRIS.HCl), and referring to a true $p{H}_T$ scale. These values are needed for the characterization of the second dissociation constant of the dye with perspective of traceability to the International System of units.