ALBERT

Description: The stoichiometric dissociation constants of carbonic acid ( and ) were determined by measurement of all four measurable parameters of the carbonate system (total alkalinity, total dissolved inorganic carbon, pH on the total proton scale, and CO2 fugacity) in natural seawater and seawater-derived brines, with a major ion composition equivalent to that Reference Seawater, to practical salinity (SP) 100 and from 25 °C to the freezing point of these solutions and –6 °C temperature minimum. These values, reported in the total proton scale, provide the first such determinations at below-zero temperatures and for SP 〉 50. The temperature (T, in Kelvin) and SP dependence of the current and (as negative common logarithms) within the salinity and temperature ranges of this study (33 ≤ SP ≤ 100, –6 °C ≤ t ≤ 25 °C) is described by the following best-fit equations: = –176.48 + 6.14528 – 0.127714 SP + 7.396×10–5 + (9914.37 – 622.886 + 29.714 SP) T–1 + (26.05129 – 0.666812 ) lnT (σ = 0.011, n = 62), and = –323.52692 + 27.557655 + 0.154922 SP – 2.48396×10–4 + (14763.287 – 1014.819 – 14.35223 SP) T–1 + (50.385807 – 4.4630415 ) lnT (σ = 0.020, n = 62). These functions are suitable for application to investigations of the carbonate system of internal sea ice brines with a conservative major ion composition relative to that of Reference Seawater and within the temperature and salinity ranges of this study.

Description: Highlights • pH of Tris buffers determined in synthetic seawater and brines with the Harned cell. • pH determination of Tris buffers to the freezing point of synthetic solutions. • pH determination of the equimolal and non-equimolal Tris buffer variants. • pH measurement is facilitated at below-zero temperatures, such as in sea ice brines. The pH on the total proton scale of the Tris-HCl buffer system (pH(Tris)) was characterized rigorously with the electrochemical Flamed cell in salinity (S) 35 synthetic seawater and S = 45-100 synthetic seawater-derived brines at 25 and 0 degrees C, as well as at the freezing point of the synthetic solutions (-1.93 degrees C at S = 35 to -6 degrees C at S = 100). The electrochemical characterization of the common equimolal Tris buffer [R-Tris = m(Tris)/m(Tris-H+) = 1.0, with m(Tris) = m(Tris-H+) = 0.04 mol kg(H2O)(-1) = molality of the conjugate acid-base pair of 2-amino-2-hydroxymethyl-1,3-propanediol (Tris)] yielded pH(Tris) values which increased with increasing salinity and decreasing temperature. The electrochemical characterization of a non-equimolal Tris buffer variant (R-Tris = 0.5, with m(Tris) = 0.02 mol kg(H2O)(-1) and MTris-H+ = 0.04 mol kg(H2O)(-1)) yielded pH(Tris) values that were consistently less alkaline by 03 pH unit than those of the equimolal Tris buffer. This is in agreement with the values derived from the stoichiometric equilibrium of the Tris-H+ dissociation reaction, described by the Henderson - Hasselbalch equation, pH(Tris) = pK(Tris)* + logR(Tris), with pK(Tris)* = stoichiometric equilibrium dissociation constant of Tris-H+, equivalent to equimolal pH(Tris). This consistency allows reliable use of other R-Tris variants of the Tris-HCl buffer system within the experimental conditions reported here. The results of this study will facilitate the pH measurement in saline and hypersaline systems at below-zero temperatures, such as sea ice brines.