Ocean Acidification

Abstract

The pH on the total proton scale of the Tris-HCl buffer system (pH_Tris) was characterized rigorously with the electrochemical Harned cell in salinity (S) 35 synthetic seawater and S = 45 – 100 synthetic seawater-derived brines at 25 and 0°C, as well as at the freezing point of the synthetic solutions (–1.93°C at S = 35 to –6°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 m_{Tris − H⁺} = 0.04 mol kg_H2O^‐1) yielded pH_Tris values that were consistently less alkaline by 0.3 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^⁎ + log R_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.