Dissociation constants of protonated cysteine species in seawater media
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The dissociation constants (pK1*, pK2* and pK3*) for cysteine have been measured in seawater as a function of temperature (5 to 45C) and salinity (S = 5 to 35). The seawater values were lower than the values in NaCl at the same ionic strength. In an attempt to understand these differences, we have made measurements of the constants in Na-Mg-Cl solutions at 25C. The measured values have been compared to those calculated from the Pitzer ionic interaction model. The lower values of pK3* in the Na-Mg-Cl solutions have been attributed to the formation of Mg2+ complexes with Cys2- anionsMg2+ + Cys2- = MgCysThe stability constants have been fitted tologK*MgCys=-0.144+0.564I0.5after corrections are made for the interaction of Mg2+ with H+. The pK 1 seawater measurements indicate that H3Cys+ interacts with SO42-. The Pitzer parameters 0(H3CysSO4), 1(H 3CysSO4) and C(H3CysSO4) have been determined for this interaction. The formation of CaCys as well as MgCys are needed to account for the values of pK2 and pK3 in seawater. The consideration of the formation of MgCys and CaCys in seawater yields model calculated values of pK1*, pK2* and pK3* that agree with the measured values to within the experimental error of the measurements. This study shows that it is important to consider all of the ionic interactions in natural waters when examining the dissociation of organic acids. 2005 Elsevier B.V. All rights reserved.
