The dissociation constants (pK 1*, pK 2* and pK 3*) for cysteine have been measured in seawater as a function of temperature (5 to 45°C) 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 25°C. The measured values have been compared to those calculated from the Pitzer ionic interaction model. The lower values of pK 3* in the Na-Mg-Cl solutions have been attributed to the formation of Mg 2+ complexes with Cys 2- anionsMg 2+ + Cys 2- = MgCysThe stability constants have been fitted tologK* MgCys=-0.144+0.564I0.5after corrections are made for the interaction of Mg 2+ with H +. The pK 1 seawater measurements indicate that H 3Cys + interacts with SO 4 2-. The Pitzer parameters β 0(H 3CysSO 4), β 1(H 3CysSO 4) and C(H 3CysSO 4) have been determined for this interaction. The formation of CaCys as well as MgCys are needed to account for the values of pK 2 and pK 3 in seawater. The consideration of the formation of MgCys and CaCys in seawater yields model calculated values of pK 1*, pK 2* and pK 3* 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.
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