Dissociation constants of protonated methionine species in seawater media

Abstract

The dissociation constants (p K 1 ⁎ and p K 2 ⁎) for methionine have been measured in artificial seawater as a function of salinity ( S = 5 to 35) and temperature (5 to 45 °C). The seawater p K 2 ⁎ values were lower than the values in NaCl at the same ionic strength while the p K 1 ⁎ values in seawater were lower only at S = 35. 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 used to determine the formation of Mg 2+ complexes and Pitzer interaction parameters for Mg 2+ with methionine. The seawater model with the interaction parameters accounts for the differences between the value of p K 1 ⁎ and p K 2 ⁎ between NaCl and seawater. This study demonstrates that it is important to consider all of the ionic interactions in natural waters when examining the dissociation of organic acids.