The interaction of amino acids with the major constituents of natural waters at different ionic strengths

The interaction of amino acids with the major constituents of natural waters has been studied potentiometrically by determining protonation constants at different ionic strengths (e.g., I≤5.6 mol (kg H2O)−1 (NaCl)) and in artificial seawater (containing Na+, K+, Ca2+, Mg2+, Cl− and SO42−) at different salinities. For glycine determinations in mixed NaCl–MgCl2, electrolyte solutions were also performed. The data included in this work, together with some already published, make it possible to calculate parameters for dependence on ionic strength using different models, i.e. an extended Debye–Hückel type equation and Pitzer equations. The results can be interpreted both in terms of specific interaction and in terms of complex formation. The formation constants of Mg2+–glycine complexes have been calculated from protonation data in mixed Na+–Mg2+ solutions at high ionic strength: a simple method for taking into account the different composition, at constant ionic strength, is reported. The overall complexing ability of major seawater components towards amino acids has been calculated from potentiometric data in artificial seawater using the single salt approximation. The relevance of the data reported in this work is discussed in connection with speciation problems. Particular attention has been paid to finding some useful predictive relationships.