Protonation Constants and Association of Polycarboxylic Ligands with the Major Components of Seawater

Abstract

Apparent protonation constants, log βjH*, of 11 carboxylic acids were determined potentiometrically ([H+]-glass electrode) in artificial seawater containing six of the major components (Na+, K+, Mg2+, Ca2+, Cl-, and SO42-) at different salinities: S (‰) = 5, 15, 25, 35, 45. Values of log βjH* were fitted by the simple polynomial equation log βjH* = log TβjH + a1S1/2 + a2S + a3S3/2 (log TβjH = protonation constants at infinite dilution; a1, a2, a3 = empirical parameters), for mono-, di-, and tricarboxylates. For carboxylic anions with charge < −3, a better fit was obtained using the equation log βjH* = log TβjH + b1I + b0z* log(1 + b2I) (b0, b1, b2 = empirical parameters, z* = square sum of reactants minus square sum of products). The potentiometric data were also interpreted in terms of complex formation between the cationic component of artificial seawater and the carboxylic anion (also partially protonated) using the single-salt approximation (the artificial seawater considered as a single salt with B1.117+ cation and A1.117- anion). Calculations were also performed, for comparison, on potentiometric data for phenolic compounds (already published) to obtain the formation constants of seasalt complexes. Generally, both empirical parameters for dependence on ionic strength and complex formation constants are dependent on the charge of the (poly)carboxylic anions. Some useful stability−charge and stability−structure relationships are reported. The interaction of carboxylic ligands with the major components of seawater is discussed in relation to speciation problems.