Speciation of low molecular weight ligands in natural fluids: protonation constants and association of open chain polyamines with the major components of seawater Original Research Article

The interaction of five open chain polyamines (ethylenediamine (en), diethylenetriamine (dien), trietylenetetramine (trien), tetraethylenepentamine (tetren), spermine (sper)) with the major components of seawater was studied potentiometrically at 25°C, in an artificial seawater (containing Na+, K+, Mg2+, Ca2+, Cl− and SO42−) at different salinities (5–45‰). Potentiometric data were interpreted in terms of both the apparent protonation constants of polyamines and the formation constants of complexes formed by unprotonated or protonated amines and the cation and the anion of seawater (the inorganic content of seawater being considered as a single 1:1 salt). Some empirical relationships, involving the polyammonium cation charge and the ratio between N and C atoms in the amine, are reported. These relationships have quite a good predictive value. The association ability of seawater salt towards amines is quite significant: at pH 8.2 and 35‰ salinity formation percentages range between 50 and 80%. Speciation problems are discussed.