Ionic strength dependence of formation constants in different ionic media

The knowledge of the formation constant values as a function of ionic strength may be very important in building chemical models for speciation of natural and biological fluids. Several approaches, known in the Iìterature, which allow to calculate activity coefficients, and consequently formation constants as a function of ionic strength, were considered: the Extended Debye-Huckel equation (EDH), the Specific ion lnteraction Theory (SIT) and the Pitzer Equations. A modified version of the SIT equation was used too, in which interaction coefficients epsilon are not constant but depend on ionic strength according to the simple relation epsilon = epsilon(infinito) + (epsilon(0) -epsilon(infinito))/(I+ 1). Different ligand classes were reported: carboxylic acids of different anionic charge (from mono to hexa-carboxylate), amines containing different number of amino groups (from ethylenediamine to pentaethylenehexamine), aminoacids. Dependence on ionic strength of ligand protonation constants were studied in different media (tetralkylammonium salts, such as (CH3)4NCl and (C2H5)4NI, sodium and potassium chlorides) and in a wide range of ionic strengths: [(CH3)4NCI: 0.1 - 3; (C2H5)4NI: 0.1-1; NaCI: 0.1-5; KCI:0.1-3 mol L-1]. For some systems the dependence on ionic strength of protonation enthalpies was also studied. The dependence of protonation thermodynamic parameters on ionic strength was analyzed by using the above cited different models, and generai empìrical relationships with a good predictive capacity were proposed as a function of the anion charge, for carboxylate, and of the number of amino groups and of protonation degree, for amines. Strong correlation was found between SIT and Pitzer parameters which allows the two models to be used with the same results. Differences in protonation constants in the different supporting electrolytes were also interpreted in terms of weak species or ion pair formation between the ligand under study and the ion of background salt.