The knowledge of the values of formation constants 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 literature, which allow to calculate activity coefficients, and consequently formation constants as a function of ionic strengths, are here considered: the Extended Debye-Hückel equation (EDH), which makes use only of few general parameters, provided that all the possible interactions in solution are taken into account, and three methods such as the Specific Interaction Theory, the Pitzer Equations and the Bromley Equation, which all make use of specific parameters of the ions involved, of the ionic medium employed and the ionic strength. There are not substantial differences among the various approaches, as regards the accuracy of the calculated stability constants, while differences can be pointed out if looking at the possibility of employing general parameters and, as a consequence, at faster and simpler calculation procedures. As regards the application of the Extended Debye-Hückel equation, the medium effect on the different ligand classes has been analyzed in order to draw general trends in the stability of ion-pair of carboxylate ligands, protonated amines, aminoacids and phenates (whose knowledge is essential for a correct chemical model, when using EDH). As regards both the ionic medium employed and the ionic strength, a literature survey is also proposed, showing which are the ionic media and ionic strengths most commonly chosen by the researchers.