Acid-base properties of etidronic acid [(1-Hydroxyethane-1,1-diyil)bis(phosphonic acid), HEDPA] in different ionic media and at different ionic strengths (NaCl, KCl: I <= 2 mol . L-1; (C2H5)(4)NI: I <= 1 mol . L-1) were studied at t = 25 degrees C, determining, by potentiometric and calorimetric techniques, protonation constants and enthalpy changes. The differences in the protonation constants in the different supporting electrolytes were also interpreted in terms of weak complex formation with MiL (with i = 1, 2), MLHj (with j = 1, 2, 3) and M2LH species (with L = HEDPA; M = Na+, K+). The formation constants for the species of Ca2+ and Mg2+, were determined by potentiometric titrations at different ionic strengths (0.1 <= I/mol . L-1 <= 1) in NaCl at t = 25 degrees C. The stability of these species is fairly high, as an example, at I = 0.1 mol . L-1 and t = 25 degrees C, for ML species, log beta = 6.52 and 6.86, for Ca2+ and Mg2+, respectively, obtained by considering simultaneously HEDPA-Na+ interactions. The dependence on ionic strength was analysed by a Debye-Huckel type equation and the SIT (Specific ion Interaction Theory) approach for protonation thermodynamic parameters and by a Debye-Huckel type equation for Mg2+ and Ca2+ complex formation. The sequestering ability of HEDPA toward Ca2+ and Mg2+ was also analysed. A comparison with literature data is given. (c) 2013 Elsevier Ltd. All rights reserved.