Quantitative parameters for the sequestering capacity of polyacrylates towards alkaline earth metal ions

The complex formation constants of polyacrylic (PAA) ligands (1.4 less than or equal to log N less than or equal to 2.4, N = number of monomer units) with calcium and magnesium ions were determined in different ionic media at different ionic strengths, 0 less than or equal to I less than or equal to 1 mol l(-1), at t = 25 degreesC. Experimental pH-metric data in the presence of Ca(2+) or Mg(2+) were firstly analysed in terms of apparent protonation constants, log K(H*), using the "three parameter model" proposed by Hogfeldt; differences in log K(H*), determined in different ionic media, were interpreted in terms of complex species formation. The only species present in the system M-PAA (M = Ca(2+) or Mg(2+)) is ML(2): attempts to find species of different stoichiometry were unsuccessful. The stability dependence of this species on ionic strength, on the degree of neutralisation (alpha) and on PAA molecular weight is discussed using empirical equations. The formation constant, log beta(2), is significantly higher for C(2+) than for Mg(2+) : at I = 0.1 mol l(-1) (NaCl), log N = 1.8 and alpha = 0.5, logbeta(2)(Ca) = 4.43 and log beta(2)(Mg) = 4.24. The formation of polyacrylate-alkaline earth metal complexes is discussed in the light of sequestering effects in natural waters. (C) 2003 Elsevier B.V. All rights reserved.