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

The complex formation constants of polyacrylic (PAA) ligands (1.4≤log N≤2.4, N=number of monomer units) with calcium and magnesium ions were determined in different ionic media at different ionic strengths, 0≤I≤1 mol l−1, at t=25 °C. Experimental pH-metric data in the presence of Ca2+ or Mg2+ were firstly analysed in terms of apparent protonation constants, log KH*, using the “three parameter model” proposed by Högfeldt; differences in log KH*, determined in different ionic media, were interpreted in terms of complex species formation. The only species present in the system M–PAA (M=Ca2+ or Mg2+) is ML2: attempts to find species of different stoichiometry were unsuccessful. The stability dependence of this species on ionic strength, on the degree of neutralisation (α) and on PAA molecular weight is discussed using empirical equations. The formation constant, log β2, is significantly higher for Ca2+ than for Mg2+: at I=0.1 mol l−1 (NaCl), log N=1.8 and α=0.5, log β2Ca=4.43 and log β2Mg=4.24. The formation of polyacrylate–alkaline earth metal complexes is discussed in the light of sequestering effects in natural waters.