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

Abstract

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 Ca 2+ or Mg 2+ were firstly analysed in terms of apparent protonation constants, log K H*, using the “three parameter model” proposed by Högfeldt; 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 ( α) and on PAA molecular weight is discussed using empirical equations. The formation constant, log β 2, is significantly higher for Ca 2+ than for Mg 2+: at I=0.1 mol l −1 (NaCl), log N=1.8 and α=0.5, log β 2 Ca=4.43 and log β 2 Mg=4.24. The formation of polyacrylate–alkaline earth metal complexes is discussed in the light of sequestering effects in natural waters.