MOLECULAR-SIZE EFFECTS ON CARBOXYL ACIDITY - IMPLICATIONS FOR HUMIC SUBSTANCES

Models of carboxyl acidity frequently applied to humic substances (HS) are examined by potentiometric titrations of polycarboxylic acids of intermediate size (N = number of acid groups = 3-70). The unique prope rties of these acids are not well described by extrapolation from smal l (N = 1 or 2) or large (N > 100) acids. The intrinsic acidity constan t, pK(int), varies with N, reaching a minimum in the intermediate size range (N = 4 to 14). The site interaction parameter Phi contains a la rge non-electrostatic term for the smaller acids, even though site het erogeneity is minimal. Models of HS acidity must explicitly consider ( 1) the size domain of the humic mixture, rather than relying on small- or large-acid models and (2) the polar effects of uncharged polar gro ups.