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 properties of these acids are not well described by extrapolation from small (N = 1 or 2) or large (N > 100) acids. The intrinsic acidity constant, pKint, varies with N, reaching a minimum in the intermediate size range (N = 4 to 14). The site interaction parameter Φ contains a large non-electrostatic term for the smaller acids, even though site heterogeneity 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 groups.