Solubility and fractionation of humic acid; effect of pH and ionic medium

Gel permeation chromatography (GPC), in conjunction with equilibrium dialysis, was used to probe the fractionation of humic substances on dissolution in several media (KNO3, Na4P2O7, and synthetic sea water) as a function of pH and ionic strength. The solubility of humic acid (and the proportion of "large" molecules in solution) increased with increase in pH and decreased with increase in ionic strength. A pH of at least 8.0 was required to solubilize a "representative" fraction of a humic acid sample (based on GPC). Pyrophosphate (0.001 mol l−1, pH 9) is proposed as a suitable solvent for the extraction of humic acid from soil.