Titration Studies on the Polynuclear, Polyacidic Nature of Fulvic Acid Extracted from Sewage Sludge‐soil Mixtures

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Abstract
Aqueous solutions of two fulvic acids extracted from sludge‐soil mixtures were studied by potentiometric titration with either KOH or HCl in an ionic medium of 0.1 mol/kg KCl. Data were obtained for a range of concentrations of FA at 25°C and for a single concentration at 35°C that duplicated one of those employed at the lower temperature. Formation functions giving the amount of H+ neutralized as a function of the free OH‐ concentration or of the pH were computed for each set of titration data. Analysis of these functions led to the conclusions that: (i) sludge‐derived FA contains functional groups ranging in acidity from very strong (ionized at pH < 2) to very weak (ionized at pH > 10); (ii) the strongly acidic groups are probably sulfonic acid groups and are relatively numerous; (iii) the more weakly acidic groups are probably carboxyls, N‐containing groups, phenolic OH, and SH groups; (iv) the strongly acidic groups can bind H+ from solution (counter‐ion condensation) when they get close to complete neutralization; (v) the H+ binding effect is enhanced by lowering the concentration of FA or by raising the temperature; and (vi) the reversibility of the titration curves is influenced greatly by the presence of the strongly‐acidic functional groups. These conclusions support the conceptualization of sludge‐derived FA as a heterogeneous polynuclear polyacid whose intrinsic acidity will show a pronounced dependence on its concentration and on the temperature. Thus the behavior of this material in soil solutions may be expected to be quite complex.

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