Protonation reactions of fulvic acids

The protonation reactions of fulvic acids are described in terms of a model involving a mixture of n monoprotic acids HA, HB, HC … pH titration data were analysed by nonlinear least squares methods, using a model having n independent 'titration' (protonation) constants, K_i i= 1 to n , and n– 1 independent acid concentrations. The best fit was obtained with n =4. The results for three fulvic acid samples were in the range log K₁ 6.5–6.7 (～8% of total carboxyl acidity), log K₂ 5.3–5.6 (～15%), log K₃ 3.9–4.8 (～20–30%) and log K₄ 2.6–2.7 (～50–70%).
The results indicate a comparatively small proportion (～ 10–20%) of polyprotic carboxyl moieties in the diverse fulvic acid structures. They give some support to the Wershaw model of fulvic acids as strongly associated aggregates of acids, each of comparatively low molecular weight.