Humic substancesʹ proton-binding equilibria: assessment of errors and limitations of potentiometric data Original Research Article

The variability of experimental procedures referred to in the literature for the potentiometric acid–base titrations of humic substances was analysed and the corresponding implications on the titration data were studied. Hysteresis phenomena were observed in the titrations of all the studied samples: two samples of fulvic acids isolated from two different points of river Vouga (Portugal), one XAD-4 fraction isolated from one of those points and the Swannee river reference fulvic acid. Due to those phenomena, carboxyl contents and charge–pH curves calculated from direct and back-titrations can be considerably different. Other sources of variability, such as changes of the moisture content of samples, different calibration procedures or different equations for the calculation of activity coefficients, are also shown to lead to large variations in the calculated carboxyl contents and charge–pH curves.