Humic substances' proton-binding equilibria: Assessment of errors and limitations of potentiometric data

Abstract

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 coef®cients, are also shown to lead to large variations in the calculated carboxyl contents and charge±pH curves.