Abstract
Carboxylic (CA), phenolic (PhA) and total (TA) acidity contents of five International Humic Substances Society (IHSS) standards and four model compounds were determined via the potentiometric titration method. Titration curves were scrutinized both by the BEST7 algorithm and the modified Henderson-Hasselbalch model (MHHM). In the case of IHSS samples, the fitting data depended on the analytical procedure undertaken. Whilst high CA and low PhA were usually recorded using the MHHM, the opposite trend was observed employing the BEST7 algorithm. In contrast, in the case of model compounds the acidity values matched well with theoretical data regardless of the procedure. In order to better understand the reasons for such discrepancies changes in the titrations procedure (e.g.: fast or slow) were also considered. General data strongly suggest that acidity determination of humic substances (HS) by potentiometric methods is extremely dependent on both the choice of mathematical model to fit experimental data points as well as the experimental conditions employed.
Highlights
Interaction between trace metals and natural organic matter (OM) is of great importance for nutritional, transport and sedimentary processes of the latter
A careful analysis of these results straightforwardly reveals that potentiometric acidity quantification in humic substances (HS) can be strongly influenced by data treatment procedure
In the case of phenolic acidity (PhA) values, fairly consistent results were observed among the fulvic acid (FA) (Table 1, entries 1 and 2), whereas for humic acid (HA) (Table 1, entries 3-5) the values generated by the BEST7 program differed deeply from those obtained by the modified Henderson-Hasselbalch model (MHHM)
Summary
Interaction between trace metals and natural organic matter (OM) is of great importance for nutritional, transport and sedimentary processes of the latter. We provided a brief outline of the method, which consists in fitting the titration data for two classes of proton-binding sites (carboxylic groups and phenolic groups) in order to extract the parameters that satisfactorily describe the curves To this end, the total charge (QTOT) in solution originated from organic compounds is calculated at each titration point on the basis of the equation (equation 1) electroneutrality and taking into account the dilutioncorrected concentrations of all species in solution, assuming that IHSS samples consist in a mixture of monoprotic acids. The acidity of the IHSS standards was evaluated by the indirect titration method formerly devised by Schnitzer and Gupta.[10] Briefly, ion exchange reactions involving HS and either calcium acetate or barium hydroxide are performed in order to determine the carboxylic acidity (CA) and the total acidity (TA), respectively The difference between these two values is ascribed to phenolic acidity (PhA).
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