Application of potentiometry to characterize acid and basic sites in humicsubstances - Testing the BEST7 program with a weak-acid mixture

Acid-base properties of seven fulvic (FA) and two humic acids (HA) from man grove sediments were investigated by potentiometry using the BEST7 program to resolve the titration curves. With the aim of testing the applicability of this software to humic substances (HS), a mixture containing three benze ne-carboxylic acids (3,5-dihydroxybenzoic acid; 2,4-dihydroxybenzoic acid a nd phthalic acid) presenting pK(a) values from 2.77 to 10.54 was studied fi rst. The acidity contents as well as the pK(a) values obtained for each of the three compounds were very close to those of theoretical and reported da ta, attesting the validity of the application. When the same methodology wa s applied to HS it was possible to detect two sites, the first being attrib uted to carboxylic and the second to phenolic groups. Acidity values were v ery uniform for the set of samples studied and they were within the range u sually shown in the literature for HS from distinct environments. Carboxyli c acidity ranged from 1.83 to 3.23 meq g(-1) and was lower than phenolic ac idity (3.23-5.18 meq g(-1)), both for FA and HA. Such an inverted trend rel ative to results normally reported is discussed in terms of the origin of t he samples as well as of the classical wet chemical methodologies customari ly employed to assess HS acid-base properties. The pK(a) values as an avera ge of HS carboxylic groups ranged from 5.51 to 5.99, the higher values, in general, being relative to HA, probably due to the higher surface electroni c density of these molecules compared to FA. The pK(a) values of phenolic g roups ranged from 9.09 to 9.85, the average being near to the pK(a) of the first catechol deprotonation (9.30). Distribution of HS species as a functi on of pH are also shown. (C) 2001 Elsevier Science B.V. All rights reserved .