Analytica Chimica Acta 445 (2001) 89–98 Application of potentiometry to characterize acid and basic sites in humic substances Testing the BEST7 program with a weak-acid mixture Maria Marta de Souza Sierra ∗ , Karine Arend, Andreia Neves Fernandes, Marcelo Giovanela, Bruno Szpoganicz Departamento de Qu´ ımica, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil Received 13 April 2001; received in revised form 30 May 2001; accepted 25 June 2001 Abstract Acid–base properties of seven fulvic (FA) and two humic acids (HA) from mangrove sediments were investigated by poten- tiometry using the BEST7 program to resolve the titration curves. With the aim of testing the applicability of this software to hu- mic substances (HS), a mixture containing three benzene-carboxylic acids (3,5-dihydroxybenzoic acid; 2,4-dihydroxybenzoic acid and phthalic acid) presenting pK a values from 2.77 to 10.54 was studied first. 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 data, attesting the validity of the application. When the same methodology was applied to HS it was possible to detect two sites, the first being attributed to carboxylic and the second to phenolic groups. Acidity values were very uniform for the set of samples studied and they were within the range usually shown in the literature for HS from distinct environments. Carboxylic acidity ranged from 1.83 to 3.23 meq g -1 and was lower than phenolic acidity (3.23–5.18 meq g -1 ), both for FA and HA. Such an inverted trend relative to results normally reported is discussed in terms of the origin of the samples as well as of the classical wet chemical methodologies customarily employed to assess HS acid–base properties. The pK a values as an average 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 electronic density of these molecules compared to FA. The pK a values of phenolic groups 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 function of pH are also shown. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Acid–base properties; Humic substances; Potentiometry; BEST7 program 1. Introduction Natural organic compounds can complex trace met- als and consequently influence their migration through continental and marine systems. Because of this there exists a great interest in measuring the concentrations of these complexing agents and in characterizing their ∗ Corresponding author. E-mail address: sierra@qmc.ufsc.br (M.M. de Souza Sierra). thermodynamic affinity for metallic ions. The most im- portant naturally-occurring organic ligands are those conventionally termed humic substances (HS) whose major components are humic (HA) and fulvic acids (FA). Humic substances are a mixture of weak-acid poly- electrolytes presenting various molar mass moieties and distinct functional groups [1,2]. The difficulties associated with quantifying trace elements taken up by HS are widely recognized, the main limitation being 0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0003-2670(01)01251-X