Application of potentiometry to characterize acid and basic sites in humic substances
| Autor: | Andreia Fernandes, Bruno Szpoganicz, Karine Arend, Marcelo Giovanela, Maria Marta de Souza Sierra |
|---|---|
| Rok vydání: | 2001 |
| Předmět: |
chemistry.chemical_classification
Catechol Titration curve Chemistry Stereochemistry Inorganic chemistry Acid–base homeostasis Biochemistry Acid dissociation constant Analytical Chemistry Phthalic acid chemistry.chemical_compound Deprotonation Electroanalytical method Environmental Chemistry Humic acid Spectroscopy |
| Zdroj: | Analytica Chimica Acta. 445:89-98 |
| ISSN: | 0003-2670 |
| DOI: | 10.1016/s0003-2670(01)01251-x |
| Popis: | Acid–base properties of seven fulvic (FA) and two humic acids (HA) from mangrove 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 benzene-carboxylic acids (3,5-dihydroxybenzoic acid; 2,4-dihydroxybenzoic acid and phthalic acid) presenting pKa values from 2.77 to 10.54 was studied first. The acidity contents as well as the pKa 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 pKa 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 pKa values of phenolic groups ranged from 9.09 to 9.85, the average being near to the pKa of the first catechol deprotonation (9.30). Distribution of HS species as a function of pH are also shown. |
| Databáze: | OpenAIRE |
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