Potentiometric acidity determination in humic substances influenced by different analytical procedures

Abstract

Os valores de acidez carboxílica (CA), fenólica (PhA) e total (TA) de um conjunto de cinco amostras padrão da Sociedade Internacional de Substâncias Húmicas (IHSS) e de quatro compostos modelo foram determinados pelo método de titulação potenciométrica. Os dados das curvas de titulação foram tratados pelo programa BEST7 assim como pela equação de Henderson-Hasselbalch modificada (MHHM) e os parâmetros obtidos mostraram-se extremamente dependentes do modelo matemático aplicado. Para as amostras de substâncias húmicas (SH), a aplicação da MHHM resultou em uma alta CA e uma baixa PhA, sendo o resultado praticamente oposto quando o programa BEST7 foi empregado. No caso dos compostos modelo entretanto, os valores de acidez convergiram aos valores esperados (teóricos) independente do modelo aplicado. Para investigar as razões das discrepâncias, variações na condução das titulações (p.e., titulação rápida ou lenta) também foram consideradas. O conjunto de informações mostrou que a determinação da acidez de SH por titulação potenciométrica é extremamente dependente do modelo matemático, assim como das condições experimentais aplicadas.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.

Pre-treatment of Fulvic Acid Plays a Stimulant Role in Protection of Soybean (Glycine max L.) Leaves Against Heat and Salt Stress

“…1). In agreement with these findings, Fernandes et al (2009) reported that leonardite, which is a derivative of humic acid, induced Ca 2+ ions significantly in olive trees. However, pretreatment with fulvic acid under heat stress did not change Na + and Cl -but decreased Ca 2+ at 6 h. This comparative result shows that fulvic acid induces a different signaling network in response to different stress conditions (salt or heat) that may depend on the requirements of the plants under stress conditions.…”

Combination of a Copper-Ion Selective Electrode and Fluorometric Titration for the Determination of Copper(II) Ion Conditional Stability Constants of Humic Substances

“…Previous studies have proven that acid-base characteristics of HA can be well modeled by the sum of a number of carboxylic and phenolic groups, with phenolic groups showing nearly five orders of magnitude higher mean values of pKa than carboxylic groups on average. 26,27 For example, the diprotic Sal ( H 2 A ), a typical compound containing both carboxylic and phenolic groups and commonly considered as a model compound in HS studies, 5,28,29 has pKa 1 = 3.0 and pKa 2 = 13.7, 5 and thus H 2 A may be neglected as compared with HA at pH values of 6 ∼ 8 which is relevant to natural water pH in most environments.…”

Characterization of pH-fractionated humic acids with respect to their dissociation behaviour

“…A major fraction of organic matter in natural ecosystems is in fact formed by humic acids, which are characterized by the presence of several functional groups with labile protons such as carboxylic acids, phenols and amines. These moieties are able to bind protons and metal ions that not only affect the geochemistry of natural systems, but also regulate the buffer capacity of waters, soils and sediments, and metal speciation and transport (Fernandes et al 2009;Masini et al 1998;Ritchie and Perdue 2003). Complexation of toxic metal ions by humic acids plays a major role in maintaining them in a bioavailable state in environmental waters.…”

“…Since the humic substance is not a single well-defined molecule but a mixture of various heterogeneous particles or aggregates containing a variety of functional groups resulting in polyelectrolyte and polyfunctional properties, it cannot be treated as an ordinary complex-forming ligand in the Responsible editor: Philippe Garrigues * Martina Klučáková klucakova@fch.vutbr.cz interaction with ions. They have a colloidal character whose size and negative charge are strictly dependent on surface functional groups (Angelico et al 2014;Fernandes et al 2009;Kirishima et al 2010;Koopal et al 2001;Tan et al 2011). The acid-base chemistry of humic acids is an important component of metal speciation models in natural systems.…”

“…However, the understanding of physical-chemical processes associated with chelation reactions still remains a challenge, fundamentally due to the dynamic nature and complexity of organic matter (Fernandes et al 2009). Since the anionic charge of humic substances plays a major role in their interactions with other components, pH and ionic strength are the most important variables controlling metal complexation (Liu and Gonzalez 2000;Michalowski and Asuero 2012;Tan et al 2011;Yonebayashi and Hattori 1990), many authors hope that fractionation of humic matter by extractions at different pH values would be useful in reducing their complexity and detail understanding of the interactions of natural organic matter with other components.…”