Utilization of flow-through electrochemical coulometry in study of dissociation ability of humic acids

abstract

English

Humic acids are operationally defined as the fraction of humic substances which is not soluble under acidic conditions. This does not mean, however, that their solid particles easily dissolve in water. Experimental results suggest that the dissolving of solid humic acids in an aqueous environment is more complex than the conventional solubility behaviour of sparingly soluble solids. In this work, the pKa value of the dissolved humic acid fraction is calculated on the basis of results of flow-through electrochemical coulometry in combination with potentiometry. While the potentiometric pH-measurement is used for determination of dissociated functional groups of humic acids, whole amount of dissolved humic acids can be measured using the coulometry. The aim of this work is to characterize the dissociation behaviour of selected humic samples by means of dissociation constants using these methods. It is shown that dissociation degree is influenced by content of dissolved humic acids. The obtained values of pKa are markedly dependent on concentration of humic acids mainly in strongly diluted systems. The values of pKa obtained in this work characterize the dissociation behaviour of humic fractions which are soluble in water and aqueous solutions and therefore they are the active fractions of humic acids in an aqueous environment. The dissociation behaviour of structurally similar humic samples can be different in water (e.g. in river) and in wet soil and a reason can be different content of humic acids in aqueous solution. Results obtained in this work confirmed that flow-through electrochemical coulometry can be used as alternative method for characterization of dissociation ability of humic acids and determination of their pKa values.

humic acids, coulometry, dissociation

KLUČÁKOVÁ, M.

1. 9. 2014

University of Ioannina

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