The 13 th Symposium on Analytical and Environmental Problems, Szeged, 25 September 2006 290 NEW METHOD FOR pH-DEPENDENT CHARGE DETERMINATION OF HUMIC ACIDS AND ITS POTENTIAL USE IN DOM ANALYSIS Erzsébet Illés, Etelka Tombácz Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Hungary e-mail: illese@chem.u-szeged.hu ABSTRACT Humic acid (HA) was obtained from brown coal. The pH-dependent dissociation of the purified sample was determined by potentiometric acid-base titration at different ionic strengths. Dilute HA solutions containing 0.005, 0.025 and 0.050 M NaCl were titrated with cetil-trimethyl-ammonium-bromide (CTABr) at pH~4, ~7, and ~9, respectively. The electromotive force (EMF) vs. specific surfactant amount (mmol CTABr/1 g of HA) functions measured at different NaCl concentrations showed a common intersection point (CIP) at each pH. The specific amounts of CTA + cations at all three CIP values could be identified with the amounts of dissociated functional groups on polyanionic HA, and so a quantitative determination of negative charges belonging to the macromolecular organic matter in dilute solutions seemed to be possible. To check the effect of small molecular dissolved organic matter (DOM) occurring in aquatic environments, comparable amounts of salicylic acid, glycine, oxalic acid were added to the HA sample and the mixtures were titrated at pH~9. The CIP appears and its position does not change in the presence of the organic molecules. INTRODUCTION Non-volatile organic acids dominate the dissolved organic matter (DOM) content in most aquatic environments. Macromolecular humic and fulvic acids are the major DOM constituents besides the chemically identified small molecular compounds [1]. There are a large variety of physical, chemical, and spectroscopic techniques to characterize these natural organic acids, most of them yield “average” information on DOM composition instead of specific information, especially on the probable chemical bounding with the functional groups of pollutants or with metal ions [2-4]. Determination of the in situ existing reactive sites such as charged functional groups of dissolved humic substances (HSs) is problematic and has not implemented yet, independently of numerous results based on mainly potentiometric acid- base titration. Since DOM contains not only humic substances, but also smaller organic molecules (e.g. oxalic acid), their presence probably disturbs the quantitative analysis of acidic functional groups bound to the aromatic carbon matrix of HSs. The dissociated acidic groups of humic acids are able to form ion-pair associates with the cationic surfactants in quantitative way [5-8]. It is highly probable that the pH-dependent charge state of humic acids, and more the HS’s content of the DOM under appropriate conditions can be determined through ion-pair formation with surfactant cations even in the presence of small organic molecules. We tested this potential analytical method by means of potentiometric titration using ionic surfactant selective electrode, and some preliminary results will be shown here.