Analytical Sciences

Spectroscopic and Potentiometric Studies on Derivatized Natural Humic Acid

Tatjana ANDJELKOVIC,* Jelica PEROVIC,* Milovan PURENOVIC,* Srdjan BLAGOJEVIC,** Ruzica NIKOLIC,* Darko ANDJELKOVIC,*** and Aleksandar BOJIC*

*Department of Chemistry, Faculty of Natural Sciences, University of Nis, Visegradska 33, 18000 Nis, Serbia
**Faculty of Agriculture, University of Belgrade, Nemanjina 16, 11081 Belgrade, Serbia
***Water Work Association, “Naissus”, Kneginje Ljubice 1/1, 18000 Nis, Serbia

Isolated soil humic acid (HA) and commercial Aldrich HA were derivatized by esterification with methanol-thionyl and acetylation with acetic anhidride, in order to obtain derivatives with selectively blocked carboxyl and phenol groups, respectively. Results obtained by FT-IR spectroscopy and potentiometry show that the methanol-thionyl procedure is a selective, specific and efficient route for blocking carboxyl groups. The good correlation between results obtained by direct potentiometry after HA esterification and by classical calcium-acetate and baryta exchange methods suggests that esterification followed by direct acid-base potentiometric titration can be used as a method for the estimation of carboxyl and phenol group contents. Phenol groups can not be specifically identified by the acetylation method, due to the low selectivity of the acetylation method. The average values of apparent and intrinsic pK of underivatized and derivatized HAs confirm decrease in ionizable groups content due to derivatization and their values are related to the different chemical structures of the acids.

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