title: Spectroscopic and potentiometric studies on derivatized natural humic acid.
abstract: 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.
authors.affiliation: Department of Chemistry, Faculty of Natural Sciences, University of Nis, Serbia.
authors.forename: Tatjana
authors.initials: T
authors.lastname: Andjelkovic
authors.forename: Jelica
authors.initials: J
authors.lastname: Perovic
authors.forename: Milovan
authors.initials: M
authors.lastname: Purenovic
authors.forename: Srdjan
authors.initials: S
authors.lastname: Blagojevic
authors.forename: Ruzica
authors.initials: R
authors.lastname: Nikolic
authors.forename: Darko
authors.initials: D
authors.lastname: Andjelkovic
authors.forename: Aleksandar
authors.initials: A
authors.lastname: Bojic
mesh_terms: D002264:Carboxylic Acids / Q000032:analysis; D004951:Esterification; D006812:Humic Substances / Q000032:analysis*; D015394:Molecular Structure; D010636:Phenols / Q000032:analysis; D011199:Potentiometry; D012987:Soil; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D002264:Carboxylic Acids; D006812:Humic Substances; D010636:Phenols; D012987:Soil