Surface Characteristics of Kaolinitic Clays and Clay-Humic Acid Complexes by Potentiometric and Conductometric Titrations

N. C. Lockhart
CSIRO Institute of Earth Resources, Physical Technology Unit, 338 Blaxland Road Ryde, New South Wales, 2112, Australia

Abstract: Acid and non-acid forms of kaolinite clays were found by potentiometric and conductometric titrations to possess a weak acid species, the concentration of which was increased by acidic preparation conditions and by the presence of soil organic matter bound to the clay. A stronger acid species was also found in untreated and in organic-free, HCl-treated samples (washed free of excess acid). The end points for organic-free and/or ion-exchanged samples were used to predict the end points for untreated samples. A surface-chemical model involving MOH2+, MOH, and MO species (where M = Al and/or Fe) was fitted to the titration data and to the cation-exchange capacities. MOH2+ is the stronger acid and MOH the weaker acid; exchange of Na+ counterions by H+ converts MONa+ sites to MOH sites. Humic acid was probably complexed or chelated to some hydroxylated Al/Fe sites through COOH and phenolic OH groups. The COOH groups contributed supplementary stronger acid species which were not exchangeable, whereas the OH groups contributed supplementary weaker acid species and also increased the CEC relative to that for organic-free samples.

Key Words: Conductometric titration • Humic acid • Kaolinite • Potentiometric titration • Surface properties

Clays and Clay Minerals; December 1981 v. 29; no. 6; p. 413-422; DOI: 10.1346/CCMN.1981.0290601
© 1981, The Clay Minerals Society
Clay Minerals Society (www.clays.org)