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Buffer Curves and Acid Strength of Humic Acid and Two Resins¹

ABSTRACT

Two humic acids, Cellex-CM, and IRC-50 were placed in 10^-1, 10^-2, and 10^-3_M NaCl or CaCl₂ and titrated potentiometrically with NaOH or Ca(OH)₂, respectively. Titration curves of Cellex-CM and IRC-50 in the NaOH-NaCl system showed parallel 1 pH unit displacements for a 10-fold change in NaCl concentration, while humic acid showed a 0.5 pH unit displacement. Buffer curves of humic acid, Cellex-CM, and IRC-50 in the Ca(OH)₂-CaCl₂ system showed a 0.4 pH unit displacement for a 10-fold change in CaCl₂ concentration. The regular displacement of buffer curves led to development of a theory using ion exchange and surface ionization of H that described the buffer curves and assigned a salt independent pK_a to the weak acid adsorbents.

¹ This investigation was supported by NIH Training Grant no. ES 0084 from the National Institute of Environmental Health Sciences, Research Triangle Park, N.C.

² Graduate Student and Professor of Soils and Plant Nutrition, respectively, Dept. of Soil Science & Agr. Eng., Univ. of California, Riverside. The senior author is now Assistant Professor of Soil Science, Univ. of Wisconsin, Madison 53706.

Received for publication September 30, 1970. Accepted for publication May 12, 1971.