Analysis of carboxyl groups in soil humic acids by a wet chemical method, Fourier-transform infrared spectrophotometry, and solution-state carbon-13 nuclear magnetic resonance. A comparative study.

摘要

Carboxyls are important functional groups that affect polarity and reactivity in humic acids (HAs). Carboxyls were analysed in eight soil HAs by three methods based on widely differing principles: (i) wet chemical analysis, (ii) Fourier-Transform Infrared spectrophotometry (FT-IR), and (iii) liquid-state 13C Nuclear Magnetic Resonance (13C NMR). The objective was to find the suitability of each of these methods for the analysis of COOH groups in HAs and the extent to which the three methods agreedwith each other in quantitative measurements of COOH groups in HAs. In regard to reaction mechanisms, the chemical Ca-acetate method is based on ion-exchange of H of COOH for Ca of Ca-acetate. From FT-IR spectra of HAs, COOH groups were determined by totalling absorbances at 1720-1710/cm (COOH) and 1620-1600/cm (COO-), whereas from 13C NMR spectra of HAs, COOH groups were computed by integration of the 175-185 ppm area. Good correlations were found between the three methods although the COOH values computed by 13C NMR were higher, as a result of the inclusion of small amounts of esters, amides, and lactones, than those obtained by the other two methods.