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

Carboxyls are important functional groups that affect polarity and reactivity in humic acids (HAs). Carboxyls were analyzed 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 C-13 Nuclear Magnetic Resonance (C-13 NMR). The objective was to uncover the suitability of each of these methods for the analysis of COOH groups in HAs and the extent to which the three methods agreed with 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 totaling absorbances at 1720-1710 cm(-1) (COOH) and 1620-1600 cm(-1) (COO-), whereas from C-13 NMR spectra of HAs, COOH groups were computed by integration of 175-185 ppm area. Good correlations were found between the three methods although the COOH values computed by C-13 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. Thus, depending on the equipment and facilities available, soil scientists have a choice of methods that can be used for determining COOH groups in HAs.