Analysis of Humic Acids by Solution and Solid‐state Carbon‐13 Nuclear Magnetic Resonance

Eighteen humic acids (HA's) were characterized by chemical analyses, infrared (IR) and electron spin resonance spectra, and by solution and solid‐state cross polarization magic‐angle spinning (CPMAS) 13 C nuclear magnetic resonance (NMR). The main objectives of this investigation were: (i) to compare the solution and solid‐state 13 C NMR spectra of the HA's; (ii) to examine effects of different delay times (D 1 ) on the solution spectra; and (iii) to find reasons for the failure of 13 C NMR to detect the presence of significant concentrations of phenolic C in unmethylated HA's. Compared to solution spectra, CPMAS spectra consisted of relatively few, broad bands, indicative of considerable overlapping of individual signals, especially in the aliphatic regions. Proportions of aliphatic C determined by solution 13 C NMR at D 1 = 0.74 s were greater than those obtained at D 1 = 1.90 s and by CPMAS 13 C NMR (where D 1 = relaxation delay). By contrast, proportions of aromatic C and aromaticities were greatest at D 1 = 1.90 s. The highest values for CO 2 H groups were obtained by CPMAS 13 C NMR and approached total acidities; values measured by solution 13 C NMR at D 1 = 1.90 s were intermediate, while those determined by solution 13 C NMR with D 1 = 0.74 s were the lowest and closest to CO 2 H concentrations determined chemically. Because the presence in HA's of significant concentrations of phenolic OH groups has been confirmed by several independent methods, it is possible that in 13 C NMR spectra of HA's, signals arising from phenolic C are shifted downfield so that they overlap with resonances of C in CO 2 H groups. The net result is that CO 2 H groups are overestimated and phenolic OH groups underestimated.