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

Abstract

AbstractEighteen 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) 13C nuclear magnetic resonance (NMR). The main objectives of this investigation were: (i) to compare the solution and solid‐state 13C NMR spectra of the HA's; (ii) to examine effects of different delay times (D1) on the solution spectra; and (iii) to find reasons for the failure of 13C 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 13C NMR at D1 = 0.74 s were greater than those obtained at D1 = 1.90 s and by CPMAS 13C NMR (where D1 = relaxation delay). By contrast, proportions of aromatic C and aromaticities were greatest at D1 = 1.90 s. The highest values for CO2H groups were obtained by CPMAS 13C NMR and approached total acidities; values measured by solution 13C NMR at D1 = 1.90 s were intermediate, while those determined by solution 13C NMR with D1 = 0.74 s were the lowest and closest to CO2H 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 13C NMR spectra of HA's, signals arising from phenolic C are shifted downfield so that they overlap with resonances of C in CO2H groups. The net result is that CO2H groups are overestimated and phenolic OH groups underestimated.