An acidic-groups detection method and its application to analysis of Chinese humic acid samples

by Pengfei Xu, Yuhao Wang, Xue Li, Qingshuang Chen, Lujiang Hao, Jie Zhang, Xiaoling Zhu, Baolei Jia

The method of non-aqueous conductivity titration (NACT) of organic weak acids was applied to quickly and accurately determine the phenolic-hydroxyl and carboxyl-groups contents in humic acid. By varying the pH of the humic-acid sample, the concentration of the titrant, and the nitrogen-gas flow rate, the optimal titration conditions were determined to be a sample pH of 4, titrant concentration of 0.05 mol/L, and nitrogen-gas flow rate of 80 mL/min. Applying the detection method to p-hydroxybenzoic acid showed that its phenolic-hydroxyl content was 758.82±111.76 cmol/kg and carboxyl content was 744.44±51.11 cmol/kg. The theoretical phenolic-hydroxyl and carboxyl-groups contents of the p-hydroxybenzoic acid were 723.96 cmol/kg respectively, indicating that the method can accurately quantify the carboxyl and phenolic-hydroxyl groups in the sample. The NACT was used to measure the phenolic-hydroxyl and carboxyl-groups contents in humic acid quickly and accurately. In addition, 29 humic acid samples from 8 provinces of China covering the main humic-acid producing areas were collected and analyzed for acidic-groups content using the reported method.