Abstract
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.
Highlights
Humic substances are complex and heterogeneous mixtures of polydispersed materials formed in coal, soils, sediments, and natural waters by biochemical and chemical reactions during the decay and transformation of plant and microbial remains [1]
The phenolic-hydroxyl and carboxyl-groups contents of p-hydroxybenzoic acid were found to be 758.82±111.76 cmol/kg and 744.44±51.11 cmol/kg. These results indicated that the non-aqueous conductivity titration (NACT) could be used quickly and accurately to measure the phenolic-hydroxyl and carboxyl-groups contents in p-hydroxybenzoic acid after optimizing the conditions
For quick and effective determination of the phenolic-hydroxyl and carboxyl-groups contents in humic acid, the measurement of the conductivity curve of humic-acid samples prepared at pH 4 was found to be the most accurate approach
Summary
Humic substances are complex and heterogeneous mixtures of polydispersed materials formed in coal, soils, sediments, and natural waters by biochemical and chemical reactions during the decay and transformation of plant and microbial remains (a process called humification) [1]. Together with fulvic acid and humins are the major component of humic substances. As the acid-insoluble fractions of humic substances, Humic acid is partially soluble in water and form micelle-like structures in neutral to acidic conditions [2]. Humic acid is polymer compound rich in active groups such as phenolic-hydroxyl, carboxyl, sulfhydryl, and carbonyl [3, 4] and is widely used in agriculture, ceramics, oil extraction, and environmental.
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