Investigation of surface acidity of coal by aqueous potentiometric titration

Surface acidic functional groups in virgin and variously extracted coal samples were determined by aqueous potentiometric acid-base titration employing KOH as a titrant. The various extractants used in this study were H2O2, HCl, KOH and distilled water. The obtained titration curve of virgin coal showed inflections at pH 4-5, 6-7 and 8-11, which indicate that the coal surface may contain carboxylic, carbonyl, phenolic and other weak acidic groups like enols and C-H. The titration curves of treated coal samples showed a normal break at pH 4-10 range and small inflections at pH 10-11, which did not show a similarity to the inflection of carboxylic acid groups but may be due to the existence of acid groups such as CO2, Phenols, enols and C-H. The aforementioned extractants oxidized (by increasing the peak intensity at 1310-1250 cm(-1) regions due to stretching vibrations of C-O-C in ether and ester) the coal sample and were verified by FT-IR spectral analysis.