Determination of acidic functional groups in low-rank coals: comparison of ion-exchange and non-aqueous titration methods

Abstract

Acidic functional groups in five low-rank coals from various parts of the world have been determined by aqueous ion exchange and non-aqueous titration procedures. The total acidity found by titration was, in general, significantly higher than that obtained by ion exchange at pH 13 for all the coals. The strongest acidity determined by titration was also higher than the carboxyl group contents obtained by ion exchange at pH 8.25 in each case. The weaker acidity determined by titration was substantially greater than the equivalent of the phenolic group content deduced from difference in ion exchange at pH 13 and pH 8.25 for two of the coals, but was in reasonable agreement for the remaining three coals. These discrepancies are believed to arise because of the swelling of the coals and enhancement of acidic strength of phenolic groups in ethylenediamine used during the titration procedure. These effects may cause some stronger phenolic groups to titrate with the carboxyl groups and also allow the titration procedure to determine weaker and less accessible phenolic groups than the aqueous-exchange method.