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Calcium acetate and sodium hydroxide solution with different concentrations mixed with three demineralized low rank bituminous coals were boiled respectively in order to ion exchange with carboxyl and total acids functional groups. The titration end of carboxyl and total acids functional groups were controlled by the pH of calcium acetate solution and phenolphthalein indicator respectively. It showed that correct carboxyl content was obtained at 0.15 mol/L calcium acetate and 2 h boiling time; total acids functional groups were obtained at 0.25 mol/L sodium hydroxide and 1 h boiling time. The carboxyl content was lower than 2 mmol/g and the phenolic hydroxyl content was between 2 and 4 mmol/g; the oxygen in total acids functional groups was more than half in three low rank bituminous coals (dry ashes free basis). An accurate method for measuring acidic functional groups in low rank bituminous coals was obtained. [ABSTRACT FROM AUTHOR]