Determination of oxalate in black liquor by headspace gas chromatography

Abstract

This study demonstrated a headspace gas chromatographic method (HS-GC) for the determination of oxalate content in black liquor (alkaline aqueous solution of inorganic chemicals and dissolved wood species from the alkaline pulping of wood). The method described in this paper is based on the reaction between oxalic and manganese dioxide in an acidic medium, in which oxalic acid is converted to carbon dioxide that is measured with a GC using a thermal conductivity detector. The challenge in developing this method was ensuring complete conversion of oxalic acid while minimizing the contribution of side reactions between carbohydrates, lignin and manganese dioxide to the carbon dioxide measured. It was found that a complete conversion of oxalate to carbon dioxide can be achieved within 3 min at a temperature of 70 °C; a MnO 2: C oxalate (concentration of H 2C 2O 4 + HC 2O 4 − + C 2O 4 2−) mole ratio of 60 and H 2SO 4 concentration of 0.005–0.01 mol/L in the headspace vial. The method can detect concentrations as low as 0.39 μg of oxalate. The standard deviation was found to be 7% while recovery experiments with black liquor showed recoveries of 93–108% which were deemed acceptable for analysis of oxalate in an industrial sample such as black liquor.