COMPARISON BETWEEN UV SPECTROPHOTOMETRY AND ANION EXCHANGE CHROMATOGRAPHY TECHNIQUES FOR QUANTIFICATION OF HEXENURONIC ACID (HEXA) IN UNBLEACHED KRAFT PULPS OF EUCALYPTUS GLOBULUS

Two different methodologies were optimized for hexenuronic acid (HexA) quantification in E. globulus kraft pulps. In one, pulps were hydrolyzed with mercuric chloride/sodium acetate and quantified by ultraviolet spectrophotometry (UV technique); in the other one, the hydrolysis was performed with sulfuric acid and the degradation products of HexA quantified by anion exchange chromatography (AEC technique). In order to determine the optimal conditions for maximum removal of HexA from pulps by acid hydrolysis, the variables time, pH, and temperature were studied. The influences of these three variables were determined by means of a response surface methodology (RSM). The optimized conditions for the hydrolysis by the UV technique were: 45 mill, pH 5.7 and 80 degrees C. While, for hydrolysis using the AEC technique, the optimized conditions were: 3.1 h, pH 0.7 and 97 degrees C. Both AEC and UV methods presented good reproducibility with 2.1% and 1.4% relative standard deviation (RSD), respectively. However, HexA values measured by the AEC technique method were 11-30% lower than those measured using the UV technique. For additional information, five selected pulps were analyzed by other methods, known as KTH and HUT. The results obtained by the four methods employed showed significant variations in the HexA quantification. The highest HexA values were obtained using the UV technique. The KTH, AEC and HUT methods provided HexA amounts in 7-16%, 17-24% and 24-30% lower than the values obtained by UV method, respectively, for the same set of pulps. The UV technique is more susceptible to be interfered with pulp hydrolysis by products, attributed mainly to lignin solubilization, while the AEC technique allows direct measurement of HexA hydrolysis products without interference from other compounds.