COLORIMETRIC DETERMINATION OF HEXURONIC ACIDS IN PLANT MATERIALS

A colorimetric reagent, 3-5-dimethylphenoi, is selective for 5-formyl-2-furancarboxylic acid, a chromogen formed from uronic acids in concentrated H2SO4 at 70 °C. Addition of the reagent at 20 °C produces within 10 min, a chromophore absorbing at 450 nm. Selectivity is critical because of interferences from neutral sugar products and lignin when uronic acids are at 1-3% levels. d-Galacturonic and 4-O-methyl-d-glucuronic acids could be measured separately from d-glucuronic acid, by adding H3BO3. About 12% more chromogen was produced from d-galacturonic and 4-O-methyl-d-glucuronic acids of polymers than from the monomers. Analyses of wood containing 3% uronic anhydride gave 0.5 standard deviation per measurement. The time for dissolution, reaction, and color formation is 30 min for fast reactors, 60 min for glucuronic acid. © 1979, American Chemical Society. All rights reserved.