Uronic acids, associated with cellulose

Hemicellulose refers to a chemically ill-defined group of heteropolysaccharides of plant cell-walls that are closely associated with cellulose. They are composed of different hexoses and pentoses, and uronic acids. The hemicelluloses may be separated into two fractions, A and B however, there is no clear demarcation of structure between the two fractions. Hemicellulose B usually contains a higher proportion of uronic acid, chiefly 4-0-methyl-D-glucuronic acid, than the A fraction. The methyl ethers of D-aldobiouronic acids containing D-glucuronic acid, D-galacturonic acid, or D-mannuronic acid are isolated chiefly as the 4-O-methylaldobiouronic acids, and are hydrolyzed with difficulty by acid. 

Kenyon and associates have compared the uronic acid values obtained by the above method with the carboxyl values obtained by the calcium acetate method. " In the latter procedure, a calcium acetate solution is added to cellulose which contains free carboxyl groups. A replacement reaction takes place, and the acetic acid liberated is titrated with sodium... 

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