Polysaccharides with Strongly Acidic Groups

Polysaccharides with strongly acidic residues, present as esters along the polymer chains (sulfuric, phosphoric acids, as in furcellaran, carrageenan or modified starch), are also very soluble in water and form highly viscous solutions. Unlike polysaccharides with carboxyl groups, in strongly acidic media these solutions are distinctly stable. 

Acidic chlorate oxidations are also carried out with a small amount of chloride, and according to the reaction conditions, different equilibria predominate.37 With these reaction systems, the oxidizing oxyhalogen species, formed in situ, promotes the oxidation of the primary alcohol groups of polysaccharides to carboxylic acids.36 Thus, the stoichiometry of the overall oxidation in strongly acidic medium (85% H3P04) is given by Eq. (24). 

The chemistry of these polysaccharides is dominated by partial esterification of the total number of carboxyl groups. The sequence of uronic acids in the primary structure is occasionally interrupted by rhamnose, and there is usually a trace of acetyl and phenolic substituents. Models developed from experimental data considered neutral side chains with DP = 2-10 (De Vries et al., 1982). These 1,4-a-linked linear uronans are susceptible to alkali—more so if the C-6 hydroxyl is esterified, but exceptionally acid-stable when this site is unsubstituted. Dispersion stability is less at higher DP. Uronans are endowed with strong dye-fixing and mineral-sequestering properties because of their charged surface. 

Humic adds. Chocolate brown, dust-like powder, poorly soluble in water with a large increase in volume, soluble in aqueous alkalis to give brown solutions, and in cone, nitric acid to give red solutions. H. forms a heteropolycondensate with Mr of 2000-500000 (mostly 20000-50000), mp. >300°C. They consist of a polycyclic core and loosely bound polysaccharides, proteins, simple phenols, and chelated metal ions which are linked to the core through carboxyl and carbonyl groups . The latter usually have aromatic character. The H. are strongly acidic (hydroxy- and polyhydroxycarboxylic acids) and mostly occur as salts. For analysis, see Lit.. ... 

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