Carboxyl group determinations heparin

Carboxyl and Sulfate Group Determinations. Conductimetric titrations of heparin (14) provided a convenient measure of sulfate and carboxylic groups. Sodium heparin was first converted into the acid form by elution of a known quantity of heparin, dissolved in neutral deionized distilled water, through a column of previously washed Amberlite IR-120 (H+) or bio-Rad AG 50W-X2 resin (i.e., 50-100 mg of heparin through a 7-mL column and diluted to a final volume of 150 mL to give a final concentration of 0.33 to 0.67 mg/mL heparin). The effluent was collected until onlv neutral water exited the column, and this then was diluted to a final volume of 150 mL. 

The titration diagrams of figs. 5 and 6 allow to determine the a parent pK value of the carboxyl group of the iduronic acid. Lowering the concentration of the polymer from 20% to 1% produced an increase of the pK from 3.93 to 5.03. This effect is to be ascribed to the polyelectrolyte character of the heparin, and it must be considered when one makes pK measurements on this kind of polymers. 

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