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To investigate acidity with pKa of water soluble chitosans, low molecular weight chitosans were prepared in mild condition by NaNO2 hydrolysis. The pKas of the water soluble chitosans (MW 3,600, pKa : 6.8, and MW : 10,000, pKa : 6.5) were slightly increased dependent on molecular weight because of decreased acidity of amino group on the chitosans compared with water insoluble chitosan (MW : 50,000, pKa : 6.2). It was suggested that chitosan of low molecular weight having lower reactivity against counter ions and biological substances than water insoluble chitosan. Furthermore, various sulfated chitosan derivatives (N-3,6-O-S-chitosan , N-desulfated 3,6-O-S-chitosan ,3,6-O-S- chitin , and 3,6-O- sulfated -N- ( o-carboxybenzoyl ) chitosan )were synthesized with sulfurtrioxide-pyridene complex in pyridine solvent. Characterization of the sulfated chitosan derivatives was carried out by 13C NMR and IR spectroscopies. To observe ionic reaction properties, pKas of the sulfated chitosan derivatives and chitosan of low molecular weight were estimated by potentiometric titration. The sulfated chitosan derivatives had high water solubility, pKas (pKa : 7.7) of N-3,6-O-S-chitosan and N-desulfated 3,6-O-S-chitosan were increased than pKa of water insoluble chitosan (pKa: 6.2). These results suggest the participation of electrostatic interaction of amino and sulfate groups on the sulfated chitosans.
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