The protonation constant of the NH2 function was determined by the method of Katchalsky and Spitnik and by the SUPERQUAD fitting procedure. Samples with higher concentrations of chitosan indicated aggregations of polymer chains, which led to a loss in the effective concentration of the ligand (L). It followed, as a result of potentiometric titrations, that an excess of L of microcrystalline chitosan (MCCh) with a deacetylation degree of 0.90 was a complexing agent toward the metal (M), which was Ni(II) or Mn(II). Species ML and ML2 were accepted by SUPERQUAD for both of the M's, where coordination occurred via the amino nitrogen. For Ni(II), however, the hydroxyl oxygen may also have been an electron-pair donor at lower excesses of MCCh and, by that, made possible the formation of five-membered chelate rings in the hydroxyl deprotonated MLH-1, species. The evaluated formation constants were compared with the values known until now for monomeric D-glucosamine. Additional confirmation of the M-L interaction was determined by the spectrophotometric titration of a Ni(II)-MCCh solution. (c) 2005 Wiley Periodicals, Inc.