Colloidal titration of chitosan and critical unit of chitosan to the potentiometric colloidal titration with poly(vinyl sulfate) using toluidine blue as indicator

Three sorts of characterized chitosan were measured by visual titration and potentiometric titration with poly(vinyl sulfate) using Toluidine Blue as the indicator, to evaluate the stoichiometry and the accuracy. The measurement values for two dialyzed chitosans by both colloidal titration methods agreed with the values obtained by the acid-base titration. The relative differences between the acid-base titration and the colloidal titration were < 2%, and the relative standard deviation of the potentiometric colloidal titration was < 1%. For a oligo-chitosan (ChO) which consisted of several D-glucosamine oligosaccharides, the value obtained by the potentiometric colloidal titration was much smaller than the value from the acid-base titration. The inadequate result was due to the low stability constant of ChO-PVS. The observed values in the colloidal titration of ChO were evaluated for the content of each D-glucosamine oligosaccharide, and a critical unit that indicated the least unit of D-glucosamine oligosaccharide that contributed to the measurement value was estimated. The critical unit was four, tetra(D-glucosamine saccharide).