When the degree of deacetylation of chitin reaches about 50% (depending on the origin of the polymer), it becomes soluble in aqueous acidic media and is called chitosan. The solubilization occurs by protonation of the –NH2 function on the C-2 position of the d-glucosamine repeat unit, whereby the polysaccharide is converted to a polyelectrolyte in acidic media. Chitosan is the only pseudonatural cationic polymer and thus, it finds many applications that follow from its unique character (flocculants for protein recovery, depollution, etc.). Being soluble in aqueous solutions, it is largely used in different applications as solutions, gels, or films and fibers. The first step in characterizing chitosan is to purify the sample: it is dissolved in excess acid and filtered on porous membranes (with different pore diameters down to 0.45 μm). Adjusting the pH of the solution to ca. 7.5 by adding NaOH or NH4OH causes flocculation due to deprotonation and the insolubility of the polymer at neutral pH. The polymer is then washed with water and dried.