The importance of a precise characterization of the polymeric material for pharmaceutical applications is exemplified by an adsorption study of the system carrageenan/amitriptyline/water performed by means of dialysis equilibrium measurements. It is shown how differences in charge density and chemical composition lead to a wide variation in adsorption and phase separation properties. Simple physico-chemical methods are suggested that provide a both rapid and precise characterization of polyelectrolyte materials. In particular, charge density, structure of the monomeric unit and accurate determination of the Donnan equilibrium are discussed.