The Isoelectric Point of Gelatin in Relation to its “Minimum Physical Properties.”

Abstract

Most investigators are of the opinion that the physical properties of gelatin (viscosity, conductivity, osmotic pressure) are at a minimum at its isoelectric point and that its opacity is a maximum at this point. This erroneous conception is partly due to the notion that ash-free gelatin is necessarily isoelectric. Gelatin, whose high degree of purity is indicated by an exceptionally low conductivity when it is free from diffusible electrolytes is not isoelectric. Cataphoresis experiments show that purified gelatin derived from lime conditioned stock migrates to the anode in an electrical field, while gelatin from acid conditioned stock migrates toward the cathode. The former must therefore be a weak acid while the latter is a weak base. The physical properties of gelatin are at a minimum in pure gelatin and not in isoelectric gelatin, which is a salt of gelatin and more highly ionized than pure gelatin.

Pure gelatin from any source should have the formula HONH₃ × COOH (where × represents the main gelatin radical and the other groups are representative of its acid and basic groups) and should accordingly furnish the ions HO^- NH₃ ⁺ × COOH⁺. In the case of gelatin, which is a weak acid, the number of ionized groups furnishing H⁺ would be greater and in the case of gelatin which is a weak base the number of groups furnishing OH^- ions would be greater, so that the 2 types of ionized gelatin may be represented by (HONH₃ × COO)^-H⁺ and HO^- (NH₃ × COOH)^- respectively.

It can be seen readily that the addition of an acid (HCl) to the type of gelatin (HONH₃ × COO)^- H⁺ will partly suppress its ionization and react with some of the OH groups of the molecule to form a chloride of gelatin ClNH₃ × COOH which would dissociate to furnish the more highly ionized Cl^- NH₃ ⁺ × COO^- H⁺, just as the addition of HCl to NH₄OH forms the more highly ionized NH₄Cl.