Shifts in the apparent ionization constant of the carboxylic acid groups of gelatin.

Titration data of Type B gelatin were used to investigate shifts of the apparent ionization constant of the carboxylic acid groups caused by changes in the degree of ionization or net molecular charge and in ionic strength. The intrinsic ionization constant was estimated by comparison between titrations in water and in 0.10 molal NaCl. This estimated value of the carboxylic acid groups of gelatin was comparable to those reported for the carboxylic acid groups of poly(acrylic acid) and of a polyampholyte based on methacrylic acid. Neighboring cationic groups of the gelatin molecule assist in the removal of the proton from a carboxylic acid group by electrostatic repulsion while neighboring carboxylate ions hinder the ionization by electrostatic attraction of the proton. These interactions result in a negative or positive electrostatic free energy of ionization, respectively. This excess free energy was calculated by two analogous procedures, from the difference between the apparent and intrinsic ionization constants, and from the slope of a plot of the apparent ionization constant versus net molecular charge. These two methods of calculation gave comparable values for gelatin. Maximum values for the electrostatic free energy of ionization, determined at nearly complete ionization, were of the same order of magnitude for gelatin and for the synthetic polyampholyte. They were an order of magnitude larger for poly(acrylic acid) due to the lack of cationic groups. Addition of 0.10 molal NaCl shifted the apparent ionization constant of the carboxylic acid groups of gelatin toward their intrinsic ionization constant and reduced the absolute value of the electrostatic free energy of ionization compared to its values in water at comparable degrees of ionization.