The infrared (IR) spectra of glycine in aqueous solutions were obtained in the pH range 0.2 to 14 in order to determine the ionic distribution of the molecule as a function of pH by factor analysis (FA). After subtraction of the water bands, FA was used to separate the spectra of each ionic species and determine their real abundance. The p K values were retrieved from the volumetric titration a as a function of pH and were used to obtain the theoretical abundance of each ionic species as a function of pH. These distribution curves were compared with the distribution curves obtained from IR. The agreement between the two curves was good. The following species were observed for glycine in water: the cation (pH 0 to 5); the zwitterion (pH 0 to 12.5); and the anion (pH 7 to 14).
Jean-Joseph Max, Michel Trudel, and Camille Chapados, "Infrared Titration of Aqueous Glycine," Appl. Spectrosc. 52, 226-233 (1998)