The Apparent Dissociation Constants of Argi- Nine and of Lysine and the Apparent Heats of Ionization of Certain Amino Acids.*

In this paper the apparent dissociation constants of arginine and of lysine at 25” and the apparent dissociation constants of arginine, lysine, histidine, aspartic acid, and glutamic acid at 0” are reported. We have made use of these data together with previously published data for the apparent dissociation constants of histidine, aspartic acid, and glutamic acid for the purpose of calculating the apparent heats of ionization of these amino acids. The technique employed for obtaining the titration curves was the same as that which has been described by Kirk and Schmidt, (1) except that for the purpose of carrying out hydrogen ion activity measurements at 0” the calomel electrode, potassium chloride solution, and the hydrogen electrode vessel were immersed in an ice-water bath. A modified electrode vessel suitable for immersion in water was used. Uniform temperature was obtained by rapid stirring. For the purpose of determining the value of the calomel electrode at 0” the electromotive force of a hydrogen electrode immersed in a 0.01 N hydrochloric acid plus 0.09 N potassium chloride solution was determined. The relation between the measured electromotive force, the electromotive force of the calomel cell, and the hydrogen’ion activity at 0” is given by the equation