David M. Kern
Dartrnouth College Hanover, New Hampshire
The Hydration of Carbon Dioxide
The neutralization of carbon dioxide is a slow process compared with that of other weak acids. This can be shown by a simple experiment. Two 70-ml portions of 0.01 M NaOH are colored with phenolphthalein. A slight excess of acetic acid is added to one, and an equal number of moles of carbonic acid (25 ml of saturated COz solution) t o the other. The first portion loses color as fast as the solutions can be mixed. No immediate change takes place in the carbonic acid mixture; then a slow fading sets in which is complete in about fifteen seconds. The reason for the delay is that only a small fraction of aqueous Cop exists in the hydrated form HzCOs. The slowness of the interconversion carbon dioxide* hydrated carbonate species is of great importance in industrial, analytical, and biological chemistry. Its kinetics have been examined over a long period and by a wide variety of techniques. It acquires additional interest from being one of the first "rapid" equilibria (tv, < 1sec) to be quantitatively studied. This article traces the work on C02 kinetics from its crude beginnings to the most recent (1958) measurements. Experimental precision has progressed so far that the system is sometimes used as a standard for testing new kinetic measuring devices. There are, however, still unresolved differences in the reported equilibrium constants. To help the reader appreciate the problems faced by early workers, the modern conception of the reaction mechanism is summarized below. The constants are for 25"; they have been adjusted where necessary to insure internal consistencv. " Neutralization of COz can occur by two paths. At
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(32) SCHEURER, P., BROWNELL, R., A N D LUVALLE, J., J. Phys. Chem., 62, 809 (1958). (33) *SIRS,J., Trans. Fa7. Soc., 54, 207 (1958). F., T ~ a n zFUT. . Soc., 47, 263 (34) 'PINSENT,B., AND ROUGHTON, 119.511~ >----,. (35) MOELWYN-HUGHES, E., "The Kinetics of Reactions in Solutions," 2nd ed., Oxford, 1947, p. 56. (36) CLARK,A., AND PERRIN,D., Biochm. J., 48, 495 (1951). (37) PINSENT,B., Disc. Far. Soe., 17, 140 (1954). (38) *PINSENT, B., PEARSON, L., A N D ROUGETON, F., Tmns. Far. Soc., 52, 1512 (1956). (39) 'Sms, J., Trans. Far. Soc., 54, 201 (1958). (40) *WISSBRUN, K., FRENCH, D., AND PATTERSON, A., J. Phys, Chem., 58, 693 (1954). H. S., A N D DAVIS,R., J. Amer. Chem. Soc., 65 (41) HARNED, 2030 (1943).
* Recommended for a brief survey of the subject.
t Reviews.
Volume 37, Number 1 , January 1960
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