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A small but pervasive error permeates the chemical
literature concerning the first ionization constant,
K1, for carbonic acid,
H2CO3. This error surfaced in this
Journal in the article by De Roo et al.
(1) on the exact solubilities of insoluble salts. The authors most likely used a
handbook value of 4.45 x 10-7 for K1, which is
incorrect. The correct value for
K1 is 1.5 x 10-4. In this regard, carbonic acid is
an exact analog of H2SeO3 and other similar oxyacids.
Clifford (2) explains the problem as follows. The
equilibrium constant K1 represents the usual process:
H2CO3< => H+ +
HCO3-
On the other hand,
CO2 bubbled into water leads to the equilibrium
CO2(aq) + H2O < = > H2CO3 < -=> H+ + HCO3-
The resulting expression is the one that is
incorrectly quoted for the first ionization constant of carbonic acid:
From these two equilibria we can derive a third:
CO2(aq) + H2O < = > H2CO3 ;
Khyd = 2.8 x 10-3
Since the maximum solubility of
CO2 is approximately 0.034 M, the maximum concentration of
H2CO3 in water is 9.5 x 10-5 M. Any time this value is exceeded, CO2 will bubble out of the solution. In De Roo's case, on the
solubility of carbonates, the error appears only in the concentration of H2CO3; it is too small to impinge on any other tabulated results.
Literature Cited:
1. Roo, S. D.; Vermeire, L.; Görller-Walrand, C.
J. Chem. Educ. 1995, 72, 419-422.
2. Clifford, A. F. Inorganic Chemistry of Qualitative
Analysis; Prentice-Hall: Englewood Cliffs, NJ, 1961; pp 150-151.
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