The Elements of Qualitative Chemical Analysis, vol. 1, parts 1 and 2. / With Special Consideration of the Application of the Laws of Equilibrium and of the Modern Theories of Solution.

by Julius Stieglitz

(1)

We may ask, however, whether both the hydrogen atoms of carbonic acid show the same tendency to ionize, or, since there is a vast difference in the ease of ionization of different acids, whether there is not also a difference in the ease of ionization of the different hydrogen atoms in a polybasic acid. As a matter of experiment, we find that a molecule of carbonic acid does ionize, first, and more readily, into one hydrogen ion and an acid carbonate ion HCO₃⁻, according to H₂CO₃ ⇄ H⁺ + HCO₃⁻.

For this reversible reaction we have[184]

[H⁺] × [HCO₃⁻] / [H₂CO₃] = K₁.

(2)

The value of this constant,[185] called the primary ionization constant of carbonic acid, is 0.3E−6.

The acid carbonate-ion HCO₃⁻, in turn, is ionized to a certain extent, producing another hydrogen ion and the carbonate-ion, CO₃²⁻. We have HCO₃⁻ ⇄ H⁺ + CO₃²⁻, and

[H⁺] × [CO₃²⁻] / [HCO₃⁻] = K₂.

(3)

The value of this constant[186], called the constant of the [p102] secondary ionization of carbonic acid, is 0.07E−9, which has about one four-thousandth of the value of the constant for the primary ionization.