Chemistry 30

Polyprotic Acids

We have seen examples of acids that contain more than one hydrogen ion that can be lost. Sulfuric acid, H₂SO₄, for example, has two hydrogen ions that it can give up. The first hydrogen ion is released as:

H₂SO_{4 (aq)} → H⁺_(aq) + HSO₄^-_(aq)

The second hydrogen will be more difficult to remove because it must now be removed from a negative ion, HSO₄^-.

HSO₄^-_(aq) → H⁺_(aq) + SO₄^2-_(aq)

For acids that can donate more than one hydrogen, it will always be easier to donate the first H⁺ than the second. If there are more hydrogens to release, each H⁺ is more difficult to remove because of the increasingly positive charge of the rest of the molecule.

Acids that can donate more than one hydrogen ion are called polyprotic

How many H⁺ can citric acid, H₃C₆H₅O₇, release?

What do you think the equations describing the release of the H⁺ will be? Make your predictions before you check . . .

Citric acid can release three H⁺. The reactions would be:

1.    H₃C₆H₅O_{7 (aq)} → H⁺_(aq) + H₂C₆H₅O₇^-_(aq)

2.    H₂C₆H₅O₇^-_(aq) → H⁺_(aq) + HC₆H₅O₇^2-_(aq)

3.    HC₆H₅O₇^2-_(aq) → H⁺_(aq) + C₆H₅O₇^3-_(aq)