Acids and Bases: Ionization Constants
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Introduction
Molecular Structure
Ionization Constants
Salts
Buffers
Lewis Theory

Ionization Constants for Polyprotic Acids and Bases

Phosphoric acid is a polyprotic acid, or an acid that has more than one acidic proton. Remember that a polyprotic acid donates its protons in steps:

Of the three acids above, which will have the largest Ka value?

How do I know which of these is the strongest acid?
H3PO4 H2PO4- HPO42-

The strongest acid will have the largest Ka value.

Good! Each ionization step of phosphoric acid will have a Ka value. The ionization constants are numbered; the first ionization is Ka1, the second Ka2 and so on. The first ionization constant is always the largest (usually by several orders of magnitude), followed by the second and so on.

The table below shows some polyprotic acids and bases and their respective ionization constants. Notice that the first ionization constant is always the largest. Also notice that there is usually a difference of several orders of magnitude between successive ionization constants (Ka1, Ka2, etc.).

Acids

 Ka1 Ka2 Ka3

Arsenic Acid

 H3AsO4

5.8 x 10-3

1.1 x 10-7

3.2 x 10-12

Carbonic Acid

 H2CO3

4.5 x 10-7

4.7 x 10-11
 

Bases

 Kb1 Kb2

Ethylene Diamine

 H2NCH2CH2NH2

8.5 x 10-5

7.0 x 10-8

Piperazine

5.4 x 10-5

2.2 x 10-9