Diprotic Acid
Definition and meaning of Diprotic Acid in chemistry.
A diprotic acid is an acid that can donate two protons (H⁺ ions) per molecule when it dissociates in solution.
In more detail
Diprotic acids ionize in two successive steps, each with its own equilibrium constant (Ka1 and Ka2), and Ka1 is always much larger than Ka2 because it is harder to remove a positively charged proton from an already negatively charged ion. This stepwise ionization means diprotic acids can form two different salts and produce buffer systems at different pH ranges. They are important in titrations, buffer chemistry, and biological systems such as carbonic acid in blood pH regulation.
Key facts
| Field | General Chemistry |
|---|---|
| Formula (example) | H2SO4 |
| Ionization steps | 2 (Ka1 and Ka2) |
| Relative strength | Ka1 >> Ka2 |
Sulfuric acid (H2SO4) is a diprotic acid: it first ionizes completely (H2SO4 -> H+ + HSO4-), then the bisulfate ion ionizes partially (HSO4- <=> H+ + SO4^2-).
Frequently asked questions
Why is Ka1 always greater than Ka2 for a diprotic acid?
After the first proton leaves, the remaining ion carries a negative charge, which electrostatically holds the second proton more tightly, making it harder to remove and lowering Ka2.
Is carbonic acid a diprotic acid?
Yes, carbonic acid (H2CO3) is a classic weak diprotic acid that ionizes stepwise to bicarbonate (HCO3-) and then carbonate (CO3^2-), and it plays a key role in blood buffering.