An acid is a substance that readily donates a proton (\(\text{H}^+\)) when dissolved in a solution. Monoprotic acids, such as hydrochloric acid (\(\text{HCl}\)), donate only a single proton per molecule. Polyprotic acids are chemical compounds that can donate more than one proton per molecule. This capability allows them to undergo multiple stages of ionization, releasing their \(\text{H}^+\) ions sequentially. They are classified based on the number of protons they can donate, such as diprotic acids (two protons) and triprotic acids (three protons).
The Sequential Release of Protons
Polyprotic acids do not release all available protons simultaneously. Instead, ionization occurs in a distinct, stepwise fashion. For a diprotic acid like \(\text{H}_2\text{A}\), the first step involves donating one proton to form a singly charged ion (\(\text{H}_2\text{A} \rightarrow \text{H}^+ + \text{HA}^-\)). The resulting intermediate ion, \(\text{HA}^-\), retains one acidic proton and can undergo a second dissociation step (\(\text{HA}^- \rightarrow \text{H}^+ + \text{A}^{2-}\)).
The second proton is more difficult to remove than the first. This difficulty arises because the remaining molecule, the conjugate base, acquires a negative charge after losing the first proton. This negative charge strongly attracts the second positively charged proton, making it less likely to dissociate.
If the acid is triprotic, a third sequential dissociation will occur. The initial ionization is the most favorable, while each subsequent step is progressively less favorable.
Quantifying Acidity: Understanding Dissociation Constants
The tendency of an acid to release its proton is measured by its acid dissociation constant, \(K_a\). Because polyprotic acids undergo multiple, sequential ionization steps, they possess multiple, distinct \(K_a\) values. Each \(K_a\) value corresponds to a specific step in the proton-releasing process.
For a triprotic acid, the constants are labeled \(K_{a1}\), \(K_{a2}\), and \(K_{a3}\). The relationship \(K_{a1} > K_{a2} > K_{a3}\) is universally true for polyprotic acids, confirming the sequential difficulty of proton removal.
The first dissociation constant, \(K_{a1}\), is always the largest, often by a factor of \(10^5\) to \(10^6\) compared to \(K_{a2}\). This large difference signifies that the first proton is released easily and contributes the most to the overall acidity. The greatly reduced value of \(K_{a2}\) confirms that the second proton is much less acidic. The contribution of the third dissociation step is often negligible due to the extremely small \(K_{a3}\) value.
Prominent Polyprotic Acids and Their Roles
Polyprotic acids play important roles in both industrial applications and biological systems.
Sulfuric Acid (\(\text{H}_2\text{SO}_4\))
Sulfuric acid is a diprotic acid widely utilized in industry. It is used for the manufacture of fertilizers, detergents, and as the electrolyte in lead-acid car batteries. Its first proton dissociates completely, making it a strong acid in that initial step.
Phosphoric Acid (\(\text{H}_3\text{PO}_4\))
Phosphoric acid is a triprotic acid used frequently in the production of fertilizers. It is also a common food additive, providing a tart flavor in soft drinks and appearing in various dietary supplements.
Carbonic Acid (\(\text{H}_2\text{CO}_3\))
Carbonic acid is a diprotic acid formed when carbon dioxide dissolves in water. This weak acid is important in the body’s primary buffer system, working with the bicarbonate ion (\(\text{HCO}_3^-\)) to regulate blood \(\text{pH}\). It also contributes to the dissolution of limestone in geology.