An acid is a substance that can donate a proton (\(H^+\)) when dissolved in water. Acids are categorized based on their ability to release these protons, a property known as proticity. The simplest acids, called monoprotic acids, possess only one ionizable hydrogen atom. However, many acids in chemistry and biology can release multiple protons, leading to the classification of polyprotic acids.
Defining Diprotic Acids
Diprotic acids are a specific type of polyprotic acid defined by their capacity to donate two protons per molecule in an aqueous solution. The prefix “di-” means two, and “protic” refers to the protons the molecule can release. This structural feature requires the acid molecule to possess two hydrogen atoms capable of dissociating.
This classification sets diprotic acids apart from monoprotic acids, such as hydrochloric acid (\(HCl\)), which only has one acidic proton. Diprotic acids are also distinct from triprotic acids, like phosphoric acid (\(H_3PO_4\)). The presence of two acidic protons allows a diprotic acid to participate in two separate acid-base reactions.
The general chemical formula for a diprotic acid is represented as \(\text{H}_2\text{A}\), where A represents the rest of the molecule. The two acidic protons may not be structurally identical, but both are available for donation.
The Sequential Ionization Process
The release of the two protons from a diprotic acid does not happen simultaneously; instead, it occurs in two distinct, sequential ionization steps. The first step involves the acid molecule (\(\text{H}_2\text{A}\)) donating its first proton, yielding the intermediate species \(\text{HA}^-\).
$\(\text{H}_2\text{A} \rightleftharpoons \text{HA}^- + \text{H}^+\)$
The resulting intermediate species, \(\text{HA}^-\), is itself an acid that can then donate the second proton in a separate equilibrium reaction. This second step forms the fully deprotonated anion (\(\text{A}^{2-}\)).
$\(\text{HA}^- \rightleftharpoons \text{A}^{2-} + \text{H}^+\)$
The first proton is always easier to remove than the second. Once the first proton leaves, the remaining intermediate species, \(\text{HA}^-\), carries a negative charge. Removing a second positive proton requires overcoming a greater electrostatic attraction, making the second ionization significantly less favorable.
Quantifying Acid Strength
The distinct acid strength of each ionization step is measured using the Acid Dissociation Constant (\(\text{K}_{\text{a}}\)). Because a diprotic acid has two separate dissociation steps, it is characterized by two constants: \(\text{K}_{\text{a}1}\) for the first step and \(\text{K}_{\text{a}2}\) for the second step.
The \(\text{K}_{\text{a}1}\) value corresponds to the equilibrium of the \(\text{H}_2\text{A}\) molecule losing its first proton. The \(\text{K}_{\text{a}2}\) value represents the loss of the second proton from the negatively charged \(\text{HA}^-\) ion.
The \(\text{K}_{\text{a}1}\) is always much larger than \(\text{K}_{\text{a}2}\) (\(\text{K}_{\text{a}1} > \text{K}_{\text{a}2}\)), often by a factor of \(10^3\) to \(10^6\) or more. This relationship reflects the chemical reality of the sequential ionization process.
The larger \(\text{K}_{\text{a}1}\) indicates a greater tendency for the initial dissociation, while the smaller \(\text{K}_{\text{a}2}\) confirms that the concentration of the fully deprotonated \(\text{A}^{2-}\) ion is typically very low in most solutions.
Common Diprotic Acids and Their Roles
Several common and important acids found in both industrial and biological settings are diprotic. Sulfuric acid (\(\text{H}_2\text{SO}_4\)) is a widely used industrial chemical, often considered a strong acid because its first proton dissociates completely in water (\(\text{K}_{\text{a}1}\) is very large). However, the second dissociation, which involves the hydrogen sulfate ion (\(\text{HSO}_4^-\)), is much weaker, with a \(\text{K}_{\text{a}2}\) of \(1.2 \times 10^{-2}\).
Carbonic acid (\(\text{H}_2\text{CO}_3\)) is a weaker diprotic acid that plays an important role in biological systems. This acid is formed when carbon dioxide dissolves in water, and it is a central component of the blood buffer system. The two dissociation steps allow the body to maintain the blood’s \(\text{pH}\) within a narrow range.
Oxalic acid (\(\text{H}_2\text{C}_2\text{O}_4\)), found naturally in many plants, is another example of a diprotic acid. It has distinct \(\text{K}_{\text{a}}\) values for its two ionizations: \(\text{K}_{\text{a}1} = 5.1 \times 10^{-2}\) and \(\text{K}_{\text{a}2} = 5.2 \times 10^{-5}\).