A triprotic acid is an acid capable of donating three protons or hydrogen ions per molecule in aqueous solution. Examples include phosphoric acid ($H_3PO_4$) and citric acid.
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Triprotic acids undergo three distinct ionization steps, each with its own equilibrium constant ($K_a$).
The first ionization step typically has the highest $K_a$ value, indicating it is the strongest of the three acidic protons.
In a triprotic acid, the subsequent ionizations occur less readily, with each $K_a$ value decreasing sequentially.
The pH of a solution containing a triprotic acid can be calculated by considering all three ionization equilibria.
Polyprotic acids like triprotic acids are important in biological systems and industrial processes for their buffering capacity.
Review Questions
How many protons can a triprotic acid donate in an aqueous solution?
Explain why the first ionization constant ($K_{a1}$) of a triprotic acid is usually higher than the second ($K_{a2}$) and third ($K_{a3}$).
Name two examples of triprotic acids and describe one application or significance of each.
Related terms
Diprotic Acid: An acid that can donate two protons or hydrogen ions per molecule in aqueous solution.
$K_a$ (Acid Dissociation Constant): A quantitative measure of the strength of an acid in solution; specifically, it measures how completely an acid dissociates into its ions.
$pH$: A scale used to specify how acidic or basic an aqueous solution is. It is logarithmically related to the concentration of hydrogen ions in a solution.