A diprotic acid can donate two hydrogen atoms to a solution, forming two bases. Sulfuric acid is a common example, with different Ka values for each dissociation. Diprotic acids have distinct titration curves and occur naturally and as man-made substances.
In chemistry, a diprotic acid is an acid that can donate two hydrogen atoms (H), or protons, for each molecule of the acid to a solution found in the aqueous state or in water. One of the most common examples of diprotic acid is sulfuric acid, which has the chemical formula of H2SO4. Sulfuric acid can lose a hydrogen atom to form hydrogen sulfate (HSO4) or lose both hydrogen atoms to form sulfate (SO4).
The term “diprotic” refers to the fact that the acid can release two hydrogen atoms or form two protons. “Di-” refers to the fact that two atoms can be released and “protic” is used because the atoms being released are protons. In some cases, dibasic is also used to describe these types of molecules as two bases are formed through the release of the hydrogen atoms. For example, hydrogen sulfate and sulfate are both bases, so two bases can be formed through the loss of one or two hydrogen atoms from sulfuric acid.
Diprotic acids are ionizable or dissociate in the presence of water. The loss of the two hydrogen atoms from diprotic acid does not occur simultaneously. Each dissociation is a separate reaction due to the fact that the strength of the acid differs based on the number of hydrogen atoms attached to the molecule. The Ka value, or acidity constant, gives the strength of an acid that is in solution. With diprotic acids, the Ka value is different for each dissociation.
The titration curves of diprotic acids have a very distinct shape which clearly shows two different points of equivalence. The equivalence points shown on a titration curve give the Ka value at each dissociation as diprotic acid loses the first and then the second hydrogen atom to the water molecules. Depending on the diprotic acid tested, the second dissociation may not fully occur, meaning that some of the acid molecules will still contain a hydrogen atom.
There are both organic and inorganic, or biological and mineral, examples of diprotic acids that occur both naturally and as man-made substances. Sulfuric acid is an example of an inorganic acid, while the acidity or flavor of some fruits, such as apples, grapes and cherries, is due to malic acid, which is an organic diprotic acid. This acid is found in most unripe fruits, but it breaks down as the fruit ripens, so the fruit becomes less acidic as it ages.
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