Diprotic
In chemistry, diprotic acid is a class of Arrhenius acids which are capable of donating two protons or hydrogen cations per molecule when dissociating in aqueous solutions. The most important chemical feature for a diprotic acid molecule is its ability to deprotonate two protons in two sequential steps during dissociation. Most diprotic acids are common acids which are used everyday in many different areas. Also, they exist everywhere in nature, such as in human bodies.
There are both organic (which are called dicarboxylic acids) and inorganic diprotic acids. Chromic acid (H2CrO4) and sulfuric acid (H2SO4) are two of the common and widely used inorganic acids. They have similar structures within each molecular has two -OH groups linked to its center atom. Some other inorganic diprotic acids such as hydrosulfuric acid (H2S) usually have two hydrogen atoms linked to an electronegative center in each molecular.Dicarboxylic acids have a general molecular formula HOOC-R-COOH.
A diprotic acid molecular dissociates in water and other aqueous solutions to be deprotonated. It forms two equilibrium step-by-step with two different equilibrium constants, which are also called acid-dissociation equilibrium constants. Usually,the constant for the first equilibrium is far larger than the second constant.
The dissociation equations of a diprotic acid in water can be written as:
H2A + H2O ⇌ H3O+ + HA−
()
HA− + H2O ⇌ H3O+ + A2−
()
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