Diprotic acid
Encyclopedia
A diprotic acid is an acid
such as H2SO4 (sulfuric acid
) that contains within its molecular structure two hydrogen
atoms per molecule capable of dissociating (i.e. ionizable) in water. The complete dissociation of diprotic acids is of the same form as sulfuric acid
:
The dissociation does not happen all at once due to the two stages of dissociation having different Ka values. The first dissociation will, in the case of sulfuric acid, occur completely, but the second one will not. Diprotic acids are of particular note in regards to titration
experiments, where a pH versus titrant volume curve
will clearly show two equivalence point
s for the acid. This occurs because the two ionization capable hydrogen atoms on the acid molecule do not leave the acid at the same time.
Common diprotic acids include malic acid
, found in apple
s and cherries
, and tartaric acid
, found in grape
s and pineapple
s.
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
such as H2SO4 (sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
) that contains within its molecular structure two hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atoms per molecule capable of dissociating (i.e. ionizable) in water. The complete dissociation of diprotic acids is of the same form as sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
:
- H2SO4 → H+(aq) + HSO4−(aq) Ka = 1 × 103
- HSO4− → H+(aq) + SO42−(aq) Ka = 1 × 10−2
The dissociation does not happen all at once due to the two stages of dissociation having different Ka values. The first dissociation will, in the case of sulfuric acid, occur completely, but the second one will not. Diprotic acids are of particular note in regards to titration
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
experiments, where a pH versus titrant volume curve
Titration curve
Titrations are often recorded on titration curves, whose compositions are generally identical: the independent variable is the volume of the titrant, while the dependent variable is the pH of the solution...
will clearly show two equivalence point
Equivalence point
The equivalence point, or stoichiometric point, of a chemical reaction when a titrant is added and is stoichiometrically equal to the amount of moles of substance present in the sample: the smallest amount of titrant that is sufficient to fully neutralize or react with the analyte...
s for the acid. This occurs because the two ionization capable hydrogen atoms on the acid molecule do not leave the acid at the same time.
Common diprotic acids include malic acid
Malic acid
Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid which is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms , though only the L-isomer exists...
, found in apple
Apple
The apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family . It is one of the most widely cultivated tree fruits, and the most widely known of the many members of genus Malus that are used by humans. Apple grow on small, deciduous trees that blossom in the spring...
s and cherries
Cherry
The cherry is the fruit of many plants of the genus Prunus, and is a fleshy stone fruit. The cherry fruits of commerce are usually obtained from a limited number of species, including especially cultivars of the wild cherry, Prunus avium....
, and tartaric acid
Tartaric acid
Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...
, found in grape
Grape
A grape is a non-climacteric fruit, specifically a berry, that grows on the perennial and deciduous woody vines of the genus Vitis. Grapes can be eaten raw or they can be used for making jam, juice, jelly, vinegar, wine, grape seed extracts, raisins, molasses and grape seed oil. Grapes are also...
s and pineapple
Pineapple
Pineapple is the common name for a tropical plant and its edible fruit, which is actually a multiple fruit consisting of coalesced berries. It was given the name pineapple due to its resemblance to a pine cone. The pineapple is by far the most economically important plant in the Bromeliaceae...
s.