diprotic acid

(noun)

one that contains within its molecular structure two hydrogen atoms per molecule capable of dissociating

Related Terms

Examples of diprotic acid in the following topics:

  • Diprotic and Polyprotic Acids

    • Diprotic and polyprotic acids contain multiple acidic protons that dissociate in distinct, sequential steps.
    • Two common examples are carbonic acid (H2CO3, which has two acidic protons and is therefore a diprotic acid) and phosphoric acid (H3PO4, which has three acidic protons and is therefore a triprotic acid).
    • Diprotic and polyprotic acids show unique profiles in titration experiments, where a pH versus titrant volume curve clearly shows two equivalence points for the acid; this is because the two ionizing hydrogens do not dissociate from the acid at the same time.
    • A diprotic acid (here symbolized by H2A) can undergo one or two dissociations depending on the pH.
    • For example, a generic diprotic acid will generate three species in solution: H2A, HA-, and A2-, and the fractional concentration of HA-, which is given by:

  • Polyprotic Acid Titrations

    • Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule.
    • Common examples of monoprotic acids in mineral acids include hydrochloric acid (HCl) and nitric acid (HNO3).
    • Certain types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).
    • For example, oxalic acid, also called ethanedioic acid, is diprotic, having two protons to donate.
    • The diprotic acid has two associated values of Ka, one for each proton.

  • Heterogeneous and Multiple Equilibria

    • Consider the case of a diprotic acid, such as sulfuric acid.
    • Diprotic acids can be written as H2A.
    • Sulfuric acid, the molecule pictured here, is an example of a diprotic acid.

  • Calculating Equilibrium Concentrations of Polyprotic Acids

    • Polyprotic acids can lose more than one proton.
    • Common polyprotic acids include sulfuric acid (H2SO4), and phosphoric acid (H3PO4).
    • For a diprotic acid for instance, we can calculate the fractional dissociation (alpha) of the species HA- using the following complex equation:
    • When a weak diprotic acid such as carbonic acid, H2CO3, dissociates, most of the protons present come from the first dissociation step:
    • The chemical structure of phosphoric acid indicates it has three acidic protons.

  • Strong Acids

    • The strength of an acid refers to the ease with which the acid loses a proton.
    • where HA is a protonated acid, H+ is the free acidic proton, and A- is the conjugate base.
    • Strong acids yield weak conjugate bases.
    • For sulfuric acid, which is diprotic, the "strong acid" designation refers only to the dissociation of the first proton:
    • p-Toluenesulfonic acid is an example of an organic soluble strong acid, with a pKa of -2.8.

  • Acid Dissociation Constant (Ka)

    • The acid dissociation constant (Ka) is the measure of the strength of an acid in solution.
    • The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution.
    • Acid dissociation constants are most often associated with weak acids, or acids that do not completely dissociate in solution.
    • Acids with a pKa value of less than about -2 are said to be strong acids.
    • Acetic acid is a weak acid with an acid dissociation constant $K_a=1.8\times 10^{-5}$ .

  • Weak Acids

    • The majority of acids are weak.
    • Examples of weak acids include acetic acid (CH3COOH), which is found in vinegar, and oxalic acid (H2C2O4), which is found in some vegetables.
    • Acids with a Ka less than 1.8×10−16 are weaker acids than water.
    • The Ka of acetic acid is $1.8\times 10^{-5}$.
    • Although it is only a weak acid, a concentrated enough solution of acetic acid can still be quite acidic.

  • The Brønsted-Lowry Definition of Acids and Bases

    • Originally, acids and bases were defined by Svante Arrhenius.
    • A wide range of compounds can be classified in the Brønsted-Lowry framework: mineral acids and derivatives such as sulfonates, carboxylic acids, amines, carbon acids, and many more.
    • The conjugate acid is the species that is formed when the Brønsted base accepts a proton from the Brønsted acid.
    • Here, acetic acid acts as a Brønsted-Lowry acid, donating a proton to water, which acts as the Brønsted-Lowry base.
    • Chemistry 12.1 What are Acids and Bases?

  • Oxoacids

    • Halogen oxoacids include hypochlorous acid (HOCl); chlorous acid(HOClO); chloric acid(HOClO2); oerchloric acid(HOClO3); oerbromic acid (HOBrO3)
    • Consider the simple oxyacids HOI (hypoiodous acid), HOBr (hypobromous acid), and HOCl (hypochlorous acid).
    • The strongest acid is perchloric acid on the left, and the weakest is hypochlorous acid on the far right.
    • Carboxylic acids are the most common type of organic acid.
    • Mellitic acid is an example of a hexacarboxylic acid.

  • Carboxylic Acids

    • Carboxylic acids are organic acids that contain a carbon atom that participates in both a hydroxyl and a carbonyl functional group.
    • As proton donors, carboxylic acids are characterized as Brønsted-Lowry acids.
    • Salts and esters of carboxylic acids are called carboxylates.
    • Generally, in IUPAC nomenclature, carboxylic acids have an "-oic acid" suffix, although "-ic acid" is the suffix most commonly used.
    • Carboxylic acids are characterized as weak acids, meaning that they do not fully dissociate to produce H+ cations in a neutral aqueous solution.