"Acid–base equilibrium" redirects here. For acid–base balance in physiology, see Acid–base homeostasis.
"pKa" redirects here. For other uses, see PKA (disambiguation).
Acids and bases
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Acid
Acid–base reaction
Acid–base homeostasis
Acid strength
Acidity function
Amphoterism
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Buffer solutions
Dissociation constant
Donor number
Equilibrium chemistry
Extraction
Hammett acidity function
pH
Proton affinity
Self-ionization of water
Titration
Lewis acid catalysis
Frustrated Lewis pair
Chiral Lewis acid
Acid types
Brønsted–Lowry
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Solid
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In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction
known as dissociation in the context of acid–base reactions. The chemical species HA is an acid that dissociates into A−, called the conjugate base of the acid, and a hydrogen ion, H+.[a] The system is said to be in equilibrium when the concentrations of its components do not change over time, because both forward and backward reactions are occurring at the same rate.[1]
The dissociation constant is defined by[b]
or by its logarithmic form
where quantities in square brackets represent the molar concentrations of the species at equilibrium.[c][2] For example, a hypothetical weak acid having Ka = 10−5, the value of log Ka is the exponent (−5), giving pKa = 5. For acetic acid, Ka = 1.8 x 10−5, so pKa is about 5. A higher Ka corresponds to a stronger acid (an acid that is more dissociated at equilibrium). The form pKa is often used because it provides a convenient logarithmic scale, where a lower pKa corresponds to a stronger acid.
Cite error: There are <ref group=lower-alpha> tags or {{efn}} templates on this page, but the references will not show without a {{reflist|group=lower-alpha}} template or {{notelist}} template (see the help page).
^Whitten, Kenneth W.; Gailey, Kenneth D.; Davis, Raymond E. (1992). General Chemistry (4th ed.). Saunders College Publishing. p. 660. ISBN 0-03-072373-6.
^Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry (8th ed.). Prentice Hall. pp. 667–8. ISBN 0-13-014329-4.
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Global Information
![{\displaystyle K_{\text{a}}=\mathrm {\frac {[A^{-}][H^{+}]}{[HA]}} ,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7cdd9efda0e3a32060020b5c9e5b2c78981b2a93)
![{\displaystyle \mathrm {p} K_{{\ce {a}}}=-\log _{10}K_{\text{a}}=\log _{10}{\frac {{\ce {[HA]}}}{[{\ce {A^-}}][{\ce {H+}}]}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d7af05bf129db2f9bc618fe809660b6e4ff8dce9)