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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
K a is variously named a dissociation constant, [3] an acid ionization constant, [2]: 668 an acidity constant [1] or an ionization constant. [2]: 708 It serves as an indicator of the acid strength: stronger acids have a higher K a value (and a lower pK a value).
Strong acids, such as sulfuric or phosphoric acid, have large dissociation constants; weak acids, such as acetic acid, have small dissociation constants. The symbol K a , used for the acid dissociation constant, can lead to confusion with the association constant , and it may be necessary to see the reaction or the equilibrium expression to ...
Acid strength is the tendency of an acid, symbolised by the chemical formula, to dissociate into a proton, +, and an anion, .The dissociation or ionization of a strong acid in solution is effectively complete, except in its most concentrated solutions.
The equilibrium constant for the protonation of a base, B, + H + ⇌ + is an association constant, K b, which is simply related to the dissociation constant of the conjugate acid, BH +. = The value of is ca. 14 at 25 °C. This approximation can be used when the correct value is not known.
The bifluoride anion (HF 2 −) encourages the ionization of HF by stabilizing the F −. Thus, the usual ionization constant for hydrofluoric acid (10 −3.15) understates the acidity of concentrated solutions of HF. The effect of homoassociation is often high in nonaqueous solutions, wherein dissociation is often low.
Bjerrum plot of speciation for a hypothetical monoprotic acid: AH concentration as a function of the difference between pK and pH. Carbonic acid is the formal Brønsted–Lowry conjugate acid of the bicarbonate anion, stable in alkaline solution. The protonation constants have been measured to great precision, but depend on overall ionic ...
For example, nitric acid has an ionization constant of 10 1.4, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid has an ionization constant of 10 10, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, they react to form solid hydronium perchlorate (H 3 O + ·ClO ...