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In chemistry and biochemistry, the acidity constant, acidionization constant or aciddissocation constant (K_{a}) is a specific type of equilibrium constant that indicates the extent of dissociation of hydrogen ions from an acid. While strong acids dissociate more or less completely in solution and consequently have large acidity constants, weak acids do not fully dissociate and generally have acidity constants significantly less than 1. Because this constant differs for each acid and varies over many degrees of magnitude, the acidity constant is often represented by the inverse of its common logarithm, represented by the symbol pK_{a} (cf. pH).
Given a weak acid HA, its dissolution into water is subject to the following equilibrium:
 HA + H_{2}O ↔ H_{3}O^{+} + A^{–} (this is sometimes written as: HA ↔ H^{+} + A^{–})
The acidity constant for the acid HA is the dissociation constant for this equilibrium. In other words,

, where [X] denotes the molar concentration of X in the solution
Basicity constant of the conjugate base
By analogy, one can define the basicity constant (K_{b}) and the pK_{b} of the conjugate base A^{–}:
This is the dissociation constant for the equilibrium
 A^{–} + H_{2}O ↔ HA + OH^{–}
Analogously to K_{a}, the magnitude of K_{b} indicates the relative strength of the base, with K_{b} > > 1 indicating a strong base.
Relationship between acidity and basicity constants
There exists a relationship between the value of K_{a} for an acid HA and the value of K_{b} for its conjugate base A^{–}. Since adding the ionization reaction for HA and the ionization reaction of A^{–} always gives the reaction for the selfionization of water, the product of the acidity and basicity constants gives the dissociation constant of water (K_{w}), which is 1.0 × 10^{14} mol^{2}.l^{2} at 20°C. In other words,
 K_{a}K_{b} = K_{w}
 pK_{a} + pK_{b} = pK_{w}
As the product of K_{a} and K_{b} remains constant, it follows that stronger acids have weaker conjugate bases, and vice versa.
pKa of some common substances
Measurements are at 25ºC:
Many more are available here: [1] http://www.uaf.edu/chem/321Fa04/pkas.html
Last updated: 02102005 13:55:46
Last updated: 04252005 03:06:01