Acidity constant
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In chemistry, given a weak acid AH, its dissolution into water is subject to the equilibrium :

AH + H₂O ↔ A^- + H₃O⁺

The acidity constant Ka of the couple AH/A^- is defined by:

the pK_A is defined by:

([AH] indicates the concentration of AH in the solution).

By analogy, one can define the basicity constant of a weak base:

the pK_B is defined by:

There is a relation between K_A and K_B:

and

where K_E is the dissociation constant of water.

The original article can be found at: http://en.wikipedia.org/wiki/Acidity_constant.
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