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# Acidity constant

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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:

$K_A = \frac{\left[A^-\right] \cdot \left[H_3 O^+\right]} {\left[AH\right]}$

the pKA is defined by:

$pK_A =-\log{K_A}$

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

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

$K_B = \frac{\left[AH\right] \cdot \left[OH^-\right]} {\left[A^-\right]}$

the pKB is defined by:

$pK_B = - \log{K_B}$

There is a relation between KA and KB:

$K_E =K_A \cdot K_B$ and $pK_E = pK_A \cdot pK_B$

where KE is the dissociation constant of water.

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