Acidity constant

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:

<math>K_A = \frac{\left[A^-\right] \cdot \left[H_3 O^+\right]} {\left[AH\right]}</math>

the pK_A is defined by:

<math>pK_A =-\log{K_A}</math>

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

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

<math>K_B = \frac{\left[AH\right] \cdot \left[OH^-\right]} {\left[A^-\right]}</math>

the pK_B is defined by:

<math>pK_B = - \log{K_B}</math>

There is a relation between K_A and K_B:

<math>K_E =K_A \cdot K_B</math> and <math>pK_E = pK_A \cdot pK_B</math>

where K_E is the dissociation constant of water.