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 pKA 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 pKB is defined by:
- <math>pK_B = - \log{K_B}</math>
There is a relation between KA and KB:
- <math>K_E =K_A \cdot K_B</math> and <math>pK_E = pK_A \cdot pK_B</math>
where KE is the dissociation constant of water.
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