Scales of Bronsted Basicity and Affinity in Solution

It is not possible to measure the absolute strength of an acid or base in solution but strengths can be measured relative to some standard pair, A°/B°. The acid strength of the 

Studied pair, A/B, is then given by the equilibrium constant of the reaction 

The standard pair, A°/B°, is usually chosen to be the acid/base pair of the solvent. In aqueous solutions, the pair H3O+/H2O is commonly preferred to the other possible pair, H20/0H. The strength of any acid A is then measured by the equilibrium constant of the reaction 

When measurements are made in dilute aqueous solution, the concentration of water remains essentially constant and its activity can be taken as unity. The strength of the acid A is then measured by the acid dissociation constant  

The strength of a neutral base B is described in terms of the X a of its conjugate acid BH+, usually denoted AiBH+  

---