Estimation of pH - More Detail About Bases
Exactly the same arguments can be made for bases that was made for acids. We have defined a base as a compound that accepts a proton when in water to form hydroxide ions (OH-). Strong bases (like NH2-) completely accept protons from water and are said to be 100% ionized. If considered in terms of equilibrium (shown below), the equilibrium lies extremely far to the right.
NH2-(aq) + H2O(l) 
NH3 + OH-(aq)
Weak bases (like ammonia) do not accept all of the protons from water and are not 100% ionized. If considered in terms of equilibrium (shown below), the equilibrium lies predominantly to the left.
NH3 + H2O(l) NH4+(aq) + OH-(aq)
Because we are dealing with an equilibrium which contains bases, the K value is called the base dissociation constant and given the symbol Kb. As the strength of the base decreases, so does its Kb value. Strong acids, have Kb values that are too high to measure. Weak acids have small Kb values, usually much less than one.
| Some Kb Values
|
| NH2-, OH-
| infinite
|
| ammonia (NH3)
| 1.8 x 10-5
|
| pyridine (C5H5N)
| 1.7 x 10-9
|
| aniline (C6H5NH2)
| 4.3 x 10-10
|