Base ionisation constant
Where;
Ka is the Acid dissociation constant,
[OH-] is the concentration of hydroxide ion,
[HB+] is the concentration of the acid conjugate to the base,
[B] is the concentraion of the base.
When an acid, HA, dissolves in water, some molecules of the acid 'dissociate' to form hydronium ions and the conjugate base, (A-), of the acid.
Values of Kb vary over many orders of magnitude, so it is common to take the negative logarithm to base ten of the value.
It is easier to compare the strengths of different acids by comparing pKb values as they vary over a much smaller range.