Possible approximations for the weak base

Approximation 1 relating to the extent of protonation of the weak base 

If B(aq) is a sufficiently weak base for the fraction protonated to be small, then very little BH (aq) will be formed and very little B(aq) will have been removed. 

As with the weak acid, the approximation generally holds for values around 4 or greater. But again, diluting the solution of the weak base will dramatically increase the fraction protonated, so that the approximation will not hold for very low stoichiometric concentrations. For stronger bases, the extent of protonation becomes sufficiently high for the approximation to be invalid. 

If the contribution to OH (aq) from the self ionisation of water is small compared with the contribution from the protonation of B(aq), then to a first approximation it can be ignored. 

There is, therefore, effectively only one source of BH (aq) and OH (aq), i.e. from the weak base, B(aq), and as they are produced in a 1 1 ratio  

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