Significance of the Henderson-Hasselbalch equation

With its help :

(1) The pH of a buffer solution can be calculated from the initial concentrations of the weak acid

and the salt provided Ka is given.

However, the Henderson-Hasselbalch equation for a basic buffer will give pOH and its pH can be

calculated as (14 – pOH).

(2) The dissociation constant of a weak acid (or weak base) can be determined by measuring the

pH of a buffer solution containing equimolar concentrations of the acid (or base) and the salt.

[salt] pH = p + log [acid]

Since

[salt] [salt] = [acid], log log1 0 [acid] = =

∴ pKa = pH

The measured pH, therefore, gives the value of pKa of the weak acid.

Likewise we can find the pKb of a weak base by determining the pH of equimolar basic buffer.

(3) A buffer solution of desired pH can be prepared by adjusting the concentrations of the salt

and the acid added for the buffer.

It is noteworthy that buffer solution are most effective when the concentrations of the weak acid

(or weak base) and the salt are about equal. This means that pH is close to the value of pKa of the acid

(or pKb of the base).

Thursday, 6 May 2021