Question

When performing titration of a weak acid with a strong base, at what point in the titration curve can you use to estimate the pK_a of a weak acid?

Equivalence point

initial point

half the equivalence of titrant

Answer 1

Suppose the weak acid is HA. The dissociation of weak acid is given as :

HA <--------------> H+ + A-

where;

H+ is the proton

A- is the conjugate base

In titration; we measure pH of the solution after adding a small amount of titrant to the titration flask.

The reaction between a strong base and a weak acid leads to the formation of a buffer. The pH of a buffer solution is given by the Henderson Hasselbalch equation. The equation is of the form:

pH = pKa + log ( [A-] / [HA] )

At the half-equivalence point; the neutralisation is half complete and

[A-] = [HA]

Therefore;

pH = pKa + log ( [A-] / [A-] )

pH = pKa + log 1

which gives;

pH = pKa

Thus; at half-equivalence point pH of solution is equal to pKa of the weak acid.

Therefore; half the equivalence of titrant  can be used to estimate the pKa of a weak acid.