Page 36 - Pharmaceutical analytical chemistry |

10-7 to 10-8 for a successful titration, an end point break is obtained for

titrating the second proton. Triprotic acids, such as H3PO4 can be titrated
similarly, but is generally too small to obtain an end point break for

the titration of the third proton. The following figure illustrates the

titration curve of a diprotic acid H2A.
The pH at the beginning of the titration is determined from the ionization

of the first proton. During titration up to the first equivalence point, an

HA-/H2A buffer region is established. At the first equivalence point, a

solution of HA- exists, and . Beyond this point, an

A2-/HA- buffer exists; and finally at the second equivalence point, the pH
is determined from the hydrolysis of A2-.Titration curves of

polyfunctional bases

A representative example of polyfunctional bases is sodium carbonate

being a salt of carbonic acid. It can be titrated as a base (replacement

titration) with a strong acid. The titration curve exhibits two inflections

corresponding to the two ionization steps. The titration curve of sodium

carbonate with HCl is shown below.

(4) Titration curves of mixtures of acids or bases

Mixtures of acids or bases can be titrated stepwise if there is an

appreciable difference in their strengths. There must generally be a
difference in Ka values of at least 10-4. If one of the acids is a strong acid,
a separate end point will be observed for the weak acid only if Ka is about
10-5 or smaller. The stronger acid will titrate first and will give a pH

break at its equivalence point. This will be followed by the titration of the

weaker acid showing a pH break at its equivalence point.

• The titration curve for a mixture of HCl and HAc versus NaOH

shows two breaks. At the equivalence point for HCl, a solution of

HAc and NaCl remains and the equivalence point is acidic. Beyond

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