Page 37 - Pharmaceutical analytical chemistry |

the equivalence point, the Ac- / HAc buffer region is established,

and this markedly suppresses the pH break for HCl. The remainder

of the titration curve is identical to the titration curve of HAc.

• If two strong acids are titrated together, there will be no

differentiation between them, and only one equivalence point

break will occur corresponding to the titration of both acids. The

same is true for two weak acids if their Ka values are not too
different. For example, a mixture of acetic acid, Ka = 1.75 × 10-5 and
propionic acid, Ka = 1.3 × 10-5, would titrate together to give a
single equivalence point.

• As for H2SO4, the first proton is completely dissociated and the
second proton has a Ka of about 10-2. Therefore, the second
proton is ionized sufficiently to titrate as a strong acid, and only

one equivalence point break is found. The same is true for a

mixture of a strong acid and weak acid with a Ka in the
neighborhood of 10-2.

• In a mixture with HCl, the first proton of H3PO4 would titrate along
with HCl and the pH at the equivalence point would be

determined by the H2PO42- remaining, i.e. .

This is followed by the titration of the second proton to give the

second equivalence point of H3PO4 whereas the third proton is too
weakly ionized to be titrated.

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