Page 38 - Pharmaceutical analytical chemistry |

Pharmaceutical applications of acid-base titrations

1) Determination of nitrogen-containing compounds
(a) Kjeldahl method: is used for the determination of an
organic nitrogen-containing compound. The nitrogen can be
obtained as an equivalent amount of ammonium hydrogen
sulfate by digesting the sample with concentrated sulfuric
acid in the presence of potassium sulfate to raise the
temperature at which the digestion is performed and by
using mercury as a catalyst to fasten the digestion reaction.
When the decomposition is complete, the mixture
containing NH4HSO4 is treated with NaOH and the liberated
ammonia is distilled into excess standard acid which is back
titrated with standard alkali.
(b) Potassium nitrite and potassium nitrate can be reduced by
Devarda's Alloy (50% Cu, 45% Al, and 5% Zn) into ammonia
which is received in a measured excess of a standard
mineral acid and the excess is titrated with a standard alkali.

2) Formol titration
This method is used for the determination of ammonium salts

4 NH4+ + 6 HCHO ↔ 4 H+ + (CH2)6N4 + 6 H2O
Hexamine

H+ is titrated with standard NaOH using phenolphthalein as indicator.

3) Direct titration of acidic substances
The general procedure involves the direct titration of acidic substances in
aqueous medium using standard base as a titrant. If the substance is

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