Page 56 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

Sample + 25 mL

Standard (Known
Excess)

In the

flask

Part react with Part remain
sample unreacted

Back (residual) titration is used in the following cases:

(1) If the sample is volatile.
[e.g. detn. of ammonia (NH ), formic acid (HCOOH)]
3

(2) If the sample is insoluble in water.

[e.g. detn. of zinc oxide (ZnO), calcium carbonate (CaCO )]
3

(3) If the reaction between the sample & the standard needs heating.

[e.g. detn. of ammonium chloride (NH Cl)]
4

(4) If the reaction between the sample & the standard is slow.

(5) If the reaction between the sample & the standard needs the presence of an excess of

the standard to achieve stoichiometry.

[e.g. detn. of lactic acid]

(6) If no suitable indicator is available for direct titration.

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