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1. Pharmaceutical Analysis - I
Topic: Gravimetry
Dept. of Pharm. Chemistry
Institute of Pharmaceutical Sciences, University of Lucknow

2. CoNtenTs
• Gravimetric analysis
• Introduction
• Principle
• Precipitation
• Purity of precipitation
• Co-precipitation
• Post-precipitation
• Steps involves in Gravimetric analysis

3. Gravimetric Analysis
• Gravimetric analysis or quantitative analysis by weight is the process
of isolation and weighing an element or a definite compound or the
element, in a pure form.
• It is based on precipitating the component of substance under test from
solution as an insoluble compound of known chemical composition.
• Gravimetric analysis can be done by following methods;
• Precipitation method
• Volatilization method
• Electro-analytical method

4. Principle of Gravimetric analysis

5. Precipitation Reaction
• Precipitation reaction is the process in which formation of mass or
weight of product occurs in the presence of any precipitating agent.
• It is in insoluble salt form or sparingly soluble precipitate.

6. Properties of Precipitate
• Precipitate should be so insoluble that no
significant loss occurs during filtration
and washing.
• It can be easily separated by filtration and
be washed free of soluble impurities.
• The precipitate should be stable to the
atmospheric conditions.
• The precipitate must be convertible to a
pure compound of definite composition.

7. Purity of Precipitate
• When the precipitate separates out from the solution, it is always not
perfectly pure, i.e. the precipitate is contaminated with varying amounts
of impurities even after extensive washing.
• The impurity appears to have precipitated along with the substance to be
analyzed although its solubility product is not exceeded and it would not
have been precipitated had the analyte been absent.
• This phenomenon is known as co-precipitate.

8. Co-precipitations
The following experiment illustrates the co-precipitation of soluble substances-
• To a mixture of barium chloride and potassium permanganate solution, excess of
sulphuric acid is added slowly in small portions and then potassium permanganate is
reduced by addition of some reducing agent.
• After the reaction, the solution becomes colourless but the precipitate appears to be
of violet color.
• This indicates that some potassium permanganates are co-precipitated together with
barium sulphate.

9. Co-precipitations
There are four types of co-precipitations;
1. Surface adsorption
2. Occlusion
3. Mixed-crystal formation
4. Mechanical entrapement

10. Surface adsorption

11. Occlusion

12. Post Precipitation
• Sometimes when the precipitate allowed to stand in presence of mother
liquor the second substance will form the precipitate with precipitating
agent, this is called post precipitation.
• Post precipitation differs from that of co precipitation in the following
aspects;
• The extent of contamination increases with the time of contact with mother liquor in
case of post precipitation while in co precipitation it decreases contamination.

13. Steps involves in Gravimetric Analysis;
1. Sampling
2. Precipitation of solution or
dissolution
3. Precipitation
4. Testing the completeness of
precipitation
5. Digestion or ageing of precipitate
6. Filtration
7. Washing of precipitate
8. Drying of precipitate
9. Weighing
10. Calculation

14. Sampling
• The sample which is weighed for the quantitative analysis should
be very small.
• It is necessary to take a representative sample correctly to ensure
that results are accurate.
• The samples should be homogenous and in the form of powder.

15. Preparation of solution
• To dissolve a sample, take a clean beaker and transfer the weighed
sample completely to a beaker.
• Add sufficient water to the sample to get a clear solution.

16. Precipitation
• Proper choice of precipitant is based on the specificity of reagent.
• For ideal precipitant, it should react with analyte to form the
precipitate which can be easily filtered, washed free of
contaminating impurities