Require no accuracy known standard solution

Classifications of Gravimetric methods

1-Precipitation gravimetry,the analyte is separated from a solutionof the sample as a precipitate and is converted to a compound of known composition that can be weighed.

2-Volatilization gravimetry, the analyte is separated from otherconstituents of a sample by conversion to a gas of known chemical composition. The weight of this gas then serves as a measure of the analyte concentration.

3-Electrogravimetry, the analyte is separated by deposition on anelectrode by an electrical current. The mass of this product then provides a measure of the analyte concentration.

Features or properties of Gravimetric Analysis

2. Traditional Method.

3. Cheap, easily available apparatus, simple to carry out.

4. Slow, especially when accurate results are required.

5. Wide range of sample concentrations (ng - kg).

6. No calibration required (except for the balance).

7. Accurate.

Procedure for gravimetric analysis (Precipitation gravimetry)

What steps are needed?

The steps required in gravimetric analysis, after the sample has been dissolved, can be summarized as follows:

1. Preparation of the solution

2. Precipitation

3. Digestion

4. Filtration

5. Washing

6. Drying or igniting

7. Weighing

8. Calculation

dissolved components

sample

precipitating agen

Properties of precipitates and precipitating reagents Properties precipitating reagents

Ideally, a gravimetric precipitating agent should react specifically or at least selectively with the analyte. Specific reagents, which are rare, react only with a single chemical species. Selective reagents, which are more common, react with a limited number of species.

Properties of good precipitates

1. Easily filtered and washed free of contaminants.

2. Of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing.

3. Unreactive with constituents of the atmosphere

4. Of known chemical composition after it is dried or, if necessary, ignited.

Particle size and filterability of precipitates Why we prefer precipitates of large particles?

Precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities. In addition, precipitates of this type are usually purer than are precipitates made up of fine particles.