Gravimetric analysis

Gravimetric analysis is an uncomplicated, easy-to-follow laboratory method that helps in the quantitative determination of chemical compounds. It is based on measuring the amount of a chemical compound depending upon its loss in mass. How is gravimetric analysis different from a volumetric analysis method such as titration? How is it performed and why is gravimetric analysis important in industry and in academia? We will discuss all these interesting concepts in this article.

What is gravimetric analysis – Definition

Gravimetric analysis or gravimetry refers to the quantitative determination of a chemical constituent using mass or weight measurements. The unknown chemical constituent (A) to be determined is converted into a substance of known composition (AB) by carrying out a chemical reaction. The substance AB is then isolated from the sample mixture and weighed. The difference in weight before and after the particular chemical reaction took place helps us in determining the amount of A present in the crude sample.

How to perform gravimetric analysis – Procedure

The following step-by-step guide can be used for determining the unknown mass of chloride (Cl) ions in a complex sample mixture by using gravimetric analysis.

Step I: The impure sample is accurately weighed.

• A clean, dry beaker with a lid placed on it is weighed on an analytical mass balance (x1).
• The sample is placed in this beaker and dried in an oven followed by cooling in a desiccator.
•  The beaker containing the sample is then reweighed (x2).
• The difference in x2 and x1 is calculated which is equal to the total mass of the sample in its crude form.
• Mass of sample = x2 -x1 = x grams.

Step II: A solvent is added that dissolves only the ion of interest.

• The chloride (Cl) ions are water-soluble. So water is added at this step and all the chloride ions present in the sample are dissolved completely.

Step III: A precipitating agent is added.

• The precipitating agent is a chemical compound that reacts with the targeted ions to form an insoluble solid known as a precipitate.
• The Cl ions can be precipitated out using AgNO3 as the precipitating agent.
• Ag+ ions react with Cl ions to yield a white AgCl precipitate.
• Complete precipitation is ensured as no more precipitate is formed on adding a small amount of AgNO3.

Step IV: The solution is heated on a hot plate.

• The solution prepared in step III is heated on a hot plate.
• It helps in increasing the particle size of AgCl precipitates so that these can be easily filtered from the sample solution. This process is called digestion.

Step V: The solution is filtered

• The solution obtained in step IV is then filtered using vacuum filtration.
• The solution is filtered by passing it through a pre-weighed (x3) filter paper placed in a Buchner funnel.
• The filtrate passes into the vacuum flask while the insoluble precipitate is collected as residue on the filter paper.
• The solution should be completely transferred from the beaker to the vacuum flask to avoid any chances of error in weight measurements.

Step VI: The residue is dried and weighed

• The residue obtained in step V is then dried and weighed (x4).
• The mass of the precipitate formed is determined by subtracting the weight of the filter paper (empty) i.e., x3 from the total weight of the filter paper and the precipitate i.e., x4.
• Weight of AgCl precipitate (x) = x4 – x3

Step VII: The unknown mass of the analyte is determined as per stoichiometric principles

As per the net ionic equation given below, 1 mole of AgCl precipitate is formed by the chemical reaction of 1 mole of Cl ions.

The gravimetric analysis performed in the above example falls under precipitation gravimetry. However, there are other different types of gravimetric analysis which we have discussed in the proceeding section.

Types of gravimetric analysis

1. Precipitation gravimetry

The gravimetric analysis involving precipitate formation.

2. Volatilization gravimetry

The gravimetric analysis in which the analyte is isolated via chemical or thermal decomposition.

3. Electrogravimetry

It is a type of gravimetric analysis in which the analyte (usually metal ions) is separated by applying an electrical potential difference. The metal ions are quantitatively electroplated on a platinum electrode.

4. Thermogravimetry

Thermogravimetry involves measuring the change in chemical and/or physical properties of chemical substances by increasing the temperature of the sample solution.

What is the difference between gravimetry and titrimetry

There are many different types of titrimetric analysis including complexometric titration, potentiometric titration, and redox titration.

Why is gravimetric analysis important- Uses and applications

Gravimetric analysis is used:

• For quantitative analysis of a given sample in a chemistry laboratory.
• To keep a check on the level of particulate matter, suspended solids, and toxic metals such as mercury (Hg) and lead (Pb) in river water.
• In the industry for quality control and quality assurance. The nutritional information printed on food and beverage items can be revalidated using gravimetric analysis.
• For determining mineral content in potable water and milk samples.
• In pharmaceutical analysis.

Check out some important uses and applications of titrimetric analysis here.

Here is a link to a more extensive gravimetric analysis experiment.

References

1. C.Harris, Daniel. 2010. Quantitative Chemical Analysis (W.H Freeman and Company ).

2. Ismail, Wan Norfazilah Wan. 2017. Gravimetric analysis.

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