Gravimetry

Gravimetry is a method used in analytical chemistry to determine the quantity of an analyte based on the mass of a solid. It’s a quantitative method used to determine the amount of a substance present in a sample by measuring its mass. This technique relies on the principles of precipitation and weighing to isolate and quantify the analyte of interest. Gravimetric analysis provides exceedingly precise analysis and is used to determine the atomic masses of many elements to six-figure accuracy. However, it usually provides only for the analysis of a single element, or a limited group of elements, at a time.

Table of Contents

Principle of gravimetry

The principle of gravimetry is based on the measurement of the mass of an analyte. The analyte is the substance or chemical constituent that is of interest in an analytical procedure.

In gravimetric analysis, the analyte is isolated from the sample as a pure compound. This isolation may be achieved by precipitation, volatilization, or electrochemical methods. The mass of the analyte, or some compound chemically related to it, is then determined.

The amount of analyte in the original sample can then be calculated based on the mass of the isolated analyte or related compound and the stoichiometry of the reaction by which the analyte was isolated.

This method is highly precise and accurate, but it is often time-consuming and typically only allows for the analysis of a single element or a limited group of elements at a time.

Steps involved in gravimetric analysis

Gravimetric analysis involves several steps to determine the amount of an analyte based on its mass. Here are the key steps involved in conducting a gravimetric analysis.

Sample Preparation: This is the first step in gravimetric analysis. It involves selecting an appropriate sample size and ensuring that it is representative of the entire batch.
Precipitation: The next step involves precipitating the analyte from the sample solution. This is typically done by adding a reagent that forms an insoluble compound with the analyte, allowing it to be easily separated from the solution.
Filtration: Once the precipitate has formed, it needs to be separated from the remaining solution. This is usually achieved through filtration, where the precipitate is collected on filter paper while the filtrate passes through.
Weighing the Isolated Constituent: After filtration, the precipitate is dried or ignited to remove water, and then weighed. This weight is used to determine the amount of the analyte in the original sample.
Computation: The final step is to compute the amount of the particular constituent in the sample from the observed weight of the isolated substance.

These steps provide a highly precise and accurate measurement of the analyte, although the process can be time-consuming.

Purity of the precipitate

In gravimetric analysis, the purity of the precipitate is crucial for accurate results. To ensure the purity of the precipitate, it’s important to control the conditions under which the precipitate forms. This includes factors such as the concentration of the solution, the temperature, and the rate at which the precipitating agent is added.

In addition, the precipitate should ideally consist of perfect crystals that are large enough to be easily washed and filtered. The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed.

Low Solubility: The precipitate must be of low solubility. If the precipitate is not sufficiently insoluble, it will remain in solution, leading to an underestimation of the analyte.
High Purity: The precipitate must be of high purity. Impurities can lead to an overestimation of the analyte. For example, if an impurity also precipitates out of solution, it will add to the mass of the precipitate, causing the analyte to appear more abundant than it actually is.
Known Composition: The precipitate must be of known composition. This is necessary to relate the mass of the precipitate back to the mass of the original analyte.

Coprecipitation

This is a phenomenon where impurities or other soluble substances are carried down with the precipitate during the precipitation process. In gravimetric analysis, coprecipitation can be a problem because undesired impurities often coprecipitate with the analyte, resulting in excess mass and inaccurate results. This problem can often be mitigated by “digestion” (waiting for the precipitate to equilibrate and form larger and purer particles) or by redissolving the sample and precipitating it again. There are three main mechanisms of coprecipitation: inclusion, occlusion, and adsorption.

Post precipitation

This occurs when a precipitate, standing in contact with the mother liquor (the remaining solution after the precipitate has been removed), becomes contaminated by the precipitation of an impurity on top of the desired precipitate. This can happen if the solution is not immediately filtered after precipitation, allowing for additional reactions to occur. Post precipitation can lead to an overestimation of the analyte, as the additional precipitate adds to the overall mass.

Both coprecipitation and post precipitation can affect the purity of the precipitate and the accuracy of the gravimetric analysis. Therefore, careful control of the precipitation conditions and prompt and proper handling of the precipitate are crucial to minimize these effects.

Estimation of barium sulphate

Sample Preparation: A pre-weighed sample of the unknown sulfate salt is dissolved in water.
Precipitation: An excess of aqueous barium chloride is added to the aqueous solution of the unknown salt. This results in the precipitation of all the sulfate ions as barium sulfate. The precipitation reaction is as follows.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Filtration: The barium sulfate precipitate is collected by filtration.
Drying and Weighing: The precipitate is then dried and weighed. This weight is used to determine the amount of the sulfate in the original sample.
Computation: The final step is to compute the amount of sulfate in the original sample from the observed weight of the precipitate. The percentage of barium can be calculated using the following formula.

%Ba=(Weight of sample×Molecular weight of BaSO4)/ (Mass of BaSO4×Molecular weight of Ba)×100

This method provides a highly precise and accurate measurement of the sulfate content in the sample.

Summary

Gravimetric analysis is a quantitative method in analytical chemistry for determining the amount of an analyte based on its mass. The process involves precipitation, filtration, drying, and weighing. The analyte is precipitated from the solution, collected by filtration, and weighed. Impurities can affect the accuracy of the results, so ensuring the purity of the precipitate is crucial. This method is highly precise and accurate but can be time-consuming. It’s commonly used to determine the atomic masses of many elements and the composition of mixtures.