Key Factors that Affect Precipitation in Gravimetric Analysis

The analysis of the composition of mixtures helps understand how to recreate the mixture and judge its purity and composition. Gravimetric analysis is the field of quantitative chemical analysis that helps determine the composition of mixtures and purity. One of the most widely used methods in quantitative chemical analysis to determine the purity of a compound is precipitation. Gravimetric analysis relies on the most basic factors or the dry weight of different components that make up a compound mixture.

Gravimetric Analysis

We know what colloids are and what is a colloidal precipitate. However, it is also important to know the constituents of a colloid or a mixture. To understand the chemical properties of a mixture, a basic understanding of its components is required. One way of doing this is through gravimetric analysis. Gravimetric Analysis deals with extracting the constituents that are present in the sample. It is chiefly concerned with the dry weight of the constituents and its contribution to the composition of the mixture.

Gravitational analysis relies on the underlying concept of changing the phase of the dissolved substance to extract it from the mixture to then measure it. There are four main methods that gravitational analysis employs to extract the required components from different mixtures. The four methods are precipitation, volatilization, electroanalytical, and other miscellaneous physical methods. From these, precipitation is the most widely used method. 

The process of gravimetric analysis using precipitation is as follows:

The sample that is to be used for the measurements is taken. If the sample is not available in the form of a solution or a colloid, a solution is made. Then a precipitating agent is mixed with the solution or the colloid to start the precipitation process. Once the precipitate is formed, it is removed from the solution. Then the precipitate is dried and weighed to achieve the required results.

Solubility of Precipitate

In gravimetric analysis, it is necessary to form the precipitate of all these components to judge the different components that make up a compound. A special reagent called the precipitating reagent is added to the mixture to form the precipitate of all these components.

Once added to a mixture, the precipitating agent causes a specific component of the mixture to form a compound that settles down due to the particle size and the effect of gravity. Gravimetric analysis relies on extracting the formed precipitate and then measuring its dry weight.

However, the component that forms the precipitate is also the solvent in the solution we are analysing. Hence, the formed precipitate may get dissolved in the solution that is being analysed. The dissolution of the precipitate in the solution will result in a loss of mass of the precipitate, resulting in the wrong weight during gravimetric analysis. Hence the solubility of the precipitate has to be very low to get accurate results during gravimetric analysis. Therefore a reagent must be selected such that the precipitate formed has low solubility in the solution that is being analysed.

Impurities in the Precipitate

A precipitate is formed in solutions with a higher amount of dissolved solids. Therefore it is possible that when a precipitate is formed, it has a high amount of impurities too. This is a source of great concern as it can interfere with the results obtained during gravimetric analysis. The surface of the precipitate formed is the most likely to be impure because it is the most prone to physical and chemical interactions with other reagents. This happens due to the structure of the precipitate that is formed. A precipitate formed in a solution always has a crystalline structure with cations and anions present in the structure. At the surface of the precipitate, there are always particles with a certain partial charge which result in the surface of the precipitate being more prone to physical and chemical interactions. If there are too many impurities in a precipitate, the best method to remove these impurities is to undergo reprecipitation, in which the precipitate is extracted from its parent solution and then dissolved in a different solution, and then precipitation is done again.

Particle Size of the Precipitate

As discussed, precipitates are formed when crystalline structures are built between the particles of a certain component. Therefore, the formation of a precipitate is dependent on the particle size. Larger particle sizes are preferred since they have purity and are easier to filter and form precipitates quicker for gravimetric analysis. In case of impurities in large particle sizes, it is easier to wash them and remove the impurities than smaller particles. Hence, in gravimetric analysis, it is preferred that when the precipitate from the solution is formed, larger particles are included in the precipitate. This ensures high purity of the precipitate, and the measurement of the weight of the precipitate does not considerably vary.

However, the production of large particles for the precipitate is a time consuming and difficult task. Hence, a balance has to be found between the size of the particle and the time that will be taken to produce a precipitate with those particles.

Conclusion 

The composition of different compounds is an important aspect of quantitative chemical analysis. An important method to perform quantitative chemical analysis is through gravimetric analysis. Gravimetric analysis deals with dry weights of the components that make up a mixture and tries to find each component’s contribution to the mixture by its weight. A popular method of performing gravimetric analysis through precipitation where a precipitate is found by adding a reagent to the analyte. The factors that affect the precipitation in a gravimetric analysis deal with the precipitate solubility, the particle size of the precipitate, and impurities present in the precipitate.