The Colloidal State in Gravimetric Analysis

The world we observe is an interaction of particles. Everything is the result of two more particles mixing with each other. Therefore it is imperative to study these mixtures and see what kind of interactions occur between these mixtures. One very important type of mixture is colloids. Colloids are a type of mixture in which the particles are dispersed in a medium but not dissolved in it.

What are Colloids?

When two substances are mixed, there can be various possibilities in their mixture. They could either form a mixture but still be distinctly separated, or they may mix together and become impossible to separate. One of these possibilities is the solvent dispersing in the solute but not dissolving in it. Dispersion means the distribution of a particular substance. If two substances, A and B, are solute and solvent, let us see how they can form a colloid.

When substance A is dispersed in substance B, it means that the particles of substance A are distributed amongst the particles of a substance to such an extent that they cannot be seen separately via a microscope; however, the particles have not mixed with each other to form one entity. This is the fundamental difference between colloids and solutions.

Colloidal Precipitate Definition

A colloidal solution is the dispersion of the particles of one type of substance amongst another substance that acts as the solute. The effect of gravity on colloidal particles is not relevant, and hence colloidal particles do not settle down easily. But that does not mean that precipitates cannot be formed in colloidal solutions.

Precipitates can be formed in colloidal solutions by adding a colloid with the opposite charge, or by adding electrolytes, or heating and stirring. When any of this is done, a solid mass is formed that is called a precipitate.

In a colloidal solution, when a precipitating agent is added, the minute particles in a colloid attach themselves to each other resulting in the formation of larger particles. The formation of larger particles results in these larger particles not being soluble in the solute anymore; hence this causes them to settle down. However, it is not possible to convert all the particles of the solute in the solvent to become a precipitate.

Therefore a colloidal precipitate definition can be given as the precipitate that is formed when the particles of the colloid collate to become larger particles.

What is Gravimetric Analysis?

We know what colloids are and what is a colloidal precipitate. But what are the constituents of a colloid or a mixture? 

One way of understanding this is through gravimetric analysis. Gravimetric Analysis deals with extracting the constituents that are present in the sample. It is concerned with the dry weight of the constituents and their contribution to the composition of the mixture.

The gravitational analysis relies on the underlying concept of changing the phase of the dissolved substance to extract it from the mixture so that it can be measured. There are four main methods that gravitational analysis employs for the extraction of the required components from different mixtures.

The four methods are precipitation, volatilization, electroanalytical and other miscellaneous physical methods, out of which precipitation is the most widely used one. The process in gravimetric analysis that uses precipitation is as follows:

A sample is taken for measurements. If the sample is not available in the form of a solution or a colloid, then 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.

Are Colloidal Suspensions Suitable for Gravimetric Analysis?

Gravimetric analysis is done to find the weight of the desired component in a mixture. But a fundamental drawback or loophole in the gravimetric analysis is that it cannot compensate for the purity of the component. In other words, all the results of the gravimetric analysis are made with the assumption that the precipitate that is obtained from the experiment is hundred percent pure.

But no precipitate is hundred percent pure due to the various components in a mixture. This brings us to a dilemma. Can we use colloidal suspensions for gravimetric analysis?

The colloidal suspensions we come across are very hard to precipitate. When a colloidal precipitate is formed, it consists of all the colloidal particles that have a greater size than the threshold for a given colloidal solution.

Hence when a precipitate is formed, it is very likely for it to contain various impurities that are present in the solution. Therefore when a gravimetric analysis is done, the experimental weight of the sample will give a false level of contribution by the substance in the sample compositions.

Conclusion

Colloidal solutions are formed when there are too many particles of the solvent such that they disperse in the solvent. The substance that is dispersed in the solvent can be separated from the solvent and made independent. This can be done by using a specific hardware tool, and by adding the precipitating agent, the solute can be extracted from the solvent, and measures can be made.

The gravimetric analysis deals with finding out the composition of a given mixture by extracting the component and then weighing it. Colloidal solutions are not good for gravimetric analysis since there are a lot of impurities in the colloidal precipitate that is collected due to various other components being present in the colloid. A colloidal state example, fog, mist, etc.