Many of the materials are obtained from natural products or are prepared in a laboratory for which it is necessary to obtain certain compounds and they must be previously separated, so knowing the methods of separation of substances in a mixture is important for chemists and their application at the industrial level. The separation processes used in the laboratory, although simple, are the same as those used in industry because they are practical, which makes them particularly important.
Among the applications of the methods are the distillation process, in the food industry for example it is carried out in the preparation of alcoholic beverages, or in the oil refining industry where fractional distillation is used. Evaporation can be applied in the production of salt from sea water, among many other examples that we can cite.
A mixture is formed by the union of two or more components where their chemical properties are maintained, these components can be solid, liquid or gaseous and are classified as homogeneous and heterogeneous. A homogeneous mixture is one in which its components cannot be differentiated at a glance and a heterogeneous mixture is one whose composition is not uniform and its components can be easily distinguished. When selecting the separation method for a given mixture, it should be done according to the particular properties of the components present in the sample, it may happen that a process is suitable for application in a given mixture while for others it is not suitable.
Among the physical properties on which the methods to be studied are based we can find:
Density: it is defined as the magnitude that expresses the relationship between the mass of a substance and the volume it occupies, it depends on the pressure and temperature, generally when there is an increase in pressure, its density increases and when the temperature increases its density decreases.
Solubility: is given by the maximum concentration of solute that can be dissolved in a volume of solvent at a given temperature. It is the fundamental basis for determining whether it is possible to obtain a precipitate (solid) when mixing two solutions.
Methods for separating the components of a mixture
Methods are nothing more than procedures that make it possible to separate two or more components of a mixture without altering their identity or causing any change in their chemical properties. They can be used for both homogeneous and heterogeneous mixtures and are basically based on physical processes. Among the methods we can find decantation, filtration, evaporation, crystallization, magnetic separation.
Let's review some of these methods.
Methods for the separation of mixtures containing solid and liquid substances (S-L)
One of the simplest, it is applicable when working with mixtures where the liquids are immiscible or when it contains solids that are not soluble in the liquid.
Principle of the method: It is based on the difference in densities of the compounds.
Equipment used: To achieve the separation, an apparatus called a separating funnel is used, which contains a valve at the bottom and a stopper at the top. The mixture is left to stand there until the substance with the highest density is deposited at the bottom of the funnel and the substance with the lowest density on top of it.
Steps for the use of the funnel
Applications: This separation process is widely used in laboratories and at industrial level. Very important, it should not be confused with sedimentation whose principle of separation is by the effect of gravity. It can be used, for example, in the treatment of wastewater, since contaminated water is usually denser than clean water due to the substances and suspended particles it contains, so that in the purification process it can be initially subjected to a filtration process and successive decantations. Another example is the separation of milk cream, where the milk at rest separates the curd, which is a yellowish and dense substance from the rest of the milk, and is then removed mechanically. It is also used in the preparation of juices, sauces, vinegars, among others.
In the production of biofuel from vegetable fats or oils such as coconut oil, it is used to separate the products of the transesterification reaction.
Principle of the method: it is based on the separation of non-soluble solids in liquids. During the filtration process, the heterogeneous mixture passes through a filter that must have a suitable pore size, the solid that is trapped in the filter paper is known as residue and the liquid that passes through the paper is called filtrate.
Equipment used: a mechanical means such as a filter or sieve is used. There is a variety of filters, they can be made of cloth, metallic or plastic nets and are also manufactured from different papers.
Applications: It is widely used both at industrial level, in laboratories and in daily life. For example, from the simplest, in the preparation of coffee we place the ground coffee in a filter, either of cloth or paper, pour hot water over it, in this way the filter allows the passage of the finest particles that have the strong flavor of the coffee and the residue remains in the filter. It is also used in the preparation of infusions, air filters, water filters, oil filters, sewage screens, among others.
At the laboratory level, filter paper is frequently used for the separation of water and easily soluble substances, it is a porous paper that retains even very small particles but allows water to pass through. There are different sizes and it is generally coupled to a funnel for its correct use.
Methods for the separation of mixtures containing liquid-liquid substances (L-L)
Principle of the method: based on the affinity of the components of the mixture either in the stationary phase (stationary phase) or mobile phase (moves through the stationary phase). There are several types including paper chromatography, gas chromatography, liquid chromatography and column chromatography.
In paper chromatography for example, the separation is achieved by washing the components along a paper with the solvent. The stationary phase is determined by a strip of filter paper and the mobile phase is a solvent containing the sample to be treated. The method consists of placing a few drops of the solvent containing the sample at one end of the paper and waiting for the liquid to advance through the paper, then allowing it to dry, and if the components of the sample have different colors, it is possible to observe the different positions on the paper.
Next I show you the initial form in which the technique was applied, in the left is the filter paper with the sample of green edible colorant and then the solvent water was added to it. Thus, as time goes by, the yellow color (low tonality) and blue color are separated. The dry filter paper corresponds to the support of the analysis known as chromatogram.
Gas chromatography works by injecting the sample into the mobile phase, which passes through the stationary phase, where the components will separate according to how well they are related to one phase or another. Once they are separated, they pass through a detector that identifies them and measures their concentration or some other property that represents a measure of their amount in the sample.
Applications: It can be applied in samples where it is more difficult to separate their components, in the analysis of blood samples, urine samples, in the environmental area to determine the levels of contamination in water or air, wine quality studies, among others.
In conclusion we can say that most of the materials that we find in daily life are made up of mixtures of substances, and that many times they have to be subjected to separation processes to obtain pure chemical substances that can be used as raw material in other processes.
It is important to select very well the separation method according to the properties of the components of the mixture and to have the proper knowledge of the technique, materials and equipment used in each one of them.
Until next time, thanks for reading!
- Whitten K. and Gailey, K. (1985). General Chemistry. Mexico: Nueva Editoral Interamericana
- Atkins, P. and Jones, L. (2006). Principles of Chemistry. The paths of discovery. Buenos Aires: Médica Panamericana