separation and purification
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What does 'separation and purification' mean in chemistry?
In chemistry, separation and purification refers to the processes of isolating and refining the components of a mixture. Separation methods change the relative amounts of substances in a mixture, and in the act of separation, some particles are either partially or totally removed from the sample. Since ancient times, people have used these methods to improve the quality of life by extracting metals from ores and medicines from plants.
These processes have numerous applications in fields like medicine and manufacturing. For instance, the pharmaceutical industry uses separation and purification in the manufacture of natural and synthetic drugs.
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Why do scientists need to separate substances in chemistry?
Scientists separate substances to isolate desired materials from mixtures, a process called purification. This is vital in drug manufacturing to ensure uniform potency and eliminate harmful components.
Separation is also essential for analyzing a sample's composition, especially when dealing with low concentrations of substances like air pollutants. Techniques like adsorption concentrate pollutants for straightforward analysis. Additionally, separations remove interfering impurities, which is crucial in analyzing trace metals in samples like river water, where organic substances can cause errors.
Various methods, including chromatography and distillation, are employed based on the quantity and properties of the materials being processed.
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What is a mixture, and how is it different from a pure substance?
A mixture is a combination of two or more substances that are physically combined. Mixtures can be either homogeneous (uniform throughout) or heterogeneous (non-uniform composition). Examples include air, salt water, and sand. Mixtures can be separated by physical methods.
A pure substance consists of only one type of element or compound with a definite chemical composition and distinct chemical properties. Elements and compounds are pure substances. Unlike mixtures, pure substances cannot be separated by physical means.
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What are some common methods used to separate mixtures?
Mixtures can be separated through methods that exploit differences in physical or chemical properties of the components. Common techniques include:
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Distillation Separates liquids based on boiling points; the vapor is collected and condensed.
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Filtration Separates particles by size using a porous material. It is often used for separating solids from liquids.
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Magnetic separation Separates substances based on their magnetic properties.
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Chromatography Separates substances as they travel at different rates through a column packed with a solid material.
Other methods include evaporation, decantation, and the use of selective solvents to dissolve one component of a mixture, leaving others behind. For azeotropic mixtures, techniques like using entrainers, pressure-swing distillation, and pervaporation are employed.
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How does filtration work to separate substances?
Filtration separates substances by using a porous material to retain solid particles from a liquid or gas. A filter medium allows the fluid to pass through while trapping the solid particles. The basic requirements for filtration are a filter medium, a fluid with suspended solids, a driving force such as a pressure difference to cause fluid to flow, and a mechanical device (the filter) that holds the filter medium, contains the fluid, and permits the application of force.
Filtration is used to purify water for cities, where the water is filtered through sand, and also has many other applications.
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How does distillation help purify a liquid, and what everyday examples use this process?
Distillation purifies liquids by exploiting differences in boiling points. The process involves boiling a liquid and converting it to vapor, which is then cooled and condensed back into a liquid, separating it from other substances.
For instance, in petroleum refining, crude oil is distilled into fractions like gasoline and kerosene based on their boiling points. Also, distillation is used to produce alcoholic liquors from fermented materials and to desalinate seawater. Chemists also use distillation to purify synthetic products. Fractional distillation is employed when boiling points are close, using a vertical column for repeated condensation and revaporization.
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separation and purification, in chemistry, the combination of processes used to isolate and refine components of a mixture. Separation and purification have a large number of applications, particularly in fields such as medicine and manufacturing.
General principles
Since ancient times, people have used methods of separating and purifying chemical substances for improving the quality of life. The extraction of metals from ores and of medicines from plants is older than recorded history. In the Middle Ages the alchemists’ search for the philosopher’s stone (a means of changing base metals into gold) and the elixir of life (a substance that would perpetuate youth) depended on separations. In the industrial and technological revolutions, separations and purifications assumed major importance. During World War II, for example, one of the main problems of the Manhattan Project, the U.S. government research project that led to the first atomic bombs, was the separation of uranium-235 from uranium-238. Many industries now find separations indispensable: the petroleum industry separates crude oil into products used as fuels, lubricants, and chemical raw materials; the mining industry is based on the separation and purification of metals.
Separations and purifications also find their places in medicine and the sciences. In the pharmaceutical industry, for example, separation and purification are used in the manufacture of natural and synthetic drugs to meet health needs. In the life sciences, many advances can be directly traced to the development of each new separation method. The first step in understanding the chemical reactions of life is to learn what substances are present in samples obtained from biological sources. The challenge and power of such separations is demonstrated in the two-dimensional gel electrophoretic separation of sulfur-35 methionine-labeled polypeptides, or proteins, from transformed epithelial amnion cells (AMA).
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Basic concepts of separations
This section is concerned with separations of the smallest subdivisions of matter, such as atoms, molecules, and minute particles (sand, minerals, bacteria, etc.). Such processes start with a sample in a mixed state (composed of more than one substance) and transform it into new samples, each of which—in the ideal case—consists of a single substance. Separation methods, then, can be defined as processes that change the relative amounts of substances in a mixture. In chemical methods, one may start with a completely homogeneous mixture (a solution) or a heterogeneous sample (e.g., solid plus liquid). In the act of separation, some particles are either partially or totally removed from the sample.
Reasons for making separations
There are two general reasons for performing separations on mixtures. First, the mixture may contain some substance that should be isolated from the rest of the mixture: this process of isolating and thus removing substances considered to be contaminants is called purification. For example, in the manufacture of synthetic drugs, mixtures containing variable proportions of several compounds usually arise. The removal of the desired drug from the rest of the mixture is important if the product is to have uniform potency and is to be free of other components that may be dangerous to the body.
The second reason for performing separations is to alter the composition of a sample so that one or more of the components can be analyzed. For example, the analysis of air pollutants to assess the quality of the air is of great interest, yet many of the pollutants are at a concentration too low for direct analysis, even with the most sensitive devices. Pollutants can be collected by passing samples of air through a tube containing an adsorbent material. By this process the pollutants are concentrated to a level such that straightforward analysis and monitoring can take place. In a second example, several impurities in a sample may interfere with the analysis of the substance of primary interest. Thus, in the analysis of trace concentrations of metals in rivers, organic substances can cause erroneous results. These interferences must be removed prior to the analysis. Several techniques for removing interferences are discussed in analysis: Interference removal.
Classification of separations
There are a variety of criteria by which separations can be classified. One is based on the quantity of material to be processed. Some methods of separation (e.g., chromatography) work best with a small amount of sample, while others (e.g., distillation) are more suited to large-scale operations.
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Classification may also be based on the physical or chemical phenomena utilized to effect the separation. These phenomena can be divided into two broad categories: equilibrium and rate (kinetic) processes. Table 1 lists some separation methods based on equilibria, and Table 2 indicates those methods based on rate phenomena.
| barrier separations | field separations |
|---|---|
| membrane filtration | electrophoresis |
| dialysis | ultracentrifugation |
| ultrafiltration | electrolysis |
| electrodialysis | field-flow fractionation |
| reverse osmosis |
| gas-liquid | gas-solid | liquid-solid | liquid-liquid | supercritical fluid-solid | supercritical fluid-liquid |
|---|---|---|---|---|---|
| distillation | adsorption | precipitation | extraction | supercritical-fluid chromatography | supercritical-fluid extraction |
| gas-liquid chromatography | sublimation | zone melting | partition chromatography | ||
| foam fractionation | crystallization | ||||
| ion exchange | |||||
| adsorption | |||||
| exclusion | |||||
| clathration |