Unit 15: Basic Separation Techniques — Long Questions

Analytical chemistry

Definition Analytical chemistry is the science of chemical characterization.

Analysis A complete chemical characterization of a compound must include both qualitative and quantitative analysis.

Types of analysis There are two types of analysis:

(i) Qualitative analysis: In qualitative analysis, the chemist is concerned with the detection or identification of the elements present in a compound.
Example: There are three types of elements in glucose which are C, H and O it is called qualitative analysis of glucose.

(ii) Quantitative analysis: In quantitative analysis, the relative amounts of elements in compounds are determined.
Example: There are six carbons, twelve hydrogen and six oxygen atoms which are total of 24 atoms in glucose (C6H12O6). It is called quantitative analysis of glucose.

Steps of analysis A complete quantitative determination generally consists of four major steps:
(i) Obtaining a sample for analysis.
(ii) Separation of the desired constituent.
(iii) Measurement and calculation of results.
(iv) Drawing conclusion from the analysis.

Filtration

Definition The process used to separate one of the components of a two-component heterogeneous mixture from a solution is called filtration.

Example A mixture of common salt and sand is dissolved in distilled water. The common salt will distribute itself throughout water to give a homogeneous mixture. This homogeneous mixture is called a solution of common salt in H2O. common salt in this solution is called a solute, while H2O is called a solvent. In a solution, solvent is that component, which is present in excess as compared to the solute. While common salt disappears in water, the other component, sand remains undissolved and after some time settles down at the bottom of solution. The process of filtration can be used to separate sand from the above solution.

Steps The first step is to select a suitable solvent in which one of the components dissolves completely while the other remains practically insoluble.

Types of filter media

It can be performed with several types of filter media. Nature of the precipitate and other factors dictate which filter medium must be used. The most convenient ways of filtration are:
(i) A filter paper.
(ii) A filter crucible.

(a) By using filter paper

Folding of filter paper is important and is done in two ways. The first one will make simple filter paper and second one will make fluted filter paper.

(i) Filtration by a glass funnel and filter paper is usually a slow process. As the mixture is poured onto the filter paper, the solvent (water) passes through leaving behind the suspended particles on the filter paper. Filter papers are available in a variety of porosities (pore sizes). Which pore size is to be used, depends upon the size of particles in the precipitate. The filter paper should be large enough so that it is one-fourth to one-half full of precipitate at the end of filtration. In this way, the filtrate runs down the side of beaker without splashing.

Folding of filtration paper

• First fold along the diameter.
• Second fold should be such that edges do not quite match and then form a cone.
• The folded filter paper may then be inserted into 60-degree funnel, moistened with water and firmly pressed down.

(ii) Fluted filter paper: The rate of filtration through conical funnel can be considerably increased using a fluted filter paper.

Preparation In preparation of such a paper ordinary filter paper in folded in such a way that a fan likes arrangement with alternate elevations and depressions at various folds is obtained.

Filtrate After a filtration the liquid which is obtained in a conical flask underneath the funnel is called the filtrate.

Residue The precipitates (sand) left behind on the funnel and collected at filter paper after the filtration is called residue.

(b) By using Crucible

Another convenient way to filter a precipitate is by suction through a crucible. Two types of crucibles are generally used

Gooch crucible

It is made of porcelain having a perforated bottom which is covered with paper pulp or a filter paper cut to its size as shown in figure. Quick filtration can be done by placing such crucible in a suction filtering apparatus. It is useful for the filtration of precipitates, which need to be ignited at high temperature. If its perforations are covered with asbestos mat, it may be used to filter solutions that react with paper e.g. concentrated HCl and KMnO4 solutions.

Sintered glass crucible

It is a glass crucible with a porous glass disc sealed into the bottom. It is very convenient to use because no preparation is needed as with the Gooch crucible.

Crystallization

Definition Crystallization is the removal of solid from solution by increasing its concentration above the saturation point in such a manner that excess solid separate out in the form of crystals.

Principle The basic principle of crystallization is the fact that the solute should be soluble in a suitable solvent at high temperature and the excess amount of the solute is thrown out as crystals when it is cooled.

First of all, a suitable solvent is selected which consist of following properties.

Choice of a Solvent

An ideal solvent should have the following features:

(i) Solubility and temperature: It should dissolve a large amount of the substance at its boiling point and only a small amount at the room temperature.

(ii) Chemical nature: It should not react chemically with the solute.

(iii) Selective solubility: It should either not dissolve the impurities or the impurities should not crystallize from it along with the solute.

(iv) Crystal formation: On cooling, it should deposit well-formed crystals of the pure compound.

(v) Economical: It should be inexpensive, safe to use and easily removable.

Common solvents

The solvents which are mostly used for crystallization

• Water
• Absolute ethanol (99.99% Ethanol)
• Rectified spirit (95% Ethanol)
• Chloroform
• Carbon tetrachloride
• Acetic acid, wood sprit (methanol), benzene etc.

S.Q. Why hot water funnel is used in crystallization?

Ans. During crystallization, sometimes the crystals start to appear during the process of filtration. This result in choking the filter paper or the runnel. To avoid this a hot water funnel may be used.

Water bath If the solvent is inflammable, then precaution should be taken while heating the solution so that it does not catch fire. In such cases, water bath is used for heating purpose.

Steps of crystallization

(i) Saturated solution.

Prepare a saturated solution of the substance in a suitable solvent at the temperature of the experiment.

(ii) Removing impurities

The insoluble impurities in the saturated solution are then removed by filtering the hot saturated solution. This avoids the premature crystallization of the solute on the filter paper or in the funnel stem. If necessary, hot water funnel should be used for this purpose.

(iii) Cooling

The hot filtered solution is then cooled at a moderate rate so that medium sized crystals are formed.

(iv) Separating crystals

When the crystallization is complete, the mixture of crystals and the mother liquor is filtered through a Gooch crucible using a vacuum pump.

S.Q. How coloured impurities are removed in crystallization?

Ans. Sometimes during the preparation of a crude substance, the coloring matter or resins products affect the appearance of product. Such impurities are conveniently removed by boiling the substance in the solvent with the sufficient quantity of finely powdered animal charcoal and then filtering the hot solution. The coloured impurities are adsorbed by animal charcoal and the pure decolorized substance crystalizes out from the filtrate on cooling.

(v) Drying of crystals

There are usually two ways for drying.

Oven The crystals may be air-dried are dried in an oven. The crystals are dried in an oven provided the substance does not melt or decompose on heating at 100° C.

Vacuum desiccators A safe and reliable method of drying crystals is through vacuum desiccators. In this process the crystals are spread over a watch glass and kept in a vacuum desiccator for several hours. The drying agents used in a desiccator are CaCl2, silica gel or phosphorus pentoxide.

Types of distillation

(i) Simple distillation

Definition A separation process that purifies a liquid by heating to its boiling point turning it to vapours, and condense them back into liquid, thus separating it from impurities.

Explanation It a two-component mixture is a solution containing a solid compound dissolved in water or in any other suitable solvent, then that mixture can be separated by simple distillation.

Example Sea water is not drinkable because it is a mixture of many soluble inorganic compounds in water. To get rid of these compounds, sea water is distilled to make it drinkable.

It is used to separate solvent form solution.

Procedure

• Take 100 cm³ sea water
• Add a few pieces of boiling chips to prevent bumping of the liquid in the flask.
• Make whole apparatus air tight to prevent vapors to escape.
• Heat the flask gently with the help of a bunsen burner
• Turn on the tap to allow cold water to circulate slowly around the condenser.
• Water or alcohol will evaporate as its boiling point is reached, and its vapours will pass through the condenser.
• The water circulating around the condenser will condense these vapours back into the liquid form.
• This is called distillate and it will be collected in the receiving flask.
• The components left behind in the distillation flask are collectively called the residue.

(ii) Fractional distillation

Definition A process used to separate components of a liquid mixture based on their different boiling points.

The process of fractional distillation is another types of distillation used to separate two liquids which are soluble or miscible with each other.

Example A mixture of water and ethyl alcohol can be separated by heating the mixture in a distillation flask.

Distillation process of Fractional distillation

Separating the two miscible liquids in this way is called fractional distillation.

Procedure

• The process of fractional distillation will be successful only if the difference in the boiling points of the two liquids being separated, is around 25°C.
• The distillation column is a glass equipment used in the distillation of mixture of liquids to separate it into its components depending upon their boiling points.
• The distillation column is filled with glass beads to increase the surface area available for condensing the vapour as shown in figure.

When a mixture of ethyl alcohol and water is heated first ethyl alcohol will be distilled over because its boiling point is 78°C Less than that of water whose boiling point is 100°C.

As long as the alcohol is being distilled over, the temperature of the thermometer will remain at 78°C.

As soon as the temperature starts rising above 78°C, replace the receiving flask with a new one as shown in figure.

Start heating again when the temperature of the thermometer rises to 100°C, water will start boiling. It is collected in the receiving flask after condensation as the second distillate.

In this way both the components will be obtained in pure form.

Interesting Information!

S.Q. What is petroleum? How is its separated?

Ans. Petroleum in natural form is a mixture of many compounds which are commonly used as fuels. Example of such fuels are petrol, diesel, kerosene oil and furnace oil. These components have boiling points not very different from one another. These components are separated as fractions by fractional distillation using a fractioning or distillation column.

Ans. Chromatography
Definition: Chromatography is a method used primarily for the separation of a sample of mixture. It involves the distribution of a solute (mixture) between a stationary phase and a mobile phase.
Stationary phase: The stationary phase may be a solid or a liquid supported as a thin film on the surface of an inert solid.
Mobile phase: The mobile phase flowing over the surface of the stationary phase may be a gas or a liquid.
Types of chromatography: The types of chromatography are two that is adsorption chromatography and partition chromatography.
(i) Adsorption chromatography: Chromatography, in which the stationary phase is a solid, is classified as adsorption chromatography. In this type, a substance leaves the mobile phase to become adsorbed on the surface of the solid phase.
Examples: Examples of adsorption chromatography are thin layer chromatography (TLC), column chromatography.
(ii) Partition chromatography: Chromatography, in which the stationary phase is a liquid, is called partition chromatography In this type, the substances being separated are distributed throughout both the stationary and mobile phases.
Examples: Examples of partition chromatography are paper chromatography and gas liquid chromatography.

Applications of chromatography
(i) Separation: The techniques of chromatography are very useful in organic synthesis for separation, isolation and purification of the products.
(ii) Analysis: They are equally important in qualitative and quantitative analyses and for determination of the purity of a substance.
(iii) Identification: Chromatography is very useful for identifying complicated chemicals such as dyes, drugs and pesticides.

Ans. Principle of chromatography
Chromatography is based on the principle that distance travelled by components of mixture depends on the distribution between stationary and mobile phase.
Paper chromatography is a type of partition chromatography.
Stationary phase: The entrapped water in cellulose fibers of paper and mobile phase which passes over the paper is immiscible. It is usually an inorganic liquid.
Mobile phase: The mobile phase is usually an organic liquid.

Arrangement of paper There are three common ways of carrying out paper chromatography namely.
(i) Ascending
(ii) Descending
(iii) Radial / Circular

Steps of paper chromatography
(i) Preparing mixture
In this technique, the solvent travels upwards by capillary action. A solvent mixture, specially composed in accordance with the sample to be separated, is poured info the chromatographic tank.

(ii) Covering the tank
Cover the tank to homogenize its inner atmosphere.

(iii) Set chromatographic paper
Take about 20 cm strip of Whatmann's chromatographic paper No.1 and draw on it a thin pencil line about 2.5 cm from one end. Spot a point. on the pencil line, with the sample mixture solution. To facilitate identification of the components of the mixture, spots of the known compounds may also be placed alongside.

(iv) Movement of components
When the spots have dried, suspend the paper with clips so that the impregnated end dips into solvent mixture to a depth of 5-6 mm. cover the tank. As the solvent front passes the spots, the solutes begin to move upward. Different components of solute will move at different rates. This separates the mixture.

(v) Drying the strip
When the solvent front has risen to about length of the paper, remove the strip, mark the solvent front with a pencil and allow the strip to dry.

(vi) Locating agents for colorless components
Once the paper is dried, the pattern on the paper is called a chromatogram. The different components of the mixture, if colored, can visually be identified.
If colorless, the chromatogram has to be developed by chemical methods or physical techniques used to identify the spots.
UV lamp: A very convenient way of visualizing the colorless chromatogram is to see it under the light of a UV lamp. The spots look colored under UV light.
Locating agents: A locating agent can be used to locate the spots. A locating agent is generally a chemical that reacts with the colorless substances like amino acids to give colored products that are visible for inspection e.g., ninhydrin.

Use of retardation factor
Each component has a specific retardation factor called the R_f value. The R_f value is given by:

R_f = Distance travelled by a component from the original spot / Distance travelled by solvent from the original spot

With reference to figure the chromatogram shows that the sample A contains both components A and B. the R_f values for B and C are given by:

R_f (A) = x/z R_f (B) = y/z

Paper chromatography is a convenient way of checking if a substance formed in a chemical reaction is pure or impure using a suitable solvent in which the substance dissolves. A pure substance produces one spot on the chromatogram while an impure substance produces more than one spot.
A paper chromatogram can also be used to identify substances by comparing them with known samples of pure substances. Two substances are likely to be identical have the same R_f values.

Ans. Checking purity of a substance
Separate methods are applied to purify compounds if they exist in liquid or solid states under normal conditions.

Purity of liquids

• Liquids are generally purified by distilling them at normal pressure or under vacuum.
• Purity of liquids can be checked by comparing their boiling points.

Aspirin

• Aspirin is prepared by reacting salicylic acid with an excess of acetic anhydride in the presence of a few drops of concentrated H₂SO₄.
• Aspirin is a very important analgesic and antipyretic.
• After the reaction is over, water is added in the reaction mixture to precipitate out aspirin. The mixture is then filtered to get aspirin as a crude product.
• It is then crystallized in water as polar solvent.

Following are the methods to check the purity of aspirin:

(i) Sharp melting point

• Aspirin obtained in the above process may be mixed with the unreacted starting material, salicylic acid.
• To check whether the product is pure or impure, its melting point is determined.
• If the product melts sharply, in this case at exactly 136°C then the product is considered as pure.
• If the product shows a melting point which is not sharp rather it melts within a range of temperature, it is considered as an impure product.

(ii) Mixed melting point
Another way of assessing the purity of the product is to get the mixed melting point. In such a case the product is mixed with a pure sample of aspirin and the melting point is then determined. If it still shows a sharp melting point, then it is a pure product.

(iii) Paper chromatography
Another way of checking the purity of aspirin is to run paper chromatography of this sample along with the pure samples of both aspirin and salicylic acid and matching the R_f value.