Chapter 1

Topic 2

Separation and Purification

1Importance of purificationIt is important that substances used in the food, pharmaceutical and chemical industries are pure.TestPure substanceImpure substanceMelting point of a solidSharp/definite temperatureMelts over a temperature range.T pure > T impureBoiling point of a liquid(the impurity is a dissolved solid)Sharp/definite temperatureBoils over a temperature rangeT pure < T impure

Boiling point of a liquid(the impurity is another liquid)Sharp/definite temperature

Boils over a temperature range - starts to boil at the b.p. of one liquid and rises to the b.p. of the otherChromatographyUsually produces only 1 spot on chromatogramProduces more than one spot2Separating and purifying substancesTo make sense of the material world around us, we need the methods for physically separating the many and varied mixtures that we come across.

The most useful separation method for a particular mixture depends on:1. the type of mixture, and2. which substance in the mixture we are most interested in3MixtureMethodExamplesolid + solid (powdered mixture)use some difference in properties (density, solubility, sublimation, magnetism)iron powder from sandsuspension of solid (insoluble) in liquid filtration or centrifugationsand from sea watertwo/more of immiscible liquidsDecantation or separating funnelwater and oilsolution of solid in liquidto obtain solid evaporation/crystallisation

to obtain liquid distillationsalt from sea water

pure water from aqueous copper sulphatetwo/more liquids mixed together (miscible)fractional distillation (based on differences in boiling point)to separate different petroleum fractionssolution of two/more solids (soluble) in a liquidchromatographya mixture of food colouring in food sample4Separating mixture of solids

5Separating insoluble solids from liquids -1-

Filtration6Separating insoluble solids from liquids -2-

Decantation

7Separating immiscible liquids

8Separating solution of solid in liquid to obtain solid -1-

Crystallisation9Separating solution of solid in liquid to obtain solid -2-

Evaporation to dryness

10Separating solution of solid in liquid to obtain liquid

Distillation

11Separating two/more miscible liquids Fractional Distillation

12ChromatographyChromatography is used toseparate the components in a sample, identify the number of components in a sample, identify the components present in a sample, determine the purity of a sample.

Chromatography is a technique of using a solvent to separate a mixture into its components.

Paper chromatography is probably the simplest form to set up and is very useful if we want to analyse the substances present in a solution.

13Paper chromatography

Solvent e.g. ethanolChromatography paperMixture to be separatedPencil line14

For example, we need to determine the purity of a food colouring (which may be made up of different dyes). A spot of the food colouring is applied to the chromatography paper. The chromatography paper is dipped in ethanol (the solvent). Ethanol travels up the paper, carrying the dyes along. A dye that is more soluble in the solvent will travel further.How does chromatography work? The chromatography paper with the separated components is called a chromatogram. The chromatogram shows that the food colouring is a mixture of two dyes. It is not pure.

How do we interpret the result of chromatography? However, if there is only one spot on the chromatogram, it means the substance is pure.

How does the chromatogram of a pure substance look like?The positions of the solvent front and spot on a chromatogram depend on how long the experiment was allowed to run.

Chromatogram after a period of time.

Chromatogram after a longer period of time.What are Rf values? -1-

This ratio is called the Rf value of the substance.

Rf value = distance moved by the substance distance moved by the solvent front

The ratio between the distance travelled by the substance

and

the distance travelled by the solvent is a constantWhat are Rf values? -2-

Chromatogram after a period of time.

Chromatogram after a longer period of time.Rf = 3 cm / 4.5 cm = 0.67Rf = 3.5 cm / 5.2 cm = 0.67Examples notice the same Rf values

The Rf value of a substance does not change as long as chromatography is carried out under the same conditions (i.e. same solvent and same temperature).

This property allows us to easily identify a substance on a chromatogram. Rf values

Chromatography was performed on a sample of food colouring (X) and 4 banned dyes ( A, B, C and D).

If X contains any of the 4 banned dyes, it is not safe to be consumed.

XABCDHow can we identify a banned substance present in food colouring? - Identical dyes produce spots at the same height. - Sample X does not contain the banned dyes A, B and D. - However, X contains the banned dye C. Therefore, it must not be consumed.

XABCDWhat conclusions can be drawn from this chromatogram?

XABCD- Dyes A and D are pure. - Both dye B and dye C are mixtures of two different dyes. - Sample X is a mixture of three dyes. What other conclusions can be drawn from this chromatogram?How do we identify colorless substances?Chromatography can also be used for colorless substances such as amino acids.

To separate and analyse colorless substances, we apply a locating agent on a chromatogram. Quote today