12.2 Separation and Purification

• Paper Chromatography is used to separate substances that have different solubilities in a given solvent
• A pencil line is drawn on chromatography paper and spots of the sample are placed on it
• The paper is then lowered into the solvent container, ensuring that the pencil line sits above the level of the solvent so the samples don't wash into the solvent container
• The solvent travels up the paper by capillary action, taking some of the colored substances with it
• Different substances have different solubilities so will travel at different rates, causing the substances to spread apart
• This will show the different components of the ink/dye
• Analysis of the composition of ink using paper chromatography
• If two or more substances are the same, they will produce identical chromatograms
• It is common practice to include a known compound as a reference spot to help match up to an unknown spot or set of spots in order to identify it
• If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
• An impure substance will show up with more than one spot, a pure substance should only show up with one spot
• Paper chromatography is the name given to the overall separation technique while a chromatogram is the name given to the visual output of a chromatography run
• Locating Agents
• Locating agents are substances which react with the sample and produce a colored product which is then visible
• The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye
• Retention Factor (Rf) Values
• These values are used to identify the components of mixtures
• The R value of a particular compound is always the same
• Calculating the R value allows chemists to identify unknown substances because it can be compared with R values of known substances under the same conditions
• The formula to calculate the retention factor is: Retention factor = Distance travelled by substance / Distance travelled by solvent
• The R value is a ratio and therefore has no units
• Using R values to identify components of a mixture
• The choice of the method of separation depends on the nature of the substances being separated
• All methods rely on there being a difference of some sort, usually in a physical property such as boiling point between the substances being separated
• Separating a mixture of solids:

  • Differences in solubility can be used to separate solids
  • For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities
  • Example: to separate a mixture of sand and salt, water is a suitable solvent to dissolve the salt, but not the sand
• Filtration:

  • Used to separate an undissolved solid from a mixture of the solid and a liquid/solution (e.g. sand from a mixture of sand and water)
  • Centrifugation can also be used for this mixture
  • Filter paper is placed in a filter funnel above another beaker
  • The mixture of insoluble solid and liquid is poured into the filter funnel
  • Filter paper will only allow small liquid particles to pass through in the filtrate
  • Solid particles are too large to pass through the filter paper so will stay behind as a residue
• Crystallisation:

  • Used to separate a dissolved solid from a solution, when the solid is more soluble in hot solvent than in cold
  • The solution is heated, allowing the solvent to evaporate and leaving a saturated solution behind
  • The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow
  • Crystals are collected by filtering the solution
  • They are then washed with distilled water to remove any impurities and then allowed to dry
• Distillation:

  • Simple & Fractional
  • Simple Distillation:
  • Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of saltwater) or a pure liquid from a mixture of liquids
  • The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask
  • The vapour passes through the condenser, where it cools and condenses, turning into pure water which is collected in a beaker
  • After all the water is evaporated from the solution, only the solid solute will be left behind
  • Fractional Distillation:
• • Used to separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two)
  • The solution is heated to the temperature of the substance with the lowest boiling point
  • This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker
  • All of the substance is evaporated and collected, leaving behind the other component(s) of the mixture
• • For water and ethanol: ethanol has a boiling point of 78ºC and water of 100ºC. The mixture is heated until it reaches 78ºC, at which point the ethanol will be separated
• Boiling point of ethanol is 78ºC and water is 100ºC
• Mixture is heated until it reaches 78ºC, at which point ethanol boils and distills out of the mixture and condenses into the beaker
• Heating should be stopped when the temperature starts to increase to 100ºC
• Water and ethanol are now separated
• Fractional distillation of a mixture of ethanol and water
• An electric heater is safer to use when there are flammable liquids present
• Separation of components in petroleum is achieved by fractional distillation on an industrial scale
• Fractional distillation of crude oil is not carried out in school laboratories due to the toxic nature of some of the components of crude oil, but it can sometimes be simulated using a synthetic crude oil made specially for the demonstration
• Student given a mixture of calcium sulfate, magnesium chloride, and water
• To obtain magnesium chloride crystals from the mixture: filtration to remove calcium sulfate, then evaporated and crystallized to obtain magnesium chloride crystals