Practical Skills and Techniques

• Describe the technique of colorimetry:

  • Colorimetry uses the relationship between colour intensity of a solution and the concentration of the coloured species present - the higher the intensity, the higher the concentration.
  • A colorimeter (or spectrophotometer) is used to measure the absorbance of light of a series of standard solutions (of varying concentrations) at specific wavelengths.
  • A filter used should be the complementary colour of the solution being analysed.
  • Blanks are used (water) in between the solutions of different concentrations.
• Describe how the results of a colorimetry experiment can be used:

  • A graph of absorbance vs concentration is plotted from the data. This is known as a calibration curve - and should form a straight line.
  • The absorbance of an unknown solution can then be measured and the calibration graph is used to determine the concentration of the solution.
  • If the measured absorbance lies outwith the calibration curve, the sample must be diluted until it falls within range.
• Describe the technique of heating under reflux:

  • Heating under reflux is a way of heating a reaction over an extended period of time without any vapours escaping.
  • The water must go in at the bottom of the condenser and come out the top. (The condenser must cover the whole of the neck of the flask). The condenser will condense any reactant or solvent that has evaporated and escaped the flask which will drip back down.
  • A heating mantle is used to heat the round bottom flask and anti-bumping granules are used to control it.
• Describe the technique of vacuum filtration:

  • Vacuum filtration is a way of separating solids and liquids. The reduced pressure speeds up the filtering and is a much faster method compared to gravity filtration.
  • This method uses a Buchner funnel (a Hirsch/sintered glass funnel can also be used) connected to a vacuum flask. The flask has an arm at the side to connect to a way of creating a vacuum, often this is done with a water pump.
• Describe the technique of recrystallisation:

  • Recrystallisation is a method of purifying the product made in an organic reaction. Purifying is important as it removes any impurities and side products formed during synthesis.
  • It involves adding the product to a hot solvent to dissolve the product and letting the product recrystallise as it cools down. Any soluble impurities will stay dissolved in the solvent.
• Describe the specific steps of recrystallisation:

  1. Add a minimal volume of hot solvent to your synthesised product.
  2. Filter the hot mixture to get rid of any insoluble impurities.
  3. Let the solvent slowly cool down which will crystallise the desired product.
  4. Filter off the product which will remove any soluble impurities left in the solvent.
• What is important to consider when choosing a solvent?

  • If the product is soluble in the solvent at high temperatures but minimally soluble at room temperature.
  • Whether it reacts with the products.
  • The solubility of the impurities.
• Describe the technique of melting point analysis:

  • The purity of a sample can be determined by working out its melting point and comparing it to a known melting point value from literature.
  • The melting point range is the temperature range over which it starts to melt to when it finishes melting.
  • Impurities in the sample lower and broaden the melting point since they disrupt the intermolecular bonding in the crystal lattice.
• Describe the technique of mixed melting point analysis:

  • This experiment requires us to mix our sample with a pure sample of what we think we synthesised. If we intended to make benzoic acid we would check by mixing it with pure benzoic acid.
  • If the melting point is at the correct literature value and the range is narrow then we have made what we intended to.
  • If we have a different product, the pure sample will act as an impurity and lower the melting point.
• Describe the technique of thin-layer chromatography:

  • TLC allows us to check if there are any impurities in our sample - chromatography separates compounds in a mixture by their polarity.
  • TLC uses a film of silica or aluminium oxide as the stationary phase.
  • The sample is spotted into the baseline and a solvent (mobile phase) carries the compound up the TLC plate. How far it travels depends on the solvent used and the temperature of the TLC plate.
  • A developing agent or UV light is used to visualise the spots on the chromatogram.
• Describe how the results of a TLC experiment can be used:

  • The Rf value can be calculated for each spot which allows us to compare different spots.
  • Rf=distance travelled by the sample (a)/distance travelled by the solvent (b)
  • If there is only 1 spot in our sample then we know there are no impurities.
  • If the Rf value for our sample is the same as the Rf value for the pure compound then we have identified what compound we have in our sample.
• Describe distillation:

  The process of separating compounds by their boiling point.

  More volatile substances with lower boiling points will pass over first in the distillation process.

  It will not work if the boiling points of both compounds are similar.

  It can be used to identify compounds as the boiling point of the fraction collected can be compared to literature values.

  It allows you to purify substances by only collecting any distillate within a certain range of the target boiling point.

  Anything collected outside this range will be impurities in the distillation process.
• Describe the process of producing results of a solvent extraction:

  • Once carried out, 2 layers are formed in the funnel. The substance being extracted will dissolve in both solvents and an equilibria is established.
  • The ratio of solute dissolved in both solvents is determined by the equilibrium constant (k).
  • k=substance in upper / substance in lower.
  • Once the flask has been shaken and left to settle into layers the lower layer is run off into a container and the upper into a second container. The process is repeated to maximise the quantity of substance extracted.
• Describe how solvent extraction can be improved:

  • It can be improved by using smaller volumes of solvent more often (instead of doing it once with 50 cm^3 it is better doing it twice with 25 cm^3 each time).
• Describe the technique of solvent extraction:

  • If our desired product is present in an aqueous solution we can use a second solvent to extract it. Often this means leaving behind any impurities, this is done in a separatory funnel.
  • The choice of the second solvent is critical. It should be:
  • Immiscible with the original solvent (usually water).
  • One in which the desired substance is more soluble.
  • Volatile to allow the desired product to be easily separated from the solvent.
  • Unreactive with the substance.