Colorimetry uses the relationship between colour intensity of a solution and the concentration of the coloured species present.
Candidates must be familiar with use of the technique of colorimetry, including
preparing a series of standard solutions of appropriate concentration → these can be used to prepare a calibration graph
A colorimeter or a spectrophotometer is used to measure the absorbance of light of a series of standard solutions, and this data is used to plot a calibration graph.
preparing a calibration graph
The concentration of the solution being tested is determined from its absorbance and by referring to the calibration curve.
→ The concentration of coloured species in the solution being tested must lie in the straight line section of the calibration graph.
choosing an appropriate colour or wavelength of filter → this is the complementary colour to the colour of the species being tested
using a blank → this is used to zero the colorimeter before taking a reading
Distillation is used for identification and purification of organic compounds.
→ Distillation can be used to purify a compound by separating it from less volatile substances in the mixture.
→ The boiling point of a compound, determined by distillation, is one of the physical properties that can be used to confirm its identity.
Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.
When carrying out heating under reflux, the reaction mixture is placed in a round-bottomed flask with anti-bumping granules and the flask is fitted with a condenser. The flask is then heated using an appropriate source of heat.
→ Heating under reflux might be used to prevent vapours escaping, to allow extended/more vigorous heating, to dissolve/extract the maximum quantity of substance possible, or because it is more efficient.
Vacuum filtration involves carrying out a filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate. A Büchner, Hirsch or sintered glass funnel can be used during vacuum filtration.
