Paper Chromatography

• Chromatography is a physical process
• Chromatography allows us to separate substances based on their solubilities
• In chromatography you:

  • Get a piece of paper and draw a line on it
  • You do a dot of each colour on the line
  • You then place the bottom of the paper in a solvent
  • The solvent makes its way up the paper
  • This will separate the colours
  • Pure compounds will make 1 dot where as not pure ones will make different coloured dots
• Chromatography works because different substances have different solubilities, so more soluble substances move further
• Why do you draw the line in Chromatography in pencil?

  because the pencil will not separate like a pen will
• Chromatography is a technique used to separate substances in a mixture so that we can identify them.
• Paper chromatography is used to separate the different dyes in ink
• STEP BY STEP CHROMATOGRAPHY:

  1. First, in pencil draw a baseline on filter paper.
  2. then add a sample of ink to your baseline
  3. Fill a beaker with a shallow amount of water or ethanol and put your paper in
  4. Be careful to not submerge the dot, and place a lid on top to stop the solvent from evaporating
  5. wait for the solvent to seep up the paper
  6. this will dissolve the different dyes in the ink, but as the dyes dissolve at different rates they will end up separating out
  7. this tells us each one is a different substance
  8. However insoluble substances won't dissolve
• The pattern left by the results of chromatography is called a chromatogram.
• The mobile phase is a substance that molecules can move in, it is always a liquid or a gas. For our chromatography experiment, it was the solvent.
• The stationary phase is a substance or material that the substance can't move in. A solid or thick liquid. In our experiment it was the paper.
• During the chromatography experiment, the ink will constantly change between the mobile and stationary phrase. Dissolving in the solvent, moving a little bit, then binding to the paper over and over again. How much time the chemicals spend in these phases determines how fast they move up the paper.

  • More soluble chemicals moving up faster as they spend more time in mobile phase
  • Less soluble chemicals moving up slower as they spend more time in stationary phase
• How far each of the chemicals travels is dependant on the properties of that substance. If we measure how far each substance moves, we can compare to a data book and identify what chemical it is.
• The distance of the chemicals travelled depends on how long you have left them for. This means you cannot just measure length but have to use the Rf value. Rf value = distance travelled by substance/distance travelled by solvent. So if the substance had travelled 6cm, and the water had travelled 10cm the Rf value would be 0.6.
• When we do chromatography with a pure chemical we get a single spot. However if we use the same substance but a different solvent and paper the chromatogram may look different as we have changed the mobile and stationary phase. When a chemist looks up an R.f value, they have to take into account the solvent and the paper type.