chromatography

• Chromatography is a method to separate and identify components from

  a mixture of soluble substances.

  We can also use it to do purity tests.

  A common use for chromatography is in the separation of different

  pigments in ink, e.g. colourings or plant pigments of chlorophyll.
• There are two main phases in paper chromatography:

  The stationary phase is the starting point where, the molecules

  can’t move on the chromatography paper (can be a solid or a really

  thick ink) as the solvent has still not reached it.

  The mobile phase is the

  solvent in which the

  sample can move. Solvent

  (liquid) that moves along

  the chromatography paper

  based on the size of the

  molecules at different rates

  of movement.
• We can use a paper chromatogram to distinguish between pure and

  impure substances.

  Pure substances only produce one spot on the chromatogram

  Impure substances will separate, producing two or more spots

  Substances that are the same will produce

  identical chromatograms.

  They will produce the same amount of

  spots, those spots will travel the same

  distances up the paper and the colours

  will match.
• The Rf value is used to identify the components of a mixture.

  A particular substance will always give you the same Rf value, as

  long as the solvent is the same.

  Changing the solvent will change the Rf value.

  You can calculate the Rf value of a spot using the following equation:
• The Rf value is the ratio between the distance travelled by the dissolved

  substance (solute) and the distance travelled by the solvent.

  So, it is a number ranging between 0 and 1.

  We can compare the Rf value of unknown substances with Rf values

  of known substances under the same conditions.

  We call these known substances reference values

  To find the distance travelled by substance, measure from the base line to

  the centre of the spot.