Chromatography

Question 1

The Principle

Answer 1

The basic principle is that a mobile phase ( a solvent), will carry a dissolved mixture through a stationary phase (paper or thin layer of aluminium oxide). The SEPARATION occurs, as some of the components of the mixture tend to be held by the stationary phase and move more slowly than those that mix well with the mobile phase.

Question 2

Experiment: To separate a mixture of indicators (inks) using chromatography.
Theory

Answer 2

• A small amount of indicator mixture placed on a strip of filter paper.The strip is placed so it JUST touches a suitable solvent.
• The paper absorbs the solvent and the solvent moves up through the paper and the indicator mixture dissolves in the solvent.
  -The degree to which these solvents dissolve differs. The components that don’t dissolve easily in the solvent will come out of the solution and appear on the paper as a spot.
  -Those that do dissolve well will be carried up the paper further and will eventually appear as spots on the paper at different intervals.
• In this way the components are separated.

Question 3

Apparatus & Materials

Answer 3

Apparatus: Filter paper, gas jar, ruler, pencil, sellotape, glass rod, hair dryer, capillary tubes.

Materials: a mixture of indicators, water/ethanol/ammonia solution

Question 4

Method

Answer 4

1. The solvent was added to the bottom of the glass to a depth of about 1cm. This was allowed to stand for a while to allow the tank to become saturated with the solvents vapour.
2. On the filter paper a line was drawn near the top of the paper with a PENCIL and another line was drawn near the bottom.
3. A small spot of the mixture of indicators was placed on the line on the bottom of the paper using the capillary tube. This was dried with a hair dryer to prevent spot from spreading. This procedure repeated several times to build up a small CONCENTRATED spot of the mixture.
4. The paper with the “spot-side down” was then placed in the gas jar so that the end of the paper was JUST touching the solvent. Do not immerse the spot. Attach top of filter paper to a glass rod using sellotape.
5. The chromatogram was allowed to run until the solvent reached the line at the top of the paper.
6. The paper was removed and dried.

Question 5

Does this experiment allow for accurate results?

Answer 5

No, the particles of the stationary phase are not small and uniform in size so this does not allow for accurate results.

Question 6

Results

Answer 6

Distance traveled by solvent front (solvent as whole)
Distance travelled by fastest component = Rf
Distance travelled by second fastest = Rf
etc…

Rf = Distance travelled by component
—————————————————-
Distance travelled by solvent front

Question 7

Why is the gas jar not used for a time after the solvent has been added?

Answer 7

To allow time for the tank to become saturated with solvent vapour.

Question 8

Why are two lines drawn on the paper?

Answer 8

One line is needed to indicate where the samples start from, and the other to indicate the distance travelled by the solvent front, which enables Rf values to be calculated.

Question 9

When is it possible to separate two components of a mixture using paper chromatography?

Answer 9

When one of the components is attracted to significantly different extents by the stationary phase and/or the mobile phase.

Question 10

When two substances are said to have two different Rf values in an experiment carried out under the same conditions what does this mean?

Answer 10

The two substances are not identical.

Question 11

Diagram

Answer 11

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