Sublimation, chromatography and crystallisation

Chromatography

Put a drop of black ink on a strip of filter paper, near one end. Stand the strip with this end in a little water, with the ink above the water. The water rises up the paper and carries the ink with it. After some time, the black spot has spread into bands of different colours. The black ink was a mixture of dyes.

This is chromatography. The dyes move up the paper at different speeds, because they dissolve in the water to different degrees, and they stick to the paper to different degrees. A dye that dissolves well in the solvent travels far. A dye that sticks to the paper travels only a little.

The Rf value

To say how far a substance moves, we use its Rf value (the retention factor).

Both distances are measured from the starting line. $R_f$ is a number between $0$ and $1$, and it has no unit, because it is a ratio of two distances. Under the same conditions, a substance always has the same $R_f$, so we can use it to identify the substance.

Example. The solvent moves $8.0\ \text{cm}$ and a spot moves $6.4\ \text{cm}$. Then $R_f = \dfrac{6.4\ \text{cm}}{8.0\ \text{cm}} = 0.80$.

You should find that each spot keeps the same fraction of the solvent's distance all the way up: its $R_f$ does not change as the solvent climbs.

Uses of chromatography

• To separate the colours of a dye, or the pigments of a leaf.
• To find the drugs in a sample of blood or urine.
• To test food for dyes that are not allowed.
• To find an impurity in a medicine.

Crystallisation

When we evaporate a solution to dryness, we get a solid, but it can be impure, and it can break down on strong heating. A better way to get a pure solid is crystallisation.

1. Make a saturated solution of the solid in hot water.
2. Filter it while it is hot, to remove any insoluble dirt.
3. Heat the solution to evaporate some of the water, until it is nearly saturated at that high temperature.
4. Leave it to cool slowly.
5. As the solution cools, its solubility falls, and the extra solid forms crystals.
6. Filter off the crystals, wash them, and dry them.

Crystallisation is used to purify salt that comes from sea water, to purify copper sulphate and alum, and to make sugar.

How much crystallises? For potassium nitrate, the solubility is $109\ \text{g}$ per $100\ \text{g}$ of water at $60\ ^\circ\text{C}$, and $32\ \text{g}$ at $20\ ^\circ\text{C}$. If we cool $100\ \text{g}$ of water that is saturated at $60\ ^\circ\text{C}$ to $20\ ^\circ\text{C}$, then $109\ \text{g} - 32\ \text{g} = 77\ \text{g}$ forms crystals.

Practise with new numbers

Try it in Python

This program finds the $R_f$ value of three coloured spots on a strip of paper.

Remember these

• Chromatography separates the dyes in a mixture, because they move at different speeds on the paper.
• $R_f = d \div D$, with no unit, and it is used to identify a substance.
• Crystallisation gives a pure solid from a hot saturated solution as it cools.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. The solvent rises $12\ \text{cm}$ and a spot rises $9\ \text{cm}$. What is the spot's $R_f$?

2. A black felt pen gives three coloured spots on the paper. What does that tell you about the ink?

3. Why is crystallisation better than evaporating a solution to dryness, to get a pure solid?

4. Can an $R_f$ value be $1.3$? Why?