Distillation and Chromatography

1Drinking water from sea water, colours from black ink

A ship far from land can make fresh drinking water from salty sea water. A forensic scientist can prove which pen wrote a threatening note. Both depend on separation methods that are more powerful than filtering: distillation, which separates a liquid from what is dissolved in it, and chromatography, which separates dissolved substances from one another.

2Experiment: simple distillation of salt water

Set up the apparatus in Figure 4.1. Heat the salt water gently until it boils. Watch the thermometer, the condenser and the receiver.

Figure 4.1: Simple distillation. Cold water enters the condenser at the lower end.

3What happens?

The water boils and turns to steam; the salt, which has a very high boiling point, stays in the flask. The steam passes the thermometer bulb, which reads the boiling point of the pure water (about $95{}^{\circ }\mathrm{C}95\backslash ,^\backslash circ\backslash mathrm\{C\}$ in Kigali), and enters the condenser. There, cold water flowing around the inner tube cools the steam so that it condenses back to liquid water, which drips into the receiver:

$${\mathrm{H}}_2\mathrm{O}(\mathrm{l})\stackrel{\text{boil}}{\to }{\mathrm{H}}_2\mathrm{O}(\mathrm{g})\stackrel{\text{condense}}{\to }{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\backslash mathrm\{H\_2O(l)\}\; \backslash xrightarrow\{\backslash text\{boil\}\}\; \backslash mathrm\{H\_2O(g)\}\; \backslash xrightarrow\{\backslash text\{condense\}\}\; \backslash mathrm\{H\_2O(l)\}$$

The liquid collected is the distillate. Taste tests are never done in the laboratory, but a drop of the distillate evaporated on a clean watch glass leaves no salt behind.

Mixtures of liquids with different boiling points, such as ethanol ($78{}^{\circ }\mathrm{C}78\backslash ,^\backslash circ\backslash mathrm\{C\}$) and water ($100{}^{\circ }\mathrm{C}100\backslash ,^\backslash circ\backslash mathrm\{C\}$ at sea level), are separated by fractional distillation, which adds a tall column packed with glass beads between the flask and the condenser. The liquid with the lower boiling point distils over first. Industry uses it to make ethanol for hand sanitiser and fuel, to split crude oil into petrol, kerosene and diesel, and to separate liquid air into oxygen and nitrogen.

4Experiment: paper chromatography

1. Draw a pencil line 2 cm from the bottom of a strip of chromatography paper (or filter paper).
2. Put a small, concentrated spot of black ink and one of green food dye on the line, well apart.
3. Hang the paper from a glass rod in a beaker containing water about 1 cm deep. The pencil line must be above the water (Figure 4.2 A).
4. Leave it until the water has risen near the top. Mark the solvent front in pencil, then let the paper dry.

Figure 4.2: Paper chromatography of black ink and green food dye.

5How chromatography works

As the water soaks up the paper, it dissolves the dyes and carries them upwards. Each dye is attracted to the paper to a different extent: a dye that dissolves well in the water and sticks weakly to the paper travels far; one that sticks strongly travels only a short distance. So the dyes in a mixture are carried different distances and separate into spots. The pattern of spots is the chromatogram. A pure substance gives only one spot.

Each substance moves a fixed fraction of the distance moved by the solvent, called its $R_{f}R\_f$ value:

$$R_f=\frac{\text{distance moved by the spot}}{\text{distance moved by the solvent}}R\_f\; =\; \backslash frac\{\backslash text\{distance\; moved\; by\; the\; spot\}\}\{\backslash text\{distance\; moved\; by\; the\; solvent\}\}$$

Table 4.1: Distillation and chromatography compared

Figure 4.3: Comparing the ink on a note (X) with three pens.