PRACTICAL

2C Separating the colours in ink Aim: to separate the dyes in ink, using chromatography You will need: n black ink n n

beaker, 100 cm3 measuring cylinder, 25 cm3

n n

capillary tube or dropper chromatography paper

Background l

l

Black ink is a mixture of dyes of different colours. Chromatography can be used to separate the dyes.

Procedure 1 Using a ruler, draw a straight pencil line across the chromatography paper, about 2 cm from the bottom. 2 Drop two spots of ink on the pencil line, using a capillary tube or dropper. Let them dry. Then place another drop on one of the spots, and let it dry. (This is in case the dyes from the single drop do not show up clearly.) 3 Pour some water into the bottom of the beaker. (Not enough to reach the ink spots – see the diagram below.)

n n n

paper clip pencil and ruler safety glasses

5 The water moves up the paper. Remove the paper when the water is about 2 cm from the fold. Mark where it reached, with a pencil line. 6 Let the paper dry. (You could wave it about to help it dry faster.) Then measure the distance between the two pencil lines. 7 Measure the distance each colour has travelled, from the lower pencil line to the centre of the colour. Record your results.

Analysing the results 1 How many different dyes does the ink appear to contain, and what colours are they? 2 Of the dyes that separate, which colour of dye: a moves furthest up the paper? b is the most soluble in water? fold

4 Straighten a paper clip. Rest it across the top of the beaker. Then fold the top of the chromatography paper over. Hang the paper from the wire so that it dips into the water, as shown here.

straightened paper clip

water

Extension questions 1

Choose one of the dyes. Calculate how far it would have travelled, if the water had reached the fold in the paper. (You will need to make one further measurement.)

2

Scientists often work with very tiny samples of compounds. See if you can suggest a way to obtain tiny samples of the separate dyes, from the chromatography paper.

Further work Use chromatography to compare inks. (Put spots of them side-by-side on chromatography paper.) Do any appear to contain the same dyes?

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