7.7 - Separation of a Mixture of Indicators

Materials and Apparatus Required

Chemicals

• Universal Indicator solution (or other mixture of indicators)
• Methyl orange solution
• Bromothymol blue solution
• Phenolphthalein solution
• Ethanol
• Ammonia solution (0.88)
• Water/ethanol/ammonia solution (5:2:1)

Apparatus

• Paper chromatography tank
• Chromatography paper
• Capillary tubes
• Solvent trough
• Hair dryer

Safety Precautions

• Wear safety glasses throughout the experiment.
• Ethanol is flammable; keep away from open flames.
• Ammonia solution is toxic and irritating to the lungs. Handle with care and ensure the experiment is conducted in a well-ventilated area.

Method

Paper Chromatography

1. Prepare a chromatography tank by adding solvent (water/ethanol/ammonia solution) to a depth of 10 mm at the bottom.
2. Cover the tank and allow it to stand for a few hours to saturate with solvent vapour.
3. Using a pencil, draw a line near the top of a sheet of chromatography paper and another line about 3 cm from the bottom.
4. Place small spots of each indicator and the mixture of indicators on the line near the bottom using a capillary tube.
5. Dry the spots with a hairdryer and repeat the spotting to intensify the spots.
6. Place the chromatogram in the tank, ensuring the solvent level is below the line where the samples are spotted.
7. Allow the solvent to rise until it reaches the top line.
8. Remove the chromatogram, dry it, and measure the distance travelled by the solvent front and by each indicator spot.
9. Calculate the Rf value for each indicator:

$$\text{Rf} = \frac{\text{Distance travelled by the component}}{\text{Distance travelled by the solvent front}}$$

10. Identify the indicators in the mixture by comparing their Rf values with known values.

Results

Indicator	Distance Travelled (cm)	Rf Value
Methyl orange	6.8	0.68
Phenolphthalein	8.4	0.84
Bromothymol blue	9.7	0.97
Fastest component of mixture	9.7	0.97
Second component of mixture	8.4	0.84
Third component of mixture	7.9	0.79
Fourth component of mixture	6.8	0.68

Indicators present in the mixture: Bromothymol blue, methyl orange, phenolphthalein, and at least one other.

Example Questions with Answers