Chromatography and Purity
Chromatography is a technique used to separate the components of a mixture, which allows us to visually check if a substance is pure. A pure substance will appear as a single, distinct spot on the resulting chromatogram.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A substance is considered pure if it produces only one spot on a chromatogram when run in a suitable solvent.
- The presence of multiple spots indicates that the original substance was a mixture, with each spot representing a different component.
- Purity can be confirmed by running a chromatogram of the test substance alongside a known pure sample. If they are the same pure substance, they will produce single spots that travel the same distance and have identical Rf values.
- Impurities affect physical properties. For example, an impure solid will melt over a range of temperatures below the sharp melting point of the pure substance.
- The Rf value of a pure substance is a characteristic property under specific conditions (same solvent and stationary phase) and can be used for identification.
Diagram
Formulae
Rf = (distance moved by spot) / (distance moved by solvent front) To calculate a unique identifier for a component on a chromatogram. This value is always less than 1 and is constant for a given substance under identical conditions.
Definitions
- Pure Substance
- A substance composed of only one type of molecule or compound, which cannot be separated into simpler substances by physical processes.
- Chromatogram
- The visual result of a chromatography experiment, typically a sheet of paper or plate, showing the separated components as spots.
- Rf value
- The Retention Factor, a ratio calculated from a chromatogram that helps identify a substance. It is the distance travelled by the substance divided by the distance travelled by the solvent front.
Worked example
A chemist analyses a sample of a food colouring 'E123' using paper chromatography. A spot of E123 is placed on a baseline drawn in pencil. The paper is placed in a beaker with a solvent, and the solvent front is allowed to travel 8.0 cm up the paper. The E123 sample separates into two spots: a red spot that travels 6.0 cm and a faint blue spot that travels 2.0 cm. Is the sample of E123 pure, and what is the Rf value of the red component?
- 1
First, determine if the sample is pure.
The sample separated into two distinct spots (red and blue).
- 2
The presence of more than one spot means the sample is an impure mixture, not a single substance.
- 3
Next, identify the values needed to calculate the Rf of the red component.
The distance moved by the red spot is 6.0 cm, and the distance moved by the solvent front is 8.0 cm.
- 4
Use the Rf formula:
Rf = (distance moved by spot) / (distance moved by solvent front) - 5
Calculate the Rf value:
Rf = 6.0 cm / 8.0 cm - 6
Simplify the fraction:
6/8 simplifies to 3/4, which is 0.75.
Answer: The sample of E123 is not pure. The Rf value of the red component is 0.75.
Common mistakes
- ×Forgetting that the baseline must be drawn in pencil, as ink would dissolve in the solvent and interfere with the results.
- ×Calculating the Rf value incorrectly by dividing the solvent front distance by the spot distance. The Rf value can never be greater than 1.
- ×Stating that two substances are identical based on their Rf values without acknowledging that the conditions (solvent, paper type, temperature) must be the same for a valid comparison.
No-calculator tips
- ✓Rf calculations often involve simple fractions that are easy to convert to decimals (e.g., 4cm/8cm = 1/2 = 0.5; 6cm/8cm = 3/4 = 0.75).
- ✓Visually estimate the Rf value to check your calculation. If a spot is about a quarter of the way up the chromatogram, its Rf value should be around 0.25.
- ✓When comparing spots on the same chromatogram, you don't need to calculate the Rf values to see if they match. If two spots are at the same height, their Rf values are identical.