Describe how paper chromatography is used to separate mixtures of soluble colourless substances, using a suitable solvent and a locating agent

📚 Paper Chromatography: Separating Colourless Mixtures

Paper chromatography is like a treasure hunt for invisible dyes. The paper acts as a sponge that carries a liquid “search party” (the solvent) up the sheet, picking up the hidden colourless molecules along the way. By the time the solvent front reaches the top, the molecules have been sorted into neat spots that can be seen after a simple treatment with a locating agent. 🔬🧪

What is Paper Chromatography?

Think of the paper as a long, thin sponge. When you dip the bottom edge into a small cup of solvent, the liquid soaks up the paper and moves upward by capillary action. The solvent carries the dissolved mixture with it. Different colourless molecules travel at different speeds because they interact differently with the paper (the stationary phase) and the solvent (the mobile phase). The result is a series of spots that are now visible after adding a locating agent. 📐

Step‑by‑Step Procedure

  1. ✏️ Prepare the paper strip: Cut a strip of chromatography paper (≈10 cm long, 2 cm wide). Mark a line 1 cm from the bottom with a pencil.

  2. 💧 Spot the sample: Using a capillary tube, apply a tiny drop (≈1 µL) of the colourless mixture onto the pencil line. Let it dry completely.

  3. 🧪 Choose a solvent: Place the paper in a shallow solvent chamber (a jar with a lid). The solvent should not reach the spot line. Common solvents: ethyl acetate, methanol, or a mixture of water and acetone.

  4. ⏱️ Develop the chromatogram: Allow the solvent to rise until it is about 1 cm from the top. Remove the paper and let it dry.

  5. 🔬 Locate the spots: Spray or dip the paper in a locating agent (e.g., iodine vapour, ninhydrin, or a silver nitrate solution). Colourless molecules will appear as coloured spots.

  6. 📏 Measure distances: Measure the distance from the pencil line to each spot (solute distance) and the distance from the pencil line to the solvent front (solvent distance).

  7. 🧮 Calculate Rf: \$R_f = \frac{\text{distance travelled by solute}}{\text{distance travelled by solvent}}\$ Spots with different Rf values are different substances.

Choosing the Right Solvent

The solvent must be able to dissolve all components of the mixture but not be so strong that it carries them all to the top at once. A good rule of thumb is to start with a solvent of moderate polarity and adjust if the spots are too close or too far apart. For colourless organic compounds, a common choice is a 1:1 mixture of ethyl acetate and methanol. 🌿

Locating Agent

Because the substances are colourless, we need a reagent that reacts with them to give a visible colour. Some popular locating agents are:

  • 🧪 Iodine vapour – turns many organic molecules yellow.

  • 🧪 Ninhydrin – gives a purple colour to amino‑containing compounds.

  • 🧪 Silver nitrate – forms coloured silver salts with halides.

Choose the locating agent that reacts with the functional groups you expect in the mixture.

Rf Value and Interpretation

The Rf value is a simple ratio that tells you how far a compound travels relative to the solvent front. A higher Rf means the compound is more soluble in the solvent (less polar), while a lower Rf indicates stronger interaction with the paper (more polar). By comparing Rf values with known standards, you can identify the components of the mixture. 📊

Solvent SystemTypical Rf Range (colourless acids)Typical Rf Range (colourless bases)
Ethyl acetate : Methanol (1:1)0.30 – 0.500.20 – 0.40
Water : Acetone (1:1)0.10 – 0.300.05 – 0.25
Hexane : Ethyl acetate (1:1)0.40 – 0.700.30 – 0.60

Exam Tips

  • 📝 Explain the principle: Mention capillary action, stationary vs mobile phase, and the role of the locating agent.

  • 📐 Show calculations: Write the Rf formula and give an example with numbers.

  • 🔍 Interpret results: Discuss how different Rf values indicate different polarities.

  • ⚙️ Describe the procedure: Use bullet points or a short paragraph to outline each step clearly.

  • 💡 Use analogies: Relate the process to everyday experiences (e.g., a sponge soaking up water).

  • 🧪 Highlight safety: Remind to wear gloves and goggles when handling solvents and locating agents.

Quick Summary

• Paper chromatography separates colourless mixtures by moving a solvent up a paper strip.

• Choose a solvent that dissolves all components but allows separation.

• Use a locating agent to reveal invisible spots.

• Calculate Rf to identify substances and compare with standards.