General Separation and Purification Techniques

Filtration and Evaporation

• Filtration separates an insoluble solid (the residue) from a liquid or solution (the filtrate).

• Method for Filtration:

  1. Line a filter funnel with a piece of fluted (folded) filter paper.
  2. Place the funnel into the neck of a beaker or conical flask.
  3. Pour the mixture into the funnel; the liquid molecules pass through the microscopic pores of the paper while the large solid particles are trapped.

• Evaporation to dryness separates a dissolved solid from a solvent by boiling the solvent entirely away. This is only for thermally stable solids.

• Method for Evaporation:

  1. Pour the solution into an evaporating basin.
  2. Place the basin on a tripod and gauze over a Bunsen burner.
  3. Heat the solution until all the liquid has evaporated, leaving the dry solid behind.

Crystallisation

• Crystallisation is used for thermally sensitive solutes that undergo thermal decomposition or lose their water of crystallisation when heated strongly.
• Method for Crystallisation:
  1. Pour the solution into an evaporating basin and heat it gently over a water bath to remove some of the solvent.
  2. Periodically dip a cold glass rod into the solution; if crystals form on the rod, you have achieved a saturated solution.
  3. Remove the basin from the heat and leave it to cool slowly, allowing large, pure crystals to form.
  4. Filter out the remaining liquid, wash the crystals with cold distilled water, and gently pat them dry with filter paper.

Simple Distillation

• Simple distillation separates liquids with significantly different boiling points or a solvent from a solution.
• Method for Simple Distillation:
  1. Place the mixture in a round-bottomed flask with anti-bumping granules to ensure smooth boiling.
  2. Connect the flask to a Liebig condenser, ensuring cold water enters the condenser jacket at the bottom and exits at the top.
  3. Insert a thermometer into the flask, ensuring the bulb is positioned exactly at the T-junction (side-arm) to measure the temperature of the vapour entering the condenser.
  4. Heat the flask; the liquid with the lower boiling point evaporates, then cools and condenses in the Liebig condenser to be collected as the distillate.

Fractional Distillation

• Fractional distillation is used for liquid mixtures with very similar boiling points.
• Method for Fractional Distillation:
  1. Set up the apparatus as for simple distillation, but add a fractionating column (filled with glass beads) vertically between the flask and the condenser.
  2. Heat the mixture gently. The glass beads create a temperature gradient (hottest at bottom, coolest at top).
  3. Vapours of higher-boiling liquids condense on the beads and drip back down, while the most volatile component reaches the top.
  4. Collect the first fraction when the thermometer reaches the boiling point of the most volatile liquid.
  5. When the temperature reading on the thermometer begins to rise, replace the collection beaker with a fresh one to collect the next fraction.

Paper and Thin-Layer Chromatography

• Chromatography separates mixtures based on their relative attraction to a stationary phase (paper or silica plate) and their solubility in a mobile phase (the solvent).
• Method for Chromatography:
  1. Draw a horizontal baseline in pencil near the bottom of the paper/plate (pencil is insoluble).
  2. Spot the sample onto the baseline and place the paper into a beaker with a small amount of solvent, ensuring the solvent level is below the baseline.
  3. Cover the beaker with a lid to prevent solvent evaporation.
  4. Once the solvent nears the top, remove the paper, mark the solvent front with a pencil, and let it dry. Use a locating agent if the spots are invisible.

Calculating the Retention Factor

• We identify substances by calculating their $R_f$ (Retention Factor), measured from the baseline to the centre of the spot.

A student performs TLC on a green food dye. The solvent front travels $8.5\text{ cm}$ and a blue spot travels $5.1\text{ cm}$ from the baseline. Calculate the $R_f$ value to 2 decimal places.

• $R_f = \frac{5.1}{8.5} = 0.60$