Purification Techniques

Name the 3 main techniques for purification

• Recrystallisation

• Thin Layer Chromatography

• Melting Point Determination

Recrystallisation

• used to purify solid, impure organic products

• desired product dissolved in chosen hot solvent - impurities can be filtered out

• colling - desired product will crystallise out

Properties of a suitable solvent

• Should dissolve the product at high temperatures

• Should not dissolve the product at low temperatures

• need to use the MINIMUM AMOUNT OF HOT SOLVENT

Explain process of recrystallisation

• Dissolve crude product in a MINIMUM AMOUNT OF HOT SOLVENT

• filter off insoluble impurities using vacuum filtration

• collect filtrate and allow to cool

• Filter off crystals

• Wash and dry crystals

Thin Layer Chromatography

• used to separate dyes from a mixture

• mixture separate as some substances more soluble in solvent than others

• more higher it is on the paper the more soluble it is

How to calculate the Rf Value

Explain the process of thin layer chromatography

1. Draw pencil-line near bottom of plate

2. place 1 drop of each mixture separate from each other on the line

3. Place play in solvent - line must be above solvent level

4. Add lid/cover

5. When solvent nears the top of plate - remove and dry plate

6. Locate spots with a UV light or iodide

7. Calculate Rf value and compare with those of standard compounds

Melting Point Determination

• provides evidence for the products purity

• small sample of compound put into the MP apparatus

• if sample melts within 0.5 of the desired products published MP value - sample is considered to be pure

Why is the solvent used important

• product isn’t soluble enough in hot solvent - wont be able to dissolve

• product is too soluble in cold solvent - most will stay in solution ad when you filter you will lost most of product (low yield)

Why are we dissolving the solid in a solvent during recrystallisation

to make a saturated solution - maximum possible amount of solid is dissolved in the solvent

as solution cool - solubility of product falls - reaches point where it cant stay in solution it forms crystals

Explain the mobile phase and the stationary phase

• mobile phase - a liquid or a gase that moves over a second material (stationary phase) - which doesnt move

What is the typically stationary phase used

• thin layer of silica (silicon dioxide) or

• thin layer of alumina (aluminium oxide)

How are colourless chemicals revealed

• UV light

• Iodine vapour - place some iodine crystals - sticks to chemicals on the plate and shows up as purple spots