CHEMISTRY A2 experimental techniques

• WHAT IS THE PURPOSE OF RECRYSTALLISATION?

• WHAT IS THE PROCEDURE OF RECRYSTALLISATION?

Purpose: Purify an impure solid

Procedure:

• Dissolve the impure solid in a MINIMUM volume of an appropriate solvent
• Allow to cool and recrystallise
• Collect crystals by vacuum filteration using a buchner funnel
• Rinse with cold solvent
• Allow to dry

Check purity by melting point test

Solid must be SOLUBLE in solvent BUT insoluble when cold

• WHAT IS THE PROCEDURE OF COLORIMETRY?

Procedure:

• Choose a suitable filter (complementary colour)
• Zero/ reference with pure solvent
• Dilute 'known solution' to range of known concentrations and measure the absorbance
• Plot a calibration curve: concentration vs. absorbance
• Measure absorbance of unknown concentration solution and use calibration curve to work out the concentration

• WHAT IS THE PROCEDURE OF USING PAPER CHROMATOGRAPHY?

Procedure:

• Draw line near bottom of paper.
• Pure amino acid samples are placed on line.
• Paper is placed in a solvent in a container, making sure that the line is above the solvent.
• Place lid on top of container.
• Wait for solvent to rise and mark where it rises to.
• Spray the chromatogram with ninhydrin and heat.
• Compare Rf values.