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What is a qualitative test
They indicate the presence or absence of a substance. They are descriptive and the results are often seen as a colour change.
How can a solid sample be tested
By grinding or crushing the solid sample and dissolving it in a suitable solvent to form a solution.
Method to test for the presence of lipids
Shake sample with ethanol which will dissolve any lipids present and then add an equal volume of cold water.
Positive a negative results for lipids
Positive result: A white emulsion will form.
Negative result: Solution remains colourless
Explanation of ethanol emulsion test
Lipids are non-polar so they dissolve in ethanol (a non-polar solvent). However the lipid comes out of solution when water (a polar solvent) is added and are dispersed in small droplets which scatter light making it appear cloudy.
Method to test for the presence of proteins
Add biuret reagent to sample, no heat is required.
Positive and negative results for proteins
Positive result: Colour of solution changes from blue to purple
Negative result: Colour of solution remains blue.
Explanation of Biuret test
Copper (II) ions form a complex with the amine groups in the peptide bonds.
Method to test for the presence of starch
Add iodine in potassium iodide solution to the sample.
Positive and negative results for starch
Positive result: Colour of solution changes from yellow/brown to blue/black.
Negative result: Colour of solution remains yellow/brown.
Explanation for iodine test
Iodine forms ions that fit inside the amylose helix.
Method to test for the presence of reducing sugars
Add benedict’s reagent to sample and heat it to 80°c in a water bath for 3 minutes.
Positive and negative results for reducing sugars
Positive result: Colour of solution changes from blue to green/yellow/orange/red (depending on concentration).
Negative result: Colour of solution remains blue.
Explanation for Benedict’s test
The sugar reduces Cu²⁺ ions to Cu⁺ ions which form a red precipitate.
Why can Benedict’s test be described as semi-quantitative
The colour of the solution indicates the concentration of reducing sugar. If equal volumes of solution and excess Benedict’s reagent are used then the concentrations can be compared (blue-none, green-trace, yellow-low, orange-moderate, red-high).
Method to test for the presence of non-reducing sugars
Carry out Benedict’s test first to confirm no reducing sugars are present. Boil a sample with acid (dilute hydrochloric acid is usually used) to hydrolyse the glycosidic bond. Allow the solution to cool and then neutralise it with an alkali (sodium hydrogen carbonate is usually used). Finally, repeat Benedict’s test.
Positive and negative results for non-reducing sugars
Positive results: Colour of solution changes from blue to green/yellow/orange/red (depending on concentration).
Negative results: Colour of solution remains blue.
Explanation of non-reducing sugar test
The non-reducing disaccharides are hydrolysed into monosaccharides which are all reducing sugars.
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