Testing for biological substances

Benedict's test for a reducing sugar

Gently boil 2 cm³ of sugar solution with an equal volume of Benedict's solution. If a 'reducing sugar' is present, a red precipitate forms, as the sugar reduces the blue copper sulphate present in the Benedict's solution to insoluble red copper oxide. All monosaccharides, and some disaccharides (such as maltose), are reducing sugars.

 

Testing for a non-reducing sugar

Gently boil 2 cm³ of sugar solution with 1 cm³ of dilute hydrochloric acid for 5 minutes, then carefully neutralise with sodium hydrogencarbonate. Carry out Benedict's test. The acid hydrolyses non-reducing sugars such as sucrose into reducing sugars, which will then give a positive result in Benedict's test.

 

Testing for starch

Add a few drops of potassium iodide solution. A bluish-black colour develops if starch is present.

 

Emulsion test for lipids

Add 2 cm³ of the sample being tested and 5 cm³ of ethanol to a test tube, and shake it to dissolve any lipid present. Add 5 cm³ of water and shake the test tube. If lipid is present, oily droplets form, which give the liquid a cloudy white appearance.

 

Biuret test for proteins

Add 2cm³ of the sample being tested and an equal volume of 5% potassium hydroxide (or sodium hydroxide) solution to a test tube. Add a few drops of dilute copper sulphate solution and mix gently. A purple colour slowly develops if peptide bonds (and hence protein) are present.

 

Separating mixtures by chromatography

Drops of mixture are spotted onto a sheet of paper, one at a time, and allowed to dry, forming a concentrated spot of the substance being tested. The end of the paper is dipped in a solvent, which travels up the paper and carries substances with it. Different substances move at different rates, causing the mixture to separate. Each substance has a particular 'Rf value', which is the distance travelled by the substance along the paper divided by the distance travelled by the solvent front.

 

Warning: appropriate safety precautions should be taken when conducting these tests.

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© Andrew Gray, 2006