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Qualitative

Biochemical

Tests

Qualitative tests are used to determine

whether a substance is present or not in

a sample

You need to be able to do these tests

independently and interpret the results

You must also be able to risk assess the

procedure

Biuret ‘reagent’

contains an alkali and

copper (II)

sulfate which react in

the presence

of peptide bonds

• Add sodium hydroxide to the

solution sample to make the

solution alkaline

• Add a few drops of copper (II)

sulfate solution

(which is blue)to the sample

(Biuret ‘reagent’ contains an

alkali and copper (II) sulfate)

Limitations

The Biuret test is qualitative - it

does not give

a quantitative value as to

the amount of protein present in a

sample

If the sample contains amino acids

or dipeptides, the result will be

negative (due to lack of peptide

To test for the presence of starch in

a sample, add a few drops

of orange/brown iodine in

potassium iodide solution to the

sample

The iodine is in potassium iodide

solution as iodine is insoluble in

water

If starch is present, iodide

ions in the solution interact

with the centre of starch

molecules, producing a

complex with a

distinctive blue-black

colour

This test is useful in

experiments for showing

that starch in a sample has

been digested by enzymes

Method:

Add ethanol to the sample to be

tested

Shake to mix

Add the mixture to a test tube

of water

Results

• If lipids are present, a milky

emulsion will form (the solution

appears ‘cloudy’); the more lipid

present, the more obvious the milky

colour of the solution

• If no lipid is present, the solution

remains clear

Limitations

This test is qualitative - it

does not give

a quantitative value as to

how much lipid may be

present in a sample

Sugars can be classified

as reducing or non-reducing ;

this classification is dependent on

their ability to donate electrons (a

reducing sugar that is able to

donate electrons is itself oxidised)

OILRIG in Chemistry