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Topics: Glucose, Vitamin C, Carbohydrate Pages: 15 (1825 words) Published: January 30, 2014
CAL Biology

Practical 2 – Identification of Biochemicals in Pure Form

Name :…………………………………………………………….

MARK:

Class :……………………………………………………………. PRACTICAL 2: IDENTIFICATION OF BIOCHEMICALS IN PURE FORM
CAUTION:
Any heating that has to be done in the following tests should be carried out in a water bath at the boiling point of water. Direct heating of test-tubes should not take place. Introduction
Biochemical Tests
Qualitative
- present/ absent

Semi-quantitative
- present in >/< amount
- a rough estimation

Quantitative
- precise numerical value of
the quantity

Aims: 1) To learn how to carry out biochemical tests to identify biochemicals in pure form. 2) To learn the basis of biochemical tests.
3) To find out the concentration of ascorbic acid in a vitamin C solution. 4) To classify biochemical tests into qualitative, semi-quantitative & quantitative tests. 5) To learn how to use the correct significant figures for numbers calculated from experimental results. 6) To learn the meaning of some scientific terms used in experiments. Materials: (as projected on screen)

Method/ Procedure:
I.

CARBOHYDRATES: REDUCING SUGARS
The reducing sugars include all monosaccharides, such as glucose and fructose, and some disaccharides, such as maltose.
Test
Benedict’s test
Add 2cm3 of a solution of the reducing
sugar to a test-tube. Add an equal
volume of Benedict’s solution. Shake
and place into boiling water. Boil for
2-5 minutes; shake continuously to
minimise spitting.
Note: Make sure you standardise the
boiling time for all specimen.

Observation

Basis of test

The initial blue colouration of the
mixture turns green, then yellowish
and may finally form a brick-red
precipitate.

Benedict’s solution contains copper
sulphate. Reducing sugars reduce
soluble
blue
copper
sulphate,
containing Cu2+ to insoluble red-brown
copper oxide, containing Cu+. The
latter is seen as a precipitate.

Note:
1) Read the colour against a white
background (e.g. white paper/
white tile);
2) Make sure you read both the
colour and opacity (due to
precipitate) of the solution.

Additional information:
• Benedict’s test is not for carbohydrate, but for reducing sugars! • The mixture is likely to bump violently during heating and extra care should therefore be taken. • The final precipitate will appear green to yellow to orange to red-brown with increasing amounts of reducing sugars. (The initial yellow colour blends with the blue of the copper sulphate solution to give the green colouration).

1

CAL Biology

Practical 2 – Identification of Biochemicals in Pure Form

II. CARBOHYDRATES: NON-REDUCING SUGARS
The most common non-reducing sugar is sucrose, a disaccharide. If Benedict’s test shows that reducing sugar is absent, a brick-red precipitate in the test below indicates the presence of a non-reducing sugar. If reducing sugars have been shown to be present, a heavier precipitate will be observed in the following test than with the Benedict’s (reducing sugar) test if non-reducing sugar is also present. Test

Hydrolysis then Benedict’s test
Add 2cm3 of sucrose solution to a testtube. Add 1cm3 dilute hydrochloric acid. Boil for one minute. Carefully
neutralise with sodium hydrogen
carbonate (check with litmus paper/
until no effervescence).
Remove 2cm3 of resulting solution and
carry out Benedict’s test.

Observation

Basis of test

(as Benedict’s test)

A disaccharide can be hydrolysed to its
monosaccharide
constituents
by
boiling with dilute hydrochloric acid.
Sucrose is hydrolysed to glucose and
fructose, both of which are reducing
sugars and give the reducing sugar
results with the Benedict’s test.

III.
CARBOHYDRATES: STARCH
This is only slightly soluble in water, in which forms a colloidal suspension. It can be tested in suspension or as a solid.
Test
Iodine test
Add 2cm3 1% starch solution to a testtube. Add a few drops of I2/KI solution. Alternatively...
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