EXPERIMENT NO.4
MOLISCH’S TEST
It is a qualitative group test for detection of
Carbohydrates in the given solution, and is
positive for all carbohydrates whether free or
bound to other substances such as proteins
(e.g glyco or muco proteins) or lipids (e.g
glycolipids).
It is named after Austrian Botanist Hans
Molisch
REAGENTS:
1. Molisch’s reagent: this is prepared by
dissolving 1gm of alpha naphthol in 95%
ethyl alcohol and making the volume upto
100ml.
2. Concentrated sulphuric acid.
PRINCIPLE:
When carbohydrates are treated with strong acids.the
polysaccharides and disaccharides are hydrolyzed into
monosaccharides.
The monosaccharides are dehydrated by acid to form
furfural or one of its derivatives.
Furfural or its derivatives condense with alpha
naphthol to form a violet coloured compound.
FURFURAL:
When sugars are treated with strong acids,on heating,
they are hydrolyzed into monosaccharides. These
monosaccharides undergo dehydration( removal of
water molecules) by acids. Depending on type of
monosaccharides these dehydrated monosaccharides
are called furfural or furfural derivatives.
PROCEDURE:
1. Take two test tubes and label them S for sample
and B for blank.
2. Take 3 ml of distilled water in test tube B and 3 ml
carbohydrate solution in test tube S.
3. Add 2 to 3 drops of molisch’s reagent in both test
tubes.
4. Mix it and incline the test tubes and run 3ml of
conc. Sulphuric acid along the wall of both test
tubes. Make the test tubes upright and rotate them
between your palms slowly and gently.
INTERPRETATION:
Appearance of a reddish violet ring at the
junction of two liquids indicate the presence of
carbohydrate in test tube S. no such ring will
appear in test tube B. in presence of excessive
alpha naphthol a green ring may also be seen
this should be ignored.
Repeat the test with different carbohydrate
solutions.
PRECAUTION:
After formation of the ring, do not shake tube
contents it may distort the ring shape.
FALLACIES:
Furfural,aldehydes and some organic acids e.g
formic acid, oxalic acid, lactic acid, citric acid
etc can also give a positive molisch’s test.