CHEMICAL TEST & IMPORTANCE OF CARBOHYDRATES
1.How can you identify carbohydrate from an unknown source?
ANSWER: The following test are done for identify carbohydrate from
an unknown source:
1.Molisch Test: specific for carbohydrates.
This test is specific for all carbohydrates Monosaccharide gives a rapid positive
test, Disaccharides and polysaccharides react slower.
Method :
1-Two ml of a sample solution is placed in a test tube.
2-Two drops of the Molisch reagent (which α-napthol in 95% ethanol) is
added.
3-The solution is then poured slowly into a tube containing two ml of
concentrated sulfuric acid so that two layers form, producing violet ring appear
as liaison between the surface separations.
FIGURE: MOLISCH TEST
2.Benedict's Test: presence of reducing sugars.
• All monosaccharides are reducing sugars; they all have a free reactive
carbonyl group.
• Some disaccharides have exposed carbonyl groups and are also reducing
sugars. Other disaccharides such as sucrose are non-reducing sugars and will
not react with Benedict's solution
-Large polymers of glucose, such as starch, are not reducing sugars, since the
concentration of hemiacetal groups is very low
Method :
• One ml of a sample solution is placed in a test tube.
• Two ml of Benedict’s reagent(complex mixture of sodium carbonate, sodium citrate
and copper(II) sulfate pentahydrate) is added.
• The solution is then heated in a boiling water bath for five minutes.
• A positive test is indicated by: The formation of a reddish precipitate.
FIGURE: Benedict’s test
3.Barfoed's Test: test used for detecting the presence
of monosaccharides.
Method:
• Place one ml of a sample solution in a test tube.
• Add 3 ml of Barfoed's reagent (a solution of cupric acetate and acetic acid).
• Heat the solution in a boiling water bath for 6 minutes(after the 3 min check
the tubes)
•Reducing monosaccharides are oxidized by the copper ion in solution to form
a carboxylic acid and a reddish precipitate of copper (I) oxide within three
minute
4. Seliwanoff's Test: distinguish between aldoses and
ketoses.
Used to distinguish between aldoses (like glucose) and ketoses (like fructose).
Method:
• One half ml of a sample solution is placed in a test tube.
• Two ml of Seliwanoff's reagent (a solution of resorcinol and HCl) is added.
• The solution is then heated in a boiling water bath for two minutes.
• A positive test is indicated by produce a cherry red product for ketohexos &
yellow to faint pink for aldohexos.
FIGURE: Seliwanoff's test
2.How will you distinguish between glucose & fructose by a chemical test?
Write down their pharmaceutical uses?
ANSWER:
Bromine water oxidizes glucose to gluconic acid when treated with it.
Fructose does not react with bromine water
.
Bromine water is a mild (very) oxidising agent. It's not strong enough
to oxidise the ketone group Fructose has. This is why they don't react.
Pharmaceutical importance of glucose:
a.In industry, glucose is used as a precursor to make vitamin C.
b.Glucose in a fluid form is a benefit in the food industry because it is
easy to mix into formulations on a big scale.
c.In jams and jellies, liquid glucose can supply the osmotic pressure
necessary to prevent bacterial growth and spoilage
d.In the medicine-making process, liquid glucose is useful as a
sweetener for syrups and tonics.
e.It is also used to coat tablets and as a granulating agent.
f.Liquid glucose can also be used as a medication for diabetic patients.
g. Ingested orally or through injection, liquid glucose can raise blood
sugar immediately.
Pharmaceutical importance of fructose:
a.High fructose corn syrup is used to sweeten foods and beverages
b.It is used as a food for diabetic people.
c.Infant feeding formula often contain fructose.
d.It is an ingredient of fructose +NaCl injection.