A systematic identification of carbohydrates
Topic 3.2- Carbohydrates, lipids and proteins
Time-2 hrs.
The object of this exercise is to devise a scheme for the systematic identification
of carbohydrates and to use it to identify four carbohydrates labeled A,B,C and D.
Reagents are provided for the identification of carbohydrates.
METHOD:
Some of the common carbohydrates including A,B ,C and D are listed in Table 1,
while some of the reagents currently used to identify carbohydrates are listed in
Table 2
TABLE 1 - Some common carbohydrates (* denotes reducing sugars)
Type of carbohydrate
Examples
Solubility in water
Monosaccharides
Glucose*
Soluble
Fructose*
Soluble
Xylose
Soluble
Disaccharides
Sucrose
Soluble
Maltose*
Soluble
Oligosaccharides
Starch
Insoluble
Glycogen
Insoluble
Cellulose
Insoluble
TABLE 2- Reagents used to test carbohydrates
Reagent
Reaction
Barfoed’s
After boiling, rapid formation of a redbrown precipitate with all reducing
monosaccharides. The reaction with
reducing disaccharides is much slower.
Benedict’s
After boiling, a red-brown precipitate
with all reducing sugars.
Bial’s
After boiling, a green coloration with
xylose
Clinistix
Pink color changes to blue with glucose
only.
Iodine
Blue-black with starch, purple-red with
glycogen and no reaction with cellulose
Schultze’s
Purple coloration with cellulose.
Seliwanoff’s
After boiling, a red coloration with
fructose and sucrose
WARNING: Bial’s and Seliwanoff’s reagents are acid. Take care when heating
carbohydrates with these reagents
TABLE 3Test for sucrose( a non- reducing disaccharide sugar)
Non-reducing sugars do not give a positive reaction to many reagents, as do
reducing sugars. However, if a sugar is suspected to be sucrose, a positive
reaction to Benedict’s solution may be produced as follows.
1. Add a few drops of dilute hydrochloric acid to a solution of the test
substance and heat in a boiling water bath in a few minutes.
2. Neutralize the acid. This may be done by adding sodium hydrogen
carbonate little by little until the fizzing stops (avoid adding too much)
3. Add a few drops of Benedict’s solution and heat in a boiling water bath.
What is expected from you?
1. Devise a logical, step-by-step, qualitative scheme for the identification of
an unknown carbohydrate. Your scheme should lead, via appropriate
tests and positive or negative results to each of the carbohydrates listed.
2. Place each of the samples A, B, C and D into labeled boiling tubes and add
10cm3 of water to each sample. Subdivide each sample into an
appropriate number of batches, each 1-2 cm3, and use your scheme to
identify carbohydrates A, B, C and D. record your results.