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Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

SIMPLE BIOCHEMICAL TESTS ON CARBOHYDRATES

Get in touch if you need answers to any of the questions below.

Materials


   Benedict's solution (not "quantitative")
   Iodine solution (I/KI)

   Waterbaths set at about 100C

   Boiling tubes
   Racks

   1% glucose soln
   1% fructose soln
   1% sucrose soln
   other sugars?

   1% starch sol

   solution X

   HCl, NaOH, litmus/universal indicator paper

   filter funnels, papers, flasks?

   measuring cylinders (10, 25, 50, 100 ml)

   plastic pipettes

   glass pipettes (1, 5, 10 ml?), pipette pumps

Objectives

   Familiarisation with    Benedict's test - for reducing sugars
         modification of Benedict's test for non-reducing sugars
         iodine test
         dilution techniques
         sensitivity of above tests

Procedure

See also Roberts Students' manual p. 42

You will be expected to write up and hand in a concise account of your techniques and the results.

There is no need to use large amounts of test substances or reagents.

SAFETY:

   IRRITANT SOLUTIONS   WEAR SAFETY GLASSES AND LAB COATS

   HOT WATER & STEAM   TAKE CARE : KEEP WORKING AREA CLEAR

1 - Carry out Benedict's test on the named sugar solutions and starch as provided.
Use fixed (noted!) quantities of test substance and reagents.
Do not contaminate stock solutions shared by the group.
Note results - colour and intensity - separately from conclusions.

- Refinement : Filter the contents of the tube in which you have been testing glucose (using paper previously labelled with pencil). Consider the colour of the filtrate, and accordingly modify the quantities of reagents used. Repeat filtration process. Explain your conclusions in your write-up.

2 - Dilute the sugar solutions (say x10, x100 etc), and carry out Benedict's test on suitable samples (i.e. amounts same as before).
Explain the dilution technique you used in your write-up.
What does this tell you about the sensitivity of the test?

3 - Using sucrose, carry out the test for non reducing sugars. Having obtained a negative result from ordinary benedict's test, a second test is performed after hydrolysing it to reducing sugars (acid hydrolysis, followed by neutralisation), then performing Benedict's test:-
Add ... drops/ml HCl - leave for ... minutes at room temperature (... C) or ... mins at 100 C - add ... drops/ml NaOH (same strength) - check neutrality with litmus (colour ......) or universal indicator paper (colour .....)

4 - Test solution X using a combination of the above tests.

5 - Test starch solution using iodine solution (I/KI). You could use drops in a recessed tile, but for sensitivity purposes it is probably best to use boiling tubes and drops of iodine.
- Dilute starch x10, x100, etc. and repeat (standardised) test.
What does this tell you about the sensitivity of the test?
- Warm the last tubes - i.e. the least blue coloured (100 C waterbath)), & observe colour.
- Cool (the lower end of) the tube under the tap.
- Conclusion?
Some points to ponder (& then explain!):
-- What is a 1% solution?
-- Why is starch provided as a sol, whereas the others are soln (solution)?
-- Why do Biologists often use percentage solutions instead of molarities?