7 Practical 3
Semi-quantitative and quantitative tests for reducing sugars
Safety
The normal safety precautions associated with the use of chemicals and heating apparatus apply.
Apparatus and materials
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15 boiling tubes
test-tube rack
10 cm3 syringes
5 cm3 syringes
1 cm3 syringes
distilled water
water bath maintained at 75 °C
six small beakers
filter funnel
filter paper
labels or marker pen
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coloured pencils
stopwatch
colorimeter and cuvettes
50 cm3 of 10% glucose solution
Benedict’s solution
20 cm3 of lemon juice
20 cm3 of unknown glucose solution, labelled A
20 cm3 of unknown glucose solution, labelled B
eye protection
pipettes
Introduction
In this practical you will:
• make a serial dilution of glucose
• test the different concentrations of glucose with Benedict’s solution
• make a colour chart
• use your colour chart to estimate the concentration of reducing sugar in some unknown solutions
• use a colorimeter to increase the sensitivity of the reducing sugar test.
Procedure
It is important to avoid contamination of solutions. Use a clean syringe for measuring out volumes of
different solutions.
A
Making a temporary preparation of onion epidermal cells
1
Label five boiling tubes 1 to 5. Using a 10 cm3 syringe, place 10.0 cm3 of 10% glucose solution in
tube 1.
2
Using a 1 cm3 syringe, take 1.0 cm3 of the solution from tube 1 and transfer it to tube 2. Using a
10 cm3 syringe, add 9.0 cm3 of distilled water to tube 2 and mix the contents. The 1.0 cm3 of 10%
glucose solution has now been diluted ten times to make a 1% solution.
3
Repeat step 2, diluting the 1% solution in tube 2, to produce a 0.1% solution in tube 3. Repeat the
process with tubes 4 and 5. Tubes 1 to 5 now contain a serial dilution of the original glucose
solution, with the following concentrations: 10%, 1%, 0.1%, 0.01% and 0.001%.
COAS Biology 1 Teacher Resources
Original material © Cambridge University Press 2008
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7 Practical 3
4
Tubes 1 to 4 have only 9.0 cm3 of solution left in them, but tube 5 has 10.0 cm3. Remove 1.0 cm3
of solution from tube 5 so that, for the Benedict’s test, all tubes start with the same volume of
solution.
5
Using a syringe, add 5.0 cm3 of Benedict’s solution to each tube, and place the tubes in a water
bath at 75 °C for 9 minutes.
6
Remove the tubes from the water bath and return them to the test-tube rack. Use coloured pencils
to make a chart of the colours.
B
Estimating the concentration of reducing sugar in some unknown solutions
1
Into three separate boiling tubes place 9.0 cm3 of either unknown solution A, unknown solution B
or the lemon juice. Label the tubes.
2
Add 5.0 cm3 of Benedict’s solution to each of the three tubes and heat in the water bath at 75 °C
for 9 minutes as in part A.
3
Compare the colours of the three tubes with those obtained from part A and estimate the
concentrations of reducing sugar present.
C
Extension: using a colorimeter to increase the sensitivity of the Benedict’s test
1
Make up a series of dilutions of the 10% glucose solution, of concentrations 0%, 0.5%, 1.0%,
1.5%, 2.0% and 2.5%, using distilled water. It is best to construct a table first to show how you
will make these dilutions. Have the table checked before you carry on.
2
Transfer 0.5 cm3 of each of your solutions to a labelled boiling tube, and add 5.0 cm3 of Benedict’s
solution to each tube. Place all the tubes in the water bath at 75 °C for 5 minutes.
3
Remove the tubes from the water bath and filter the contents of each tube into a clean, labelled
test tube. Using a pipette, transfer some of each filtrate to labelled colorimeter cuvettes.
4
Using an orange filter in the colorimeter, place the cuvette containing the filtrate from the
2.5% solution into the colorimeter. Set the colorimeter to zero absorbance using this solution.
Now read the absorbance of the other filtrates.
5
Process your results so that you could use the information to determine the concentration of
glucose in an unknown solution.
COAS Biology 1 Teacher Resources
Original material © Cambridge University Press 2008
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