Equipment

• Benedict’s reagent

• 2 dropping pipettes

• 8 boiling tubes

• 8 syringes

• Samples A,B,C,D

• Water bath at 100 degrees C

• 2.0 mol/dm³ hydrochloric acid

• Sodium hydrogen carbonate

• Small spatula

• pH paper

• Marker pen

Health and Safety

• Eye protection must be worn due to boiling water and acid

• Benedict’s reagent is low hazard but contact with eyes should be avoided

• The boiling water bath prevents a scalding hazard

• 2.0 mol/dm³ hydrochloric acid is an irritant

Method 1 : Testing for a reducing sugar

1. Label each boiling tube A,B,C,D and place 4cm³ of the corresponding sample into each, use a fresh syringe each time

2. Use a dropping pipette to add 2cm³ Benedict’s reagent to each of the four samples

3. Record your observations of each sample immediately after adding Benedict’s reagent

4. Place all four tubes into water bath

5. After 2 mins record observations of each sample

6. After a further 2 mins (4 total) remove tubes from water bath and record final observations

Method 2 : Testing for a non-reducing sugar

1. Put 4cm³ of each sample A-D into its own labelled boiling tube using a fresh syringe each time

2. Use a dropping pipette to add 2cm³ of 2 mol/dm³ hydrochloric acid to each of the four samples

3. Place all four boiling tubes into the water bath and incubate for 2 mins

4. Remove tubes from water bath and allow to cool

5. Neutralise the acid by GRADUALLY adding sodium hydrogen carbonate to each sample until no more effervescence is observed ( or check the pH with pH paper and ensure it is 7 or slightly above

6. Use a dropping pipette to add 2cm³ Benedict’s reagent to each of the four samples

7. Record observations immediately after adding Benedict’s

8. Place all four tubes into the water bath

9. After 2 min record your observations of each sample, replacing the samples in the water bath

10. After a further 2 mins (4 total) remove tubes from water bath and record final observations of each sample

Extension Questions

1. What improvements can you suggest to the method you were given for this qualitative test?

   More accurate measuring of the Benedict’s reagent would be an improvement. Additional repeats might be suggested. Depending on the way in which the boiling tubes were labelled, students might comment on difficulties with labels becoming smudged or illegible in the water bath.

2. What changes would you make to obtain semi-quantitative results

   The results are already semi-quantitative

   - colour change given has a range (red, orange, yellow, green)
   - colour indicates approximation of reducing sugar concentration
   - therefore range (conc number) indicated semi-qualitative results

3. Outline a method for a fully quantitative test for reducing sugars

1) Measure 4cm³ of 5 different glucose concentration each into boiling tubes using a pipette
2) Using a pipette, measure 2cm³ of Benedict's solution and place this into all 5 boiling tubes
3) Place all 5 boiling tubes into a water bath and heat it for 4 mins at 100°C
4) If reducing sugar is present, solution will change colour from clear blue to orange-red
5) Colour change may be very subtle, as lower glucose concentration leaves more Cu2+ ions, therefore colour of solution appears blue prominently and conceals precipitates
6) to fix this use colorimeter (lamp and photoelectric cell)
7) Plot a graph of results (calibration curve)

4. Suggest how you could distinguish a sample of glucose from a sample of maltose using Benedict’s reagent

Perform the test for reducing sugars on both, glucose and maltose. Maltose, after filtering the solution will have produced more precipitate than glucose. You need to hydrolyse maltose with HCL before adding Benedict's reagent and then heat it. You will have x2 conc. of reducing sugars and x2 as much precipitate