PAG 9.3 Qualitative Testing for Glucose

• Qualitative test


  Gives a positive or negative result to tell us whether a particular substance is present, does not tell us how much of the substance is present
• Positive result for Benedict's test for reducing sugars


  Colour change due to the reduction of Cu2+ to Cu1+
• Equipment


  • Benedict's reagent
  • 2 dropping pipettes
  • 8 boiling tubes
  • 8 x 5 cm3 syringes
  • Samples in beakers labelled 'A, B, C and D'
  • Water bath set to 100˚C
  • 2.0 mol dm-3 hydrochloric acid
  • Sodium hydrogen carbonate
  • Small spatula
  • pH paper
  • Marker pen
• Testing for reducing sugar


  1. Label four boiling tubes A - D and place 4 cm3 of the corresponding samples into each

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

  3. Record observations immediately after adding Benedict's reagent

  4. Place all four boiling tubes into the water bath

  5. After 2 minutes record observations, replace samples in water bath

  6. After 4 minutes total incubation time, remove tubes and record final observations and conclusions about presence/absence of reducing sugar
• Testing for non-reducing sugar

  1. Put 4 cm3 of each sample A-D into labelled boiling tubes

  2. Use dropping pipette to add ~2 cm3 2 mol dm-3 hydrochloric acid to each sample

  3. Place tubes in water bath and incubate for 2 minutes

  4. Remove tubes, allow to cool, then neutralise acid by adding sodium hydrogen carbonate until no more effervescence

  5. Check pH, repeat neutralisation if needed

  6. Use dropping pipette to add ~2 cm3 Benedict's reagent to each sample

  7. Record observations immediately after adding Benedict's reagent

  8. Place tubes in water bath for 2 minutes, record observations, replace in water bath

  9. After 4 minutes total incubation time, remove tubes and record final observations and conclusions about presence/absence of non-reducing sugar