PAG 5.2 Determining Glucose Concentration

• Quantitative test


  Tells us not only whether a particular substance is present, but at what concentration
• Reducing sugars such as glucose

  1. Reduce the Cu2+ in Benedict's reagent to Cu1+

  2. Causing a precipitate to form

  3. Resulting in a colour change
• Equipment


  • Benedict's reagent
  • Distilled water
  • 1% glucose solution
  • Unknown glucose solution
  • 5 x 50 cm3 beakers
  • 7 boiling tubes
  • 6 x 5 cm3 syringes
  • Water bath set at 100˚C
  • 14 dropping pipettes
  • Centrifuge
  • 8 centrifuge tubes
  • 8 clean cuvettes
  • Colorimeter
• Health and Safety


  • Eye protection must be worn throughout the activity due to the presence of boiling water
  • Benedict's reagent is low hazard but contact with eyes should be avoided
  • The boiling water bath presents a scalding hazard
• Procedure


  1. Collect 10 cm3 of 1% glucose solution

  2. Make a dilution series of glucose in distilled water to give the following concentrations: 1.00%, 0.50%, 0.25%, 0.13%, 0.06%

  3. Add 1 cm3 of the corresponding glucose concentration to each boiling tube

  4. Add 10 cm3 quantitative Benedict's reagent to each tube

  5. Incubate for 15 minutes in water bath

  6. Cool the boiling tubes

  7. Pipette 2 cm3 of each solution into a labelled centrifuge tube

  8. Centrifuge all 8 centrifuge tubes for 2 minutes

  9. Set the colorimeter to 'red light' and use distilled water to set 100% transmission

  10. Measure the transmission of the supernatant from each centrifuge tube

  11. Create a calibration curve using the results

  12. Use the calibration curve to find the glucose concentration of the Unknown glucose solution
• High concentration of glucose

  • High concentration of Cu+ but low concentration of CU2+
  • Lower concentration of Cu2+ means less red light is absorbed causing a higher transmission
• Centrifuging the sample before taking colorimeter readings ensures that the precipitate has been separated from the sample
• If the reading for the unknown glucose solution did not fall within the range of the calibration curve, the curve could be extended following the pattern to reach a higher concentration