practical 11

• Quantitative Benedict's test


  Used to find an unknown concentration of glucose in a sample by creating a calibration curve, using known concentrations
• Reducing sugar


  Glucose is a reducing sugar
• Cu2+ ions


  Reduced to insoluble copper (I) oxide in Benedict's solution
• Colour change


  From blue to brick red, depending on glucose concentration
• Absorbance


  Measured in a colorimeter to determine glucose concentration
• Equipment


  • 10 mmol dm-3 glucose solution
  • Distilled water
  • "Urine" samples of unknown glucose concentration
  • Benedict's solution
  • Boiling tubes
  • Boiling tube rack
  • Water bath
  • Colorimeter
  • Cuvettes
• Dilution series


  Create a dilution series of glucose using distilled water, with concentrations ranging from 0 to 10 mmol dm-3
• Procedure


  1. Place 2cm3 of each unknown sample in separate boiling tubes

  2. Add 2cm3 of Benedict's solution to all boiling tubes

  3. Place boiling tubes in a water bath at 90°C for four minutes

  4. Zero the colorimeter using distilled water, set to red filter

  5. Measure absorbance of known samples using colorimeter

  6. Make a calibration curve, plotting absorbance against known glucose concentrations

  7. Measure absorbance of unknown samples and use calibration curve to determine glucose concentrations
• Risk assessment


  • Hazards: Benedict's solution (irritant), broken glass (cuts), hot liquids (scalding)
  • Safety precautions: wear eye protection, handle glassware carefully, use tongs for hot liquids
  • Emergency procedures: wash eyes/skin, elevate cuts, apply pressure, run burns under cold water, seek medical assistance
• For calibration curve: plot absorbance against known glucose concentrations, draw line of best fit
• High glucose concentration in urine sample indicates high risk of diabetes, as lack of insulin and insulin sensitivity leads to higher than usual blood glucose levels, which are filtered into the urine