CIE AS Biology (9700) exams from 2022

Revision Notes

2.1.2 The Benedict's Test

The Benedict's Test for Reducing Sugars

  • Benedict’s solution can be used to carry out a semi-quantitative test on a reducing sugar solution to determine the concentration of reducing sugar present in the sample
    • It is important that an excess of Benedict’s solution is used so that there is more than enough copper (II) sulfate present to react with any sugar present
  • The intensity of any colour change seen relates to the concentration of reducing sugar present in the sample
    • A positive test is indicated along a spectrum of colour from green (low concentration) to brick-red (high concentration of reducing sugar present)
  • A semi-quantitative test can be carried out by setting up standard solutions with known concentrations of a reducing sugar (such as glucose)
  • These solutions should be set up using serial dilution of an existing stock solution
  • Each solution is then treated in the same way: add the same volume of Benedict’s solution to each sample and heat in a water bath that has been boiled (ideally at the same temperature each time) for a set time (5 minutes or so) to allow colour changes to occur
    • Remember it is really important thing is to ensure that an excess of Benedict’s solution is used
  • Any colour change observed for each solution of a known concentration in that time can be attributed to the concentration of reducing sugar present in that solution
  • The same procedure is carried out on a sample with an unknown concentration of reducing sugar which is then compared to the stock solution colours to estimate the concentration of reducing sugar present

Alterations

  • It is also possible to standardise this test but instead of waiting a fixed amount of time for a range of colours to be observed, time how long it takes for the first colour change to occur (blue to green)
    • The higher the concentration of reducing sugar in a sample, the less time it would take for a colour change to be observed
  • To avoid issues with human interpretation of colour, a colorimeter could be used to measure the absorbance or transmission of light through the sugar solutions of known concentration to establish a range of values that an unknown sample can be compared against a calibration curve

Serial dilutions

  • Serial dilutions are created by taking a series of dilutions of a stock solution. The concentration decreases by the same quantity between each test tube
    • They can either be ‘doubling dilutions’ (where the concentration is halved between each test tube) or a desired range (e.g. 0, 2, 4, 6, 8, 10 mmol dm-3)
  • Serial dilutions are completed to create a standard to compare unknown concentrations against
    • The comparison can be:
      • Visual
      • Measured through a calibration/standard curve
      • Measured using a colorimeter
    • They can be used when:
      • Counting bacteria or yeast populations
      • Determining unknown glucose, starch, protein concentrations

Colorimeter

  • A colorimeter is an instrument that beams a specific wavelength (colour) of light through a sample and measures how much of this light is absorbed (arbitrary units)
  • They provide a quantitative measurement
  • They contain different wavelengths or colour filters (depends on the model of colorimeter), so that a suitable colour can be shone through the sample and will not get absorbed. This colour will be the contrasting colour (eg. a red sample should have green light shone through)
    • Remember that a sample will look red as that wavelength of light is being reflected but the other wavelengths will be absorbed
  • Colorimeters must be calibrated before taking measurements
    • This is completed by placing a blank into the colorimeter and taking a reference, it should read 0 (that is, no light is being absorbed)
    • This step should be repeated periodically whilst taking measurements to ensure that the absorbance is still 0
  • The results can then be used to plot a calibration or standard curve
    • Absorbance against the known concentrations can be used
    • Unknown concentrations can then be determined from this graph

 

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.
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