CIE A Level Biology复习笔记2.1.2 The Benedict's Test

Method

• Add Benedict's reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube
  • 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
• Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes
• If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water
• A positive test result is, therefore, a colour change somewhere along a colour scale from blue (no reducing sugar) to brown/brick-red (a high concentration of reducing sugar)
  • This test is semi-quantitative as the degree of the colour change can give an indication of how much (the concentration of) reducing sugar present

Semi-quantitative Benedict's test

• A semi-quantitative test can be carried out by setting up standard solutions with known concentrations of reducing sugar (such as glucose)
• These solutions should be set up using a 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
  • It is important 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 colourimeter 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 colourimeter
  • They can be used when:
    • Counting bacteria or yeast populations
    • Determining unknown glucose, starch, protein concentrations

Making serial dilutions

Colourimeter

• A colourimeter 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 colourimeter), 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
• Colourimeters must be calibrated before taking measurements
  • This is completed by placing a blank into the colourimeter 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

A colourimeter is used to obtain quantitative data that can be plotted to create a calibration curve to be used to find unknown concentrations