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2.1.22 · Biological Molecules

The Benedict’s Test — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

2.1.2 The Benedict's Test — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Carry out the Benedict's test and use it semi-quantitatively with standards, timing and a colorimeter.Add an excess of Benedict's reagent to the sample solution in a test tube.Heat in a water …

  • Excess reagent
  • Positive result
  • Absorbance
  • Calibration
  • Keep constant
  • Non-reducing sugars

Part 1 of 7Method for the Benedict's test

Method for the Benedict's test
Add an excess of Benedict's reagent to the sample solution in a test tube.
Heat in a water bath brought to the boil for a few minutes.
Observe the colour.
Excess reagent
  • Excess reagent there must be more than enough copper(II) sulfate to react with all the sugar.
Positive result
  • Positive result blue changes to brown or brick-red, and the colour shows the concentration of reducing sugar.

Part 2 of 7Semi-quantitative method with standards

1
Make standard glucose solutions by serial dilution of a stock solution.
2
Add the same volume of Benedict's reagent to each standard.
3
Heat each in a boiled water bath at the same temperature for the same time, about 5 minutes.
4
Record the colour of each standard.
5
Treat the unknown sample in the same way.
6
Compare its colour with the standards to estimate the concentration.

Part 3 of 7Alternative approaches

Timing
  • Timing measure how long it takes for the first colour change from blue to green.
  • A higher concentration of reducing sugar gives a shorter time.
Colorimeter
  • Colorimeter avoids human error in judging colour by measuring absorbance against a calibration curve.

Part 4 of 7Serial dilutions

Serial dilutions
Meaning
  • Meaning a series of dilutions of a stock solution, with concentration changing by the same amount between tubes.
Types
  • Types doubling dilutions or a set range such as 0, 2, 4, 6, 8, 10 mmol dm-3.
    • (concentration halved each time)
Uses
  • Uses visual comparison, calibration curves, colorimeter readings, and counting bacteria or yeast.

Part 5 of 7Using a colorimeter

Principle
  • Principle a chosen wavelength is passed through the sample and the absorbed light is measured.
Filter
  • Filter the filter colour contrasts with the sample; a blue filter suits an orange Benedict's product.
Absorbance
  • Absorbance brick-red absorbs more blue light than orange-green, so absorbance measures colour strength.
Calibration
  • Calibration a blank in the colorimeter should read 0, and this is repeated periodically.
Calibration curve
  • Calibration curve plot absorbance against known concentrations and read unknowns from the graph.

Part 6 of 7Control variables

  • Keep constant
    • volume of sample, volume of Benedict's, temperature and heating time.

Part 7 of 7More from the specification

Starch
  • An insoluble polysaccharide of glucose used by plants for energy storage.
Lipids
  • Concentrated energy, insulation, cell membranes.
Precipitate
  • An insoluble solid formed in a solution.
Iodine
  • Element needed to make thyroxine.
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
Non-reducing sugars
  • Non-reducing sugars they give no change with Benedict's reagent alone,
    • so boil with dilute hydrochloric acid, neutralise,
    • and test again for a positive result.

Key terms

TermMeaning
Semi-quantitativeGiving an estimate of amount rather than an exact value.
Serial dilutionA series of dilutions that changes concentration by the same factor each time.
ColorimeterA device measuring how much light a solution absorbs.
AbsorbanceA measure of the light absorbed by a sample.
Calibration curveA graph of known concentrations against readings.
Stock solutionThe starting concentrated solution used for dilutions.

Avoid these mistakes

1
Using too little Benedict's reagent — an excess is needed so sugar is the limiting factor.
2
Not keeping heating time and temperature constant — colour depends on both.
3
Judging colour by eye only for quantitative results — a colorimeter removes subjectivity.
4
Forgetting to zero the colorimeter with a blank — the blank should read 0.

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