Preparing and Using of Benedict's Reagent

Yellowing of the reaction mixtures containing reducing sugars (aldoses and ketoses)

Scientific name: The redox reaction between the aldehyde groups of the sugars and copper cations in an alkaline medium

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Safety

Wear eye protection goggles.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

RCHO + 2 Cu2+ + 5 OH- → RCOOH + Cu2O + 3 H2O

Step-by-step instruction

  1. Pour 100 ml of water into a beaker. Magnetic stirring should be turning on.
  2. Add 17.3 g of sodium citrate, 10 g of sodium carbonate, then, 17.3 g of copper sulfate pentahydrate into the beaker.
  3. Prepare five samples in 5 test tubes. The first sample is water. The second sample is sucrose solution. The third sample is glucose solution. The fourth sample is hydrolysed sucrose solution, which can be obtained by heating a mixture with hydrochloric acid and sucrose, followed by neutralization with soda. The fifth sample is a mixture of corn syrup with lemon-lime soda.
  4. Heat the samples using a water bath.
  5. Observe the change in color of different solutions.

Scientific background

Benedict's reagent, as well as the Tollens and Fehling reagents, are applied for the detection of reducing sugars (aldoses and ketoses), which is classified as such by the positive reaction with this reagent. Unlike non-reducing sugars, which include glycosides, reducing sugars contain a free aldehyde group, or may form it as a result of its isomerization in alkaline conditions.

Its action is based on a redox reaction between the aldehyde groups of the sugars and cations of copper in an alkaline medium. Wherein the copper-cations are reduced to copper (I) oxide, which settle as a red-colored precipitate.

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Published on 30 June 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst