Fehling's test

Color change of the solution from blue to brown

Scientific name: Oxidation of glucose in the presence of Cu(II)

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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

2 KNaC4H4O6 + CuSO4 + 2 NaOH → K2Na2[Cu(C4H4O6)2] + Na2SO4 + 2H2O

RCHO + K2Na2[Cu(C4H4O6)2] → Cu2O + RCOOH

Step-by-step instruction

  1. Prepare the first solution. Dissolve 4 g of copper sulfate in 20 ml water.
  2. Prepare the second solution. For this, dissolve a little potassium sodium tartrate, and sodium hydroxide in water.
  3. Mix the first and the second solutions. Thus, we have obtained the Fehling's solution.
  4. Take two test tubes. Put a small amount of table sugar into the first test tube and a small amount of glucose into the second test tube. Add some water into both test tubes with stirring, achieving complete dissolution.
  5. Add Fehling's solution into both tubes and mix.
  6. Place both tubes in a water bath.
  7. Watch the color change in one of the test tubes (with glucose).

Scientific background

Copper cations can form complexes with tartrate anions that have a deep-blue color. In this case, this complex is an oxidant which can oxidize aldehydes to carboxylic acids.

Table sugar consists essentially of sucrose, which has not an aldehyde groups. Glucose has an aldehyde group which exhibits reducing properties in this case.

Danger:
Coolness:
Difficulty:

Published on 13 June 2015

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