The Determination of the Presence of Sugar Using Fehling's Reagent

The Determination of the Presence of Sugar Using
Fehling's Reagent
Aims of an experiment
Learning about the properties and the preparation of copper(I) oxide through the redox reaction between the bistartratocuprate(II) complex and the reducing sugar.
Introduction
Copper(I) oxide, Cu2O is a red crystalline solid. It can appear either yellow or red, depending on the procedural details and the size of the particles. Copper(I) oxide can be prepared by heating of the copper(II) oxide. Other way of preparation is when a base is added to the aqueous copper(I) chlorine containing complex. It's presumed that primarily the yellow precipitate of copper(I) hydroxide is formed, even if there is little evidence that it really happens, since it undergoes rapid change to red copper(I) oxide caused by continued heating.
Formation of copper(I) oxide is the basis of the Fehling's test. It was developed by German chemist Hermann von Fehling in 1849. The test is used to differentiate between an aldehyde and a ketone functional group. The difference between them is the presence of the hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. That hydrogen atom makes aldehydes a very strong reducing agents while the ketones are much less reactive. An aldose is the monosaccharide with one aldehyde functional group (e.g. glucose), while the ketose is the monosaccharides with one ketone functional group (e.g. fructose). Ketones can't be detected with this test in contrast to 2-ketoses which change to aldoses under the alkaline conditions and thus can be detected. Fehling's test was one of the first tests used to screen for glucose in the urine detecting diabetes mellitus. Sugar is not normally found in the urine, but when blood sugar levels rise above the tolerable limit, the kidneys release some of the excess sugar from the blood into the urine. Fehling's reagent is always prepared fresh in the laboratory. It is initially made as two separate solutions. Fehling's I is a light blue aqueous solution of copper(II) sulfate pentahydrate crystals, while Fehling's II is a colourless solution of aqueous potassium sodium tartrate and a strong alkali.
Materials and methods
Chemicals: Fehling's I solution, Fehling's II solution, glucose, distilled water
Laboratory equipment: pipette, boiling tube, Bunsen burner
Procedure:
1. Pour 5ml of freshly prepared Fehling's I solution (7g of copper(II) sulfate pentahydrate, CuSO4 x5H2O/ 100ml H2O) to a boiling tube and dilute it just a little with distilled water.
2. Add an equal amount of Fehling's II solution (35g of Seignette's salt, differently named Rochelle salt, i. e. potassium sodium tartrate, KNaC4H4O6 and 11g of sodium hydroxide, NaOH diluted in 100ml of H2O) and 0.2mg of sugar.
3. Heat the solution gently over a Bunsen flame.
Results
When a colourless Fehling's II solution is added to the light blue Fehling's I solution the colour of the mixture turns to transparent royal blue.
Soon after the start of the heating the solution becomes opaque and the dark yellow precipitate is formed.
As the heating proceeds the colour of the mixture becomes darker orange until it changes colour to a bright red.
Discussion
Copper(II) ions are mixed with tartrate ions under the alkaline conditions. The presence of tartrate ions prevents precipitation of copper(II) hydroxide because the royal blue bistartratocuprate(II) complex is formed. Cu(II) ions from the bistartratocuprate(II) complex are reduced to Cu(I) ions which react with hydroxide ions forming yellow precipitate of copper(I) hydroxide, while the aldehyde functional group of the glucose molecules is oxidized forming gluconic acid. Further boiling causes yellow precipitate to change to a bright red precipitate of copper(I) oxide.
Conclusion
The bistartratocuprate(II) complex in Fehling's solution is an oxidizing agent in the reaction with the reducing sugar glucose. Red copper(I) oxide precipitate indicates that the sugar is present in the solution .
References:
Greenwood, Norman N.; Earnshaw, Alan . Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. 1997.
Solomons, T. W. Graham, Craig B. Fryhle, and Scott A. Snyder. Organic Chemistry. 11th ed. John Wiley & Sons Canada, Limited, 2013. 744. Print.
Wikipedia contributors. "Hermann von Fehling." Wikipedia, The Free Encyclopedia. Wikipedia,