Fehling's solution

Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone functional groups, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling in 1849. Fehling's solution is a mixture of two solutions: Fehling's "A" which is a blue aqueous solution of copper(II) sulfate and Fehling's "B" which is a clear solution of potassium sodium tartrate (Rochelle salt) and a strong alkali (usually sodium hydroxide). When mixed in equal volumes, they react to form a deep blue solution that is partially reduced to a brick-red precipitate of copper(I) oxide (Cu2O) when heated with a reducing sugar. The presence of a reducing sugar is indicated by the formation of a red precipitate, whereas non-reducing sugars do not react.

Chemistry

The chemical reaction underlying the Fehling's test involves the reduction of the complexed copper(II) ion (Cu^2+) in the Fehling's solution to copper(I) oxide (Cu2O), which precipitates out of solution, and the corresponding oxidation of the reducing sugar. The active component in Fehling's "A" solution, copper(II) sulfate, serves as the source of Cu^2+ ions. In Fehling's "B" solution, potassium sodium tartrate (Rochelle salt) complexes with the Cu^2+ ions to stabilize them in solution, allowing for a more uniform reaction. The strong alkali in Fehling's "B" provides the necessary alkaline conditions for the reaction.

Application

Fehling's solution is widely used in carbohydrate chemistry for the qualitative analysis of reducing sugars. It is particularly useful in distinguishing between aldose and ketose sugars, as aldoses are typically oxidized by Fehling's solution, while most ketoses are not, unless they are converted to aldoses through isomerization. This property makes Fehling's test a valuable tool in the identification and characterization of carbohydrates in various biological and food samples.

Preparation

To prepare Fehling's solution, equal volumes of Fehling's "A" and "B" solutions are mixed together. The mixture should be used immediately or stored in a dark, cool place to prevent deterioration. The exact concentrations of the components can vary, but a typical preparation involves dissolving 69.28 grams of copper(II) sulfate pentahydrate in water to make 500 mL of solution (Fehling's "A") and dissolving 173 grams of potassium sodium tartrate tetrahydrate and 50 grams of sodium hydroxide in water to make 500 mL of solution (Fehling's "B").

Limitations

While Fehling's solution is a useful tool for detecting reducing sugars, it has several limitations. It cannot distinguish between different reducing sugars, as the test is qualitative rather than quantitative. Additionally, other substances that can be oxidized under alkaline conditions may also give a positive result, leading to potential false positives. The test also requires the sugar to be in solution, and it cannot detect sugars that do not readily dissolve in water.

Safety

Handling Fehling's solution requires caution. Both solutions are corrosive and can cause burns upon contact with skin. The use of personal protective equipment, such as gloves and safety goggles, is recommended when preparing and using Fehling's solution. Proper ventilation is also important to avoid inhalation of fumes.

See Also

• Benedict's solution
• Tollens' reagent
• Biuret test

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