Fehling solution

Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrates 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" and Fehling's "B". Fehling's "A" is a blue aqueous solution of copper(II) sulfate, while Fehling's "B" 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 which is then used to test for the presence of aldehydes.

Preparation

To prepare Fehling's solution, equal volumes of Fehling's A and Fehling's B are mixed together. The resulting mixture is then ready for use in testing for reducing sugars. The test is based on the reduction of complexed copper(II) ions (from the copper sulfate in Fehling's A) to copper(I) oxide (Cu2O), which precipitates out of solution as a red solid. This reaction occurs in the presence of reducing sugars, such as glucose or fructose, under alkaline conditions provided by the sodium hydroxide in Fehling's B.

Application

Fehling's solution is primarily used in the Fehling's test, a qualitative test in organic chemistry for the presence of reducing sugars. The test is significant in food industry for sugar analysis and in medical diagnostics for diabetes through glucose detection in urine samples. When a reducing sugar is heated with Fehling's solution, a red precipitate of copper(I) oxide is formed, indicating a positive result. Non-reducing sugars do not react, as they cannot directly donate electrons to the copper(II) ions to reduce them to copper(I) oxide.

Limitations

While Fehling's solution is useful for detecting the presence of reducing sugars, it does have limitations. It cannot distinguish between different reducing sugars, and it may give false positives with other substances that can reduce copper(II) ions under alkaline conditions. Additionally, the test requires careful control of reaction conditions, as the presence of other reducing agents can interfere with the results.

Safety Considerations

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