Fehling

Fehling's solution, also called Fehling reagent, is a chemical reagent used to detect reducing sugars in a solution. It is named after the German chemist Hermann von Fehling, who developed the test in the 19th century. The reagent is supplied as two separate solutions, Fehling's A and Fehling's B, which are mixed immediately before use. Fehling's A contains an aqueous solution of copper(II) sulfate, while Fehling's B is a strongly alkaline solution containing sodium potassium tartrate (Rochelle salt) and sodium hydroxide.

Procedure and reaction: When Fehling's solution is heated with a solution containing reducing sugars, the blue Cu(II) ions are reduced to copper(I) oxide (Cu2O), which forms a brick-red precipitate. A positive result is indicated by the appearance of this brick-red precipitate and the disappearance of the blue color.

Chemistry and scope: The test detects reducing sugars because their aldehyde group (or enediol form after tautomerization) reduces Cu2+ to Cu+. The reaction is favored under alkaline conditions; aldoses and reducing disaccharides typically yield positive results, while many ketoses may also be detected due to tautomerization. Results can be influenced by the sample composition; substances such as ascorbic acid or other strong reducing agents can cause false positives, and very high sugar concentrations can affect precipitation.

History and use: The test is named after Hermann von Fehling and was widely used in clinical chemistry to detect glucose and other reducing sugars in urine and solutions. In modern practice, Fehling's test has largely been superseded by Benedict's test and enzymatic methods, but it remains of historical importance and is still used in some teaching laboratories.