Fehlingtype

Fehlingtype is a term used in chemistry to describe a class of qualitative tests based on Fehling's reaction. Named after Hermann von Fehling, these tests detect reducing agents, most notably reducing sugars, by observing the reduction of copper(II) ions to copper(I) oxide in an alkaline solution, which yields a brick-red precipitate.

Fehling's solution is prepared as two separate reagents, Fehling A and Fehling B. Fehling A typically contains copper(II) sulfate; Fehling B contains potassium sodium tartrate (Rochelle salt) as a complexing agent and sodium hydroxide to create the alkaline medium. When equal parts of Fehling A and B are mixed and heated with a sample containing a reducing group, the Cu2+ is reduced to Cu2O, causing the blue solution to turn brick red.

Scope: The test is sensitive to aldehydes and certain reducing ketones; most reducing monosaccharides (for example glucose) and some reducing disaccharides give a positive result. Nonreducing sugars such as sucrose require hydrolysis to reveal a reducing sugar. The test is qualitative, not highly specific, and can be affected by other reducing agents or by sample composition.

History and status: Fehling developed the test in 1848 as a method for identifying reducing carbohydrates. In many modern laboratories Benedict's test or enzymatic methods have supplanted Fehling's test due to greater stability and ease of use, but Fehling-type reactions remain a standard example in the teaching of redox chemistry and carbohydrate analysis.