Fehlingin

Fehlingin is not recognized as a standard chemical substance in contemporary chemical literature. The term may be a misspelling or misremembering of Fehling's reagent, a classic diagnostic solution used to detect reducing sugars in solution. If Fehlingin appears in a text, it is important to verify whether the author meant Fehling's reagent, Fehling's test, or another similarly named compound.

Fehling's reagent comprises two separate solutions that are mixed before use: a copper(II) sulfate solution and a solution containing Rochelle salt (sodium potassium tartrate) in a strong alkali such as sodium hydroxide. When mixed and heated with a reducing sugar, the Cu2+ ion is reduced to copper(I) oxide, which forms a characteristic brick-red precipitate. The intensity of the precipitate correlates with the concentration of reducing sugar, enabling qualitative and semi-quantitative analyses. The test is historically significant in carbohydrate chemistry and has been used to distinguish aldoses from ketoses.

In modern practice, Fehling's test has largely been supplanted by more convenient or safer methods, such as Benedict's test or enzymatic assays, although Fehling-type principles remain relevant in some teaching laboratories and historical discussions. If encountering the term Fehlingin, readers should consult the primary source to determine whether it refers to Fehling's reagent, a proprietary formulation, or a typographical error.