Benedict's test

Benedict's test

Positive Benedict's test
Classification	Colorimetric method
Analytes	Reducing sugars

Benedict's reagent (often sold as Benedict's Qualitative Solution or Benedict's Solution) is a chemical reagent named after an American chemist, Stanley Rossiter Benedict.

Benedict's reagent is a chemical reagent commonly used to detect the presence of reducing sugars, however other reducing substances also give a positive reaction. This includes all monosaccharides and many disaccharides, including lactose and maltose. Such tests that use this reagent are called the Benedict's tests.

Generally, Benedict's test detects the presence of aldehydes, alpha-hydroxy-ketones, also by hemiacetal, including those that occur in certain ketoses. Thus, although the ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone, and gives a positive test because it is converted to the aldoses glucose and mannose by the base in the reagent.

A positive test with Benedict's reagent is shown by a colour change from clear blue to a brick-red precipitate.

One litre of Benedict's reagent can be prepared from 100 g of anhydrous sodium carbonate, 173 g of sodium citrate and 17.3 g of copper(II) sulfate pentahydrate. It is often used in place of Fehling's solution.

The principle of Benedict's test is that when reducing sugars are heated in the presence of an alkali they get converted to powerful reducing species known as enediols. Enediols reduce the cupric compounds (Cu²⁺) present in the Benedict's reagent to cuprous compounds (Cu⁺) which get precipitated as insoluble red copper(I) oxide(Cu₂O).