Benedict's reagent (often called Benedict's qualitative solution or Benedict's solution) is a chemical reagent and complex mixture of sodium carbonate, sodium citrate, and copper(II) sulfate pentahydrate. It is often used in place of Fehling's solution to detect the presence of reducing sugars and other reducing substances. Tests that use this reagent are called Benedict's tests. A positive result of Benedict's test is indicated by a color change from clear blue to brick-red with a precipitate.

Generally, Benedict's test detects the presence of aldehyde groups, alpha-hydroxy-ketones, and hemiacetals, including those that occur in certain ketoses. In example, although the ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone which results to a positive test because the base component of Benedict converts it into aldoses glucose and mannose. Oxidizing the reducing sugar by the cupric (Cu<sup>2+</sup>) complex of the reagent produces a cuprous (Cu<sup>+</sup>), which precipitates as insoluble red copper(I) oxide (Cu<sub>2</sub>O).

The test is named after American chemist Stanley Rossiter Benedict.

Composition and preparation

Benedict's reagent is a deep-blue aqueous solution. Each litre contains:

  • 17.3 g copper sulfate
  • 173 g sodium citrate
  • 100 g anhydrous sodium carbonate or, equivalently, 270 g sodium carbonate decahydrate

Separate solutions of the reagents are made. The sodium carbonate and sodium citrate are mixed first, and then the copper sulfate is added slowly with constant stirring.

Sodium citrate acts as a complexing agent which keeps Cu<sup>2+</sup> in solution, since it would otherwise precipitate. Sodium carbonate serves to keep the solution alkaline. In the presence of mild reducing agents, the copper(II) ion is reduced to copper(I), which precipitates in the alkaline conditions as very conspicuous red copper(I) oxide.

Organic analysis

To test for the presence of monosaccharides and reducing disaccharide sugars in food, the food sample is dissolved in water and a small amount of Benedict's reagent is added. During a water bath, which is usually 4–10 minutes, the solution should progress through the colors of blue (with no reducing sugar present), orange, yellow, green, red, and then brick red precipitate or brown (if a high concentration of reducing sugar is present). A color change would signify the presence of a reducing sugar.

See also

  • Dextrose equivalent

Other oxidizing reagents

  • Fehling's solution
  • Tollens' reagent

Other reducing reagents

  • Jones reductor
  • Walden reductor

References