Benedict's reagent

Benedict's reagent (also called Benedict's solution or Benedict's test) is a chemical reagent named after an American chemist, Stanley Rossiter Benedict.^[1]

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Benedict's reagent is used as a test for the presence of reducing sugars such as glucose, fructose, galactose, lactose and maltose, or more generally for the presence of aldehydes (except aromatic ones). It can be prepared from sodium carbonate, sodium citrate and copper(II) sulfate.^[2] It is often used in place of Fehling's solution.

Benedict's reagent contains blue copper(II) ions (Cu²⁺) which are reduced to copper(I) (Cu⁺). These are precipitated as red copper(I) oxide which is insoluble in water.

Chemical test

To test for the presence of reducing sugars in food, the food sample is dissolved in water, and a small amount of Benedict's reagent is added. The mixture is heated in a boiling water bath, and any precipitate formed is recorded as a positive result for the presence of reducing sugars in the food.

Sucrose (table sugar) is a non-reducing sugar and thus does not react with Benedict's reagent. Sucrose can produce a positive result with Benedict's reagent if heated with dilute hydrochloric acid prior to the test. The acidic conditions and heat break the glycosidic bond in sucrose through hydrolysis. The products of sucrose decomposition are glucose and fructose, which can be detected by Benedict's reagent as described above.

Benedict's reagent can be used to test for the presence of glucose in urine. Glucose found to be present in urine is an indication of diabetes. Once a reducing sugar is detected in urine, further tests have to be undergone in order to ascertain which sugar is present. Only glucose is indicative of diabetes.

Quantitative reagent

Benedict's quantitative reagent is used to determine how much reducing sugar is present. This solution forms as white precipitate rather than a red one and so can be used in a titration. The titration should be repeated with 1% glucose solution instead of the sample for calibration.

References

1. ^ Benedict, S. R. (1908). "A Reagent For the Detection of Reducing Sugars". J. Biol. Chem. 5: 485–487.
2. ^ Robert D. Simoni, Robert L. Hill, and Martha Vaughan (2002). "Benedict's Solution, a Reagent for Measuring Reducing Sugars: the Clinical Chemistry of Stanley R. Benedict". J. Biol. Chem. 277 (16): 10-11.