Benedict's reagent
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Benedict's reagent (also called Benedict's solution or Benedict's test) is a reagent named after an American chemist, Stanley Rossiter Benedict. It is used as a test for the presence of reducing sugars such as glucose, fructose and maltose, or more generally for the presence of aldehydes (except aromatic ones). It is often used in place of Fehling's solution.
Benedict's reagent contains blue copper(II) sulfate (CuSO4) which is reduced to red copper(I) oxide (Cu2O) by aldehydes, also oxidizing them to carboxylic acids. The copper(I) oxide is insoluble in water and so precipitates.
Benedict's reagent can be made from 100 g sodium carbonate and 173 g sodium citrate dissolved in 850 mL water, to which a solution of 17.3 g copper(II) sulfate in 100 mL of water is slowly added, and the overall reagent made up to 1 litre.
[edit] Chemical test
To test for the presence of reducing sugars in food, the food sample is dissolved in water and about 5ml of the sample solution is added to 5ml of Benedict's reagent. The mixture is placed in a boiling water bath for 5 minutes and any precipitate formed is recorded as a positive result for the presence of reducing sugars in the food. Sucrose (household sugar) is a non-reducing sugar and thus does not react with Benedict's reagent.
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. 5ml of Benedict's reagent is mixed with 0.5ml of urine and the mixture is put in a boiling water bath for 5 minutes. The results are recorded like this:
| no precipitate | — |
| yellow | a trace |
| green | + |
| orange | ++ |
| red | +++ |
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.
[edit] Quantitive 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 as follows:
- Accurately measure 25ml of Benedict's quantitative reagent and pour into a 100ml conical flask.
- Add 6g of anhydrous sodium carbonate, to keep the solution alkaline.
- Add a few anti-bumping granules and bring the solution to a gentle simmer.
- Pour the sample into a burette and allow the sample to run from the burette into the conical flask until all the blue colour has disappeared.
- Repeat twice more, but this time allow the bulk of the sample to run into the conical flask all at once; then when the end point is near let it drip in one drop at a time, and boil for 30 seconds between each addition.
- Average the results.
The titration should be repeated with 1% glucose solution instead of the sample in order to calibrate the Benedict's. The concentration of reducing sugar in the sample can be calculated by comparing the volume of 1% glucose solution needed to discolour the Benedict's with the volume of the sample that was needed to do the same.
