Benedict's reagent
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Benedict's reagent (also called Benedict's solution or Benedict's Test) is a reagent used as a test for the presence of reducing sugars (such as glucose, lactose, and fructose, but not sucrose) in a solution.
Benedict's reagent contains blue copper(II) sulfate (CuSO4) · 5H2O which is reduced to red copper(I) oxide. The copper oxide is insoluble in water and so forms a precipitate.The colour of the final solution ranges from green to brick red depending on how many of the copper(II) ions are present.
When Benedict's is used as a chemical 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 qualitative reagent. The mixture is placed in a boiling waterbath for 5 minutes and any precipitate formed is recorded as a positive for the presence of sugar in the food.
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. 5.0ml of Benedict's qualitative solution is mixed with 0.5ml of urine and the mixture is put in a boiling water bath for 5 minutes. The results are recorded thus:
no precipitate - Green a trace yellow + orange ++ red +++
Once 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.
Benedict's quantitative reagent is used to determine how much 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 qualitative 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 glucose in the sample can be worked out by comparing the volume needed to discolour the Benedict's with the volume of 1% glucose solution that is needed to discolour the Benedict's.
Benedict's reagent is also used in organic chemistry to determine the presence of aldehydes. Any present aldehyde is oxidised to form a carboxylic acid, while the Benedict's reagent is reduced to red copper oxide.
Benedict's qualitative and quantitative solutions can be purchased from chemical suppliers, but to make them up from scratch see making up solutions
Benedict's reagent is named after an American chemist, Stanley Rossiter Benedict (1884 – 1936).