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what is the action of Benedict's solution on the different kinds of sugars?

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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 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 glucose in the sample can be calculated 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.