KEGS Biology Blog


Monday 17th September 2012 Section 1
September 16, 2012, 5:36 pm
Filed under: Uncategorized

Non-reducing sugar Benedicts test and calibration curve

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5 Comments so far
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but for the last lesson we did a test for non-reducing sugars. The purpose of heating sucrose with HCL(aq) at the start was to break down the disaccharide molecule (sucrose) into fructose and glucose using a hydrolysis reaction.
Sodium hydrogencarbonate was added to neutralise the acid.
the Benedict’s control test showed a negative result as there was no change of colour, proving that sucrose was a non reducing sugar.

Comment by stefansoyombo

“Non-reducing Sugars (Benedict’s test). Sucrose is called a non-reducing sugar because it does not reduce copper sulphate, so there is no direct test for sucrose. However, if it is first hydrolysed (broken down) to its constituent monosaccharides (glucose and fructose), it will then give a positive Benedict’s test. So sucrose is the only sugar that will give a negative Benedict’s test before hydrolysis and a positive test afterwards. First test a sample for reducing sugars, to see if there are any present before hydrolysis. Then, using a separate sample, boil the test solution with dilute hydrochloric acid for a few minutes to hydrolyse the glycosidic bond. Neutralise the solution by gently adding small amounts of solid sodium hydrogen carbonate until it stops fizzing, then test as before for reducing sugars.” – http://www.mrothery.co.uk

Comment by roobennithi

I thought this was a good video from you tube on the experiment http://www.youtube.com/watch?v=QU0VBcHnQOk but they use Sodium Hydroxide to neutralise the acid instead of sodium hydrogencarbonate.

Comment by eilishsmyth

Benedict solution is composed of copper sulfate. When used in the non-reducing sugars test, a negative reaction would show a blue colour, whereas a positive reaction would see a change to orange then brown. 1cm of sucrose would be added to a boiling tube, then 1cm of HCL(aq) would be added to the tube and swirled. The tube would be heared in a hot water bath for five minutes. Two spatulas of sodium hydrogencarbonate would then be added and we would wait until the fizzing stops. The same test would be done again but no sodium hydrogencarbonate would be added or HCL(aq) for that tube to be a controlled tube.

Comment by stefansoyombo




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