Explanation for the test for reducing and non reducing sugars

[lightoftruth]

Could someone please explain to me why we do each step in the tests for reducing sugars and non reucing sugars...

i.e why do we have to create a solution....why do we need to heat the solution when the benedicts reagent has been added etc..

Thanks in Advance

[blue orchid]

Reducing sugars such as maltose and lactose have the ability to carry out the chemical reaction known as reduction.All monosaccharides are reducing sugars and some disaccharides.
Benedict's solution contains copper sulphate. Reducing sugars reduce blue copper sulphate to red brown copper sulphide, which is seen as the precipitate. Non-reducing sugars cannot do this.

The heating of the solution produces kinetic energy to assist the chemical reaction.

[theprof]

With all due respect blue orchid, I have to point out that you have over-simplified the chemical reaction that occurs here.

What happens is that the cupric ion that is complexed with the citrate ion in the Benedict's Solution is reduced to a cuprous ion by the oxidised functional aldehyde group (-CHO) in the reducing sugar.

This does indeed precipitate out but you should note that the test is semi-quantitative as well as qualitatitive. This is because the precipitate goes through colour changes from blue, to green, to yellow, to orange, to red and finally to brick-red depending on the concentration of the reducing sugar in the sample under analysis. Because of this the final precipitate may not necessarily be red-brown in colour - it can be any of the other colours in the sequence under certain circumstances.

This variation is sometimes known as the Benedict's Quantitative Test although you should note that the same reagant is used.

Concentrations of reducing sugars from around 0.05% to 3% can be roughly measured in this way and for many years, this was the only means of testing the percentage concentration of reducing sugars in urine.

Finally, the reduced cuprous ion precipitates out as cuprous (1) oxide, not "copper sulphide".

[Peter Pedant]

Blimey I remember my GP doing this in his surgery in the fifties

You need toheat it toget the reaction going - striaghforward phys chem - if it is hotter then it goes faster.