chemical test for carbohydrates-quanlitive

Question 1

What test do you use to test for reducing sugars

Answer 1

the benedict tets

Question 2

What is contained in the benedict solution

Answer 2

It is a blue, alkaline solution of copper (two) sulfate.

-it is the Cu2+ ions that give it its blue colour

Question 2

Describe what happens when there is reducing sugar present and explain why

Answer 2

In the presence of a reducing sugar, a brick-red precipitate forms, and a colour change occurs anywhere from blue to green, orange,brown/brick-red

Explanation
In the presence of a reducing sugar, the reducing sugar reacts with the Cu2+ ions, which results in the addition of electrons to the Cu2+ reducing them to Cu + ions -in other words the copper 2 sulfate is reduced to copper oxide

Question 2

why does a precipitate form

Answer 2

It forms as the copper sulfate is reduced to copper oxide, and copper oxide is insoluble in water, so a precipitate forms

Question 3

the method of the Benedict test

Answer 3

-add equal volume of Benedict reagent to a sample
-Put an excess of benedict to ensure there is more than enough present to react with all the reducing sugar present
-Then heat it in a thermostatically set water bath for 5 min to allow for a colour change to occur
-if a reducing sugar is present, a coloured precipitate will form as copper sulfate has been reduced to copper oxide

-A positive result is a colour change anywhere from blue top green to orange for the medium concentration of reduced sugar
and brown/brick red for high concentration of reducing sugar

Question 4

What is a limitation of the benedict test

Answer 4

-it gives only an estimate of the concentration of reducing sugar present and does not give a quantitative value

Question 5

what is the non-reducing sugar you must known of

Answer 5

sucrose, everything else is a reducing sugar

Question 5

what happens when we carry out the benedict test on a non-reducing sugar

Answer 5

-negative result,the colour of the solution stays blue,this is because the non-reducing sugar cant donate electrons to reduce the Cu2+ ions to Cu+ ions and thus allowing for a brick red precipitate of copper oxide to form

Question 6

how can i make the beenedict test work on a non-reducing sugar

Answer 6

first we need to hydrolyse the glycosidic bond in order to break the disaccharide into alpha glucose and fructose,both reducing suagrs which give a positive result in a benedict test

Question 7

method for non-reducing sugar and explain where needed

Answer 7

-heat the non-reducing sgar sample in dilute hydrochloric acid.This will provide the water needed to break the glycosidic bonds, resulting in the formation of alpha glucose and fructose.They are reducing sugars thus able to donate electrons and reduce the soluble copper sulfate into insoluble copper oxide,allowing for a coloured precipitate to form

Neutralise the solution with sodium hydrogen carbonate ,use a red litmus paper to indicate when the solution is neutralised and add a bit more sodium hydrogen carbonate after as the benedict test requires slightly alkaline conditions to work.

Then do the test as normal add benedict reagent and heat the sample in a thermostatically set water bath for 5 minutes-if a colour change occurs ,reducing sugar is present

Question 8

how do you test for starch

Answer 8

-add a couple of drops of iodine dissolved in potassium iodide to the sample

-wait for a colour change to occur

Question 9

what is apostive result for starch

Answer 9

orange/brown to blue/black

Question 10

what is a negative result for starch

Answer 10

colour stays orange/brown

Question 11

what do we get when starch is broken down and how will that effect test for starch

Answer 11

if enzyme is added and starch is broken w eget maltose and maltose will give a negative result in latest for starch,meaning that the colour of the solution will remain orange/brown if all starch has been broken down into maltose