Testing For Carbohydrates

TesAng for carbohydrates.
These tests are QUALITATIVE this means they do not tell us any exact data, just yes or no answers i.e. is there a reducing sugar present or not.

Tes:ng for reducing sugars –

Reducing sugars are sugars that can donate electrons, and become oxidised.
- (oxidised means it gains electrons, reduced means it loses electrons. The transfer of

electrons is essenAal in biological reacAons). BENEDICT’S TEST FOR REDUCING SUGARS:

1- Add an excess of benedict’s reagent to the sample being tested.
Benedict’s soluAon is a blue soluAon that contains CuSO4 ions. When these ions gain electrons from the reducing sugars that are donaAng the electrons. When CuSO4 ions gain electrons they form a precipitate which is brown to brick red in colour.

2- Heat the sample and Benedict’s mixture unAl it boils.

3- Observe a colour change.

If there is a lot of reducing sugar present we will see the colour change from blue to brown/brick red.
If there is a low amount of reducing sugar present we will see a colour change to green, yellow or orange.

NOTE- it is important to use an excess of Benedict’s so that there is more than enough CuSO4 ions for the reacAon.
NOTE- galactose, glucose, fructose and maltose are all reducing sugars. Sucrose is a non- reducing sugar.

Tes:ng for non-reducing sugars –

1- Add dilute hydrochloric acid to the sample and boil.
2- Add sodium hydrogencarbonate to neutralise the soluAon.

a. Test the soluAon using a pH indicator to ensure it is neutralised.

b. The benedict’s soluAon needs to be slightly alkaline to work. 3- Carry out the test for reducing sugars.

By adding hydrochloric acid we are making the non-reducing sugar a reducing sugar. This is because the acid hydrolyses the glycosidic bonds and form aldehyde of ketone groups – which are able to donate electrons.

Iodine test for starch -

When starch is present in a sample and you add iodine it will turn a blue-black colour.