Testing For Carbohydrates

Question 1

What is reduction

Answer 1

Is a reaction involving the gain of electrons. All monosaccharides and some disaccharides(eg maltose and lactose) are reducing sugars. This means they can donate electrons, or reduce another molecule or chemical.

In the chemical test for a reducing sugar, this chemical is Benedict’s reagent, an alkaline solution of copper(II) sulfate.

Question 2

How is Benedict’s test carried out

Answer 2

1. Place sample to be tested in a boiling tube. If it is not in liquid form, grind it up or blend it in water.
2. Add an equal volume of Benedict’s reagent.
   3.Heat the mixture gently in a boiling water bath for five minutes

Question 3

How do reducing sugars work in the Benedict’s test

Answer 3

Reducing sugars will react with the copper ions in Benedict’s reagent. This results in the addition of electrons to the blue Cu2+ ions, reducing them to brick red Cu+ ions. When a reducing sugar is mixed with benedicts reagent and warmed, a brick-red precipitate is formed indicating a positive result.

The more reducing sugar present, the more precipitate formed and the less blue Cu2+ ions are left in solution, so the actual colour seen will be a mixture of brick-red(precipitate) and blue(unchanged copper ions) and will depend on the concentration of the reducing sugar present. This makes the test qualitative.

Question 4

Using Benedict’s test for non- reducing sugars

Answer 4

Non-reducing sugars do not react with Benedict’s solution and the solution will remain blue after warming, indicating a negative result. Sucrose is the most common non-reducing sugar.

If sucrose is first boiled with dilute hydrochloric acid it will then give a positive result when warmed with Benedict’s solution. This is because the sucrose has been hydrolysed by the acid to glucose and fructose, both reducing sugars.

Neutralise solution with sodium hydrogen carbonate because Benedict’s reagent needs slight alkaline conditions to work

Question 5

Iodine test for starch

Answer 5

The iodine test is used to detect the presence of starch. To carry out the test, a few drops of iodine dissolved in potassium iodide solution are mixed with a sample. If the solution changes colour from yellow/brown to purple/black starch is present in the sample. If iodine solution remains yellow/brown it is a negative result and starch is not present.

Question 6

Reagent strips

Answer 6

Manufactured reagent test strips can be used to test for the presence of reducing sugars, most commonly glucose. The advantage is that, with the use of a colour-coded chart, the concentration of the sugar can be determined.