3.4 Testing for Carbohydrates

Question 1

What does reduction involve?

Answer 1

Gain of electrons

Question 2

What saccharides are reducing sugars? What does this mean?

Answer 2

Disaccharides
Monosaccharides
Means they can donate electrons or reduce another molecule/chemical

Question 3

What test is used to test for reducing sugars?

Answer 3

Benedict’s test

Question 4

What is the Benedict’s reagent?

Answer 4

Alkaline solution of copper(II)sulfate

Question 5

How is the Benedict’s test carried out?

Answer 5

1. Place sample in a boiling tube. If sample isn’t liquid, grind it up or blend it in water
2. Add equal volume of Benedict’s reagent
3. Gently heat the mixture in a boiling water bath for 5 mins

Question 6

What happens during the Benedict’s test?

Answer 6

• Reducing sugars react w/ copper ions in Benedict’s reagent
• This results in electrons being added to the blue copper(II) ions, reducing them to brick red copper ions

Question 7

What indicates that reducing sugars are present during the Benedict’s test?

Answer 7

A brick-red precipitate

Question 8

During the Benedict’s test, how can you differentiate between the different concentrations of reducing sugars present?

Answer 8

Like traffic lights:

Green = Low conc.
Yellow/amber/orange= Medium conc.
Red = High conc.

Question 9

What happens when non-reducing sugars and Benedict’s solution are mixed?

Answer 9

No reaction - solution remains blue after warming indicating a negative result

Question 10

What is the most common non-reducing sugar?

Answer 10

Sucrose

Question 11

How do you test sucrose with Benedict’s?

Answer 11

• First boil sucrose with dilute hydrochloric acid
• This will give a positive result when warmed with Benedict’s solution

This is because sucrose has been hydrolysed by the acid into glucose and fructose (reducing sugars).

Question 12

What is the Iodine test used to test for?

Answer 12

Starch

Question 13

How is the Iodine test carried out?

Answer 13

Mix a few drops of iodine dissolved in potassium iodide solution with the sample

Question 14

What indicates the presence of starch during the Iodine test?

Answer 14

Solution turns from yellow/brown to purple/black

Solution = remains brown/yellow = negative result - no starch present

Question 15

What are reagent strips used for?

Answer 15

To test for presence of reducing sugars

Question 16

What is the advantage of reagent strips?

Answer 16

Concentration of sugar can be determined with the use of a colour-coded chart

Question 17

What is a colorimeter used for?

Answer 17

To quantitatively measure absorbance/transmission of light by a coloured solution
(The more concentrated the solution is, the less light will be transmitted)

Question 18

What can the results of a colorimeter calculate?

Answer 18

Concentration of the reducing sugar present

Question 19

How do you carry out the procedure to determine the concentration of a glucose solution?

Answer 19

1. Place a filter in the colorimeter
2. Calibrate colorimeter using distilled water
3. Conduct Benedict’s test on a range of known concentrations of glucose
4. Filter resulting solutions to remove precipitate
5. Measure % transmission of each glucose solution using the colorimeter
6. Repeat steps 3-6 using the solution with unknown concentrations of glucose