3.4 Tests For Reducing & Non-Reducing Sugars Flashcards
(22 cards)
Describe the method for testing for reducing sugars.
- 1) Set up a water bath at 80 degrees C using a thermometer
- 2) -If substance is solid: grind into powder using pestle and mortar, add 1cm^3 of powder, 2cm^3 distilled water. If substance is liquid: add 2cm^3.
- 3) Add 0.5cm^3 of Benedict’s reagent to each test solution (making sure the pipette doesn’t touch the test tubes)
- 4) Place all in the water bath for 5 minutes
- 5) At 5 minutes, retrieve the test tubes, record the colour changes.
What does a positive result for reducing sugars look like?
A colour change from blue, to green, orange or red.
What does a negative result for reducing sugars look like?
No colour change, the solution stays blue.
What is the safety risk of using Benedict’s reagent?
Benedict’s reagent is alkaline and an irritant, so can irritate skin. So, wear safety glasses to prevent it from entering eyes.
What is a qualitative result?
A result based on what you see
What is a quantitative result?
A result based on numerical measurements.
Is the Benedict’s test for reducing sugars qualitative or quantitative?
Qualitative, because it tells us whether a substance is present or not, and roughly how much based on observation only.
Why is Benedict’s Reagent blue?
Because it contains Copper ions (Cu2+).
Why does Benedict’s Reagent change colour when subjected to reducing sugars?
-Reducing sugars are able to donate electrons to the copper ions
-Copper ions in the Benedict’s solution are blue
-The copper ions are reduced, and a coloured precipitate is formed
-The more reducing sugar is present, the more of the blue copper ions will be reduced.
The colour of the precipitate indicates the concentration of reducing sugar present
What does the colour of the precipitate indicate?
The concentration of reducing sugar present
What is a reducing sugar?
Any sugar that is capable of giving electrons away (acting as a reducing agent)
What does a reagent strip test for?
Reducing sugars
What substance is used to test for reducing sugars?
Benedict’s reagent
Describe the method for testing for non-reducing sugars.
1) Test for reducing sugars using Benedict’s solution + method
2) If any solutions remain blue, that indicates that there are no reducing sugars
3) Boil the sugar solution with dilute hydrochloric acid
4) Neutralise the mixture with sodium hydrogencarbonate (an alkali)
5) Add Benedict’s reagent and heat in a water bath
6) Look for a positive result (green/yellow/orange/brick red precipitate)
What is Acid Hydrolysis?
Using acid to break down chemical bonds
How come sucrose gives a positive result for reducing sugars after acid hydrolysis?
The acid hydrolyses the glycosidic bonds in sucrose and it is broken down into glucose and fructose. Glucose and fructose are both reducing sugars, so they give a positive result
Why do we have to neutralise an acidic solution before performing a Benedict’s test?
To prevent the acidic environment from interfering with the reaction
What substance do you use to test for the presence of starch? (What makes the substance a solution?)
Iodine solution (iodine dissolved in a potassium iodide solution)
How to test for the presence of starch?
Mix a sample with a few drops of iodine dissolved in a potassium iodide. If starch is present, solution changes from yellow/brown to purple/black
What is the colour change of iodine solution if starch is present?
Yellow/brown to purple/black.
What is colorimetry?
A piece of equipment used to quantitatively measure the absorbance or transmission of light by a coloured solution.
What does a biosensor do?
Uses biological components to determine the presence and concentration of molecules.
