L8 - Sugar Tests Flashcards
What are the 2 types of sugar tests?
reducing sugar test
non-reducing sugar test
How do you do the reducing sugar test?
3
Add 2cm3 of the sugar + an equal quantity of Benedict’s reagent
Heat to approx. 80c
The colour of the solution will change from blue if a reducing sugar is present
What colours can you get as a result of a reducing test and what do they mean?
blue - no reducing sugar green yellow brown brick red (increasing quantity of reducing sugar going down)
How do you know you have a really positive reducing sugar test?
the test will produce a brick red precipitate
How do you know you have a really negative reducing sugar test?
the solution will remain blue
Why do reducing sugars change from blue to brick red?
copper (II) sulphate (blue solution) is reduced to copper (I) oxide (brick red precipitate)
When should you carry out a non-reducing sugar test?
after testing for reducing sugars as the solution may contain a mixture of both reducing + non-reducing sugars
How do you test for non-reducing sugars?
Place 2cm3 of the test solution into a boiling tube
Add 1cm3 of dilute hydrochloric acid + boil it to hydrolyse the disaccharides
Cool + neutralise with excess sodium hydroxide (test with litmus as Benedict’s test only works in alkali conditions. Red litmus goes blue)
Repeat Benedict’s test
how do you know non-reducing sugars were present?
there will be a brick red precipitate
How do you know non-reducing sugars were not present?
the solution will remain blue
What chemical is used to test for sugars?
benedict’s reagent
What are examples of reducing sugars?
3
glucose
fructose
maltose
What is an example of a non-reducing sugar?
sucrose
Is the Benedict’s test quantitive?
Why?
Semi-quantitive
No numbers but do have colour differences which can indicate level of glucose (sugar)
