9.3 Benedict's Test Flashcards
How do we determine the result of a Benedict test
Reducing sugars lose electrons (are oxidised) to Cu2+ ions forming Cu+, this is seen as a colour change from blue to yellow/red (which is the highest concentration)
How do we test for reducing sugars?
- Prepare 4 samples: 1% glucose, 1% sucrose, 1% protein and distilled water
- Label 4 boiling tubes A to D and place 2cm3 of the corresponding samples into each using a fresh syringe every time
- add the same volume of Benedict’s regent to each four samples & record observations
- Place boiling tubes into 100.C water bath
- After two minutes record observations
The precipitate copper oxide which would be green-red indicates reducing sugars.
Cu2+ + Aldehyde group(cho) —> carboxylic acid(cooh) + Cu2O (cu+ x2 = o-2)
How can we observe the results qualitatively and quantitively?
Qualitatively- observe the colour change
Quantitively -use a colorimeter and a red filter to measure how much Cu 2 is left after the reaction to estimate/calculate the sugar concentration
How to test for non-reducing sugar e.g. sucrose
Carry out Benedict’s test one for negative results to ensure no reducing sugars.
1. Add 2cm3 of 2 mol/dm of HCl to samples to breakdown into monomers
2. Place the boiling tubes into a 100.C water bath for 2 minutes, then remove and cool
3. neutralise the added acid by adding NaHCO3 until no effervescence
4. check with indicator paper that PH is 7+
5. Now do steps 2-6 from reducing sugars
6. Positive test proves a non-reducing sugar
What is a reducing sugar?
- A sugar that can act as a reducing agent /is oxidised (loses electrons),
- any sugar that has a C with an OH group (hemiacetal group) next to the O can act as reducing sugar
- In water, the ring of sugar will open exposing the aldehyde group which will be oxidised as its not involved in a glycosidic bond
Why is sucrose a non-reducing sugar?
The C next to the O does not contain an OH as it is involved in a glycosidic bond so It lacks any hemiacetal groups and so doesn’t open up exposing an aldehyde group
Why does Benedict reagent turn red in the presence of reducing sugar?
The aldehyde group in reducing sugar loses electrons to form Cu+ from Cu2+ causing a precipitate Cu2O to form which is seen as a colour change
What are the conditions required for Benedict reagent to work effectively?
Water bath at 100°
Under five minutes
Not acidic
same vol as sample
Why can you not do the test for non reducing sugars Before testing for reducing sugar
a reducing sugar could show present in a test for non reducing sugar therefore you have to make sure it’s not a reducing sugar first by filtering out the precipitate
How could you make this test semi-quantitative
Measure the ph colour at each stop eg every min
How could you tell the difference between glucose and maltose using Benedict’s reagent
- glucose turns into rapid red colour quickly
- maltose only turns yellow and turns red after further heating as it has to break down into glucose first
