3.4 : Testing For Carbohydrates Flashcards
What are reducing sugars?
All monosaccharides and some disaccharides are reducing sugars. This means that they can donate electrons, or reduce another molecule or chemical.
What is Benedicts Reagant?
An alkaline solution of copper (II) sulfate, used in the chemical test for reducing sugars.
Describe how the chemical test for reducing sugars is carried out.
- Place the sample to be tested in a tube. If it is not in liquid form, grind it up or blend it in water
- Add an equal volume of Benedict’s reagent
- Heat the mixture gently in a boiling water bath for five minutes.
How will a reducing sugar react in the chemical test ? What will the results be?
They 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 mixed with BR and warmed, a brick- red precipitate is formed, indicating a positive result.
How does the colour vary?
The more reducing sugar present , the more precipitate formed and the less blue Cu2+ ions are left in solution.
The shade of red / orange depends on the concentration of the reducing sugar present.
Is the Benedict’s test for reducing sugar qualitative or quantitative ?
Qualitative - dealing with colours, which is subjective , instead of actual numerical values and measurements we can analyse.
How do you use Benedict’s test for non - reducing sugars?
They do not react with BR and the solution will remain blue, which is negative.
If sucrose is first boiled with dilute hydrochloric acid it will then give a positive result when warmed with BR.
This is because the sucrose has been hydrolysed by the acid to glucose and fructose, both reducing sugars.
How do you test for starch?
The iodine test. A few drops of iodine dissolved in potassium iodide solution are mixed with a sample.
If it CHANGES colour from yellow / brown to purple / black , starch is present.
If it stays yellow / brown it is a negative result and starch is not present.
What is the advantage to using reagent strips?
Manufactured reagent strips can be used to test for the presence of reducing sugars.
With the use of a colour - coded chart, the concentration of the sugar can be determined.
