PAG 9: Qualitative Testing Flashcards
What is the Biuret test used for? (1)
The Biuret test is used to test for proteins
Describe the method for the Biuret test. (2)
Add Biuret solution (sodium hydroxide and copper (II) sulfate) to the sample
A positive result turns the solution from blue to purple
How can the Biuret test give a false positive? (1)
The presence of enzymes can also produce a positive result
What is the Benedict’s test used for? (1)
The Benedict’s test is used to detect reducing sugars
Describe the method for testing for reducing sugars. (3)
- Add Benedict’s reagent to a sample
- Heat the sample
- If a reducing sugar is present, a coloured precipitate forms, changing from blue to brick red
How does the colour change in Benedict’s test indicate sugar concentration? (1)
The precipitate colour varies from blue → green → yellow → orange → brick red depending on the concentration of the reducing sugar
How can non-reducing sugars be detected? (4)
- Add dilute hydrochloric acid to hydrolyse glycosidic bonds
- Neutralise with sodium hydrogen carbonate
- Add Benedict’s reagent and heat
- If a precipitate forms, a non-reducing sugar was present
What is the iodine test used for? (1)
The iodine test is used to detect starch
Describe the iodine test method and result. (2)
- Add iodine solution and shake gently to mix
- If starch is present, the solution turns from browny-orange to blue-black
What is the emulsion test used for? (1)
The emulsion test is used to detect lipids
Describe the method for the emulsion test. (3)
- Add ethanol and shake to dissolve lipids
- Add water
- A positive result forms a white/milky emulsion
How could the results of a Benedict’s test be improved with a colorimeter? (1)
A colorimeter provides quantitative results rather than subjective colour interpretation
How could the Benedict’s test be modified for semi-quantitative results? (2)
Prepare at least five different glucose concentrations
Carry out Benedict’s test and use colour standards to estimate glucose concentration
Why should Benedict’s solution be added in excess? (1)
To ensure all reducing sugar reacts
