Biochemical Tests Flashcards
Biuret test =
Proteins
Iodine test =
Starch
Emulsion test =
Lipids
Benedicts test =
Sugars
Describe how you would use a biochemical test to show that a reducing sugar is present. (2)
- Add Benedict’s (reagent) and heat / warm.
- Red / orange / yellow / green (colour); accept brown.
A quantitative Benedict’s test produces a colour whose intensity depends upon the concentration of reducing sugar in a solution. A calorimeter can be used to measure the intensity of this colour.
A scientist used the test to produce a calibration curve of calorimeter reading against concentration of maltose. Describe how the scientist would have produced the results in the graph.
- Use maltose solutions of different known concentrations.
- Carry out Benedicts test on each of these.
- Use calorimeter to measure the intensity of colour for each solution.
- Plot the results on a graph, creating a calibration curve.
- Find concentration of sample from calibration curve.
Describe how to carry out a test for lipids.
- Shake the test substance with ethanol for a minute to dissolve it.
- Then pour the solution into water.
- Milky emulsion shows presence of lipids.
What is the positive result for a lipids test?
A milky emulsion.
What are reducing sugars?
All monosaccharides (e.g. glucose) and some disaccharides (e.g. maltose and lactose).
How do you carry out a test for reducing sugars?
- Add Benedict’s reagent (blue) to a sample and heat it in a thermostatic water bath at the boil.
- A positive test will result in a brick red colour.
- A negative test will result in a blue colour.
How do you carry out a test for non-reducing sugars?
- Add dilute HCl and heat carefully in a water bath at the boil.
- Neutralise the solution with sodium hyrogencarbonate.
- ## Then carry out the Benedicts test as you would with a reducing sugar test.Benedicts test:
- Add Benedict’s reagent (blue) to a sample and heat it in a thermostatic water bath at the boil.
- A positive test will result in a brick red colour.
- A negative test will result in a blue colour.
What is the test for starch?
Iodine test.
Describe how to test for starch:
- Add iodine dissolved in potassium iodide solution to the test sample.
- If there is starch present, the sample changes from browny-orange to a dark, blue-black colour.
What is the positive result for iodine?
Browny-orange changes to a dark, blue-black.
Describe how to carry out a test for protein:
- Add a few drops of sodium hydroxide solution ( to make it alkaline).
- Then add copper (II) sulfate solution.
- If protein is present solution turns purple.
- If no protein is present solution will stay blue.
