Use a semi-quantitative method with Benedict’s reagent to estimate the concentrations of reducing sugars and with iodine solution to estimate the concentrations of starch, using colour standards.
OBJECTIVES
● To understand what is meant by a semi-quantitative test
● To be able to estimate concentrations of reducing sugars using Benedict’s reagent
● To be able to estimate concentrations of starch using iodine solution
● To develop the skills needed to plan an investigation using dilutions
SAFETY
● Wear eye protection.
● Avoid skin contact with Benedict’s reagent, iodine and hot solutions.
● Handle the test tubes with tongs to avoid burns.
● Do not taste the fruit juice.
MATHS SKILLS
● Solve algebraic equations.
EQUIPMENT
● eye protection
● test tubes, tongs and test tube rack
● small beakers
● Benedict’s reagent
● iodine solution
● 2% glucose solution
● 2% starch solution
● ‘unknown’ starch solution
● 1/10 dilution of fruit juice 1 cm3 and 5 cm3 syringes
● 3 cm3 pipettes
● distilled water
● waterproof marker pen
● water bath at 60–80 °C or a large beaker of recently boiled water
● timer or stop clock
PROCEDURE
PROCEDURE – PART 1: BENEDICT’S REAGENT METHOD
1. Plan how you will use the stock 2% glucose solution to make the following five concentrations of glucose solution: 2%, 1%, 0.5%, 1.5% and 0.25%. Write your plan in the space below.
2. Use the waterproof pen to label six test tubes and five small beakers with the different glucose concentrations they will contain. Label the sixth test tube ‘fruit juice’.
3. Use the syringes, the distilled water and the 2% glucose solution to create 5 cm3 of each solution in the corresponding labelled beaker.
4. Use a clean syringe to add 2 cm3 of Benedict’s reagent to each of the six labelled test tubes.
5. Using a clean syringe each time, add 1 cm3 of each glucose solution to the corresponding labelled test tubes.
6. Add 1 cm3 of fruit juice to the last labelled test tube.
7. Give each tube a gentle shake to ensure the contents are mixed.
8. Place all six test tubes in the water bath and start the timer.
9. After 2 minutes, use tongs to remove the test tubes from the water bath and place them in the test tube rack. Take care: the tubes will be hot.
10. Observe the tubes and record the results in a suitable table.
PROCEDURE – PART 2: IODINE SOLUTION METHOD
1. Plan how you will use the stock 2% starch solution to make the following five concentrations of starch solution: 2%, 1%, 0.5%, 0.2% and 0.1%. Write your plan in the space below.
2. Use the waterproof pen to label five test tubes and five small beakers with the different starch concentrations they will contain. Label the sixth test tube ‘unknown’
3. Use the syringes, the distilled water and the 2% starch solution to create 5 cm3 of each solution in the corresponding labelled beaker.
4. Use a clean syringe to add 0.5 cm3 of iodine to each of the six labelled test tubes. Add a further 10 cm3 of distilled water to each tube.
5. Using a clean syringe each time, add 5 cm3 of each starch solution to the corresponding labelled test tube.
6. Add 5 cm3 of the ‘unknown’ starch solution to the sixth test tube. Compare the colour produced in this tube with the colour of the diluted test tubes.
7. Record your results in a suitable table.
ANALYSIS OF RESULTS
1. Record your results for glucose in a suitable table.
2. If possible take a photograph of your standard solutions and keep it with your data
3. Plot a suitable graph to display your data
4. Record your results for starch in a suitable table
5. If possible take a photograph of your standard solutions and keep it with your data
6. Plot a suitable graph to display your data
LEARNING TIPS
● When recording the results of the Benedict’s test, use the correct descriptive language. Simply stating ‘green’ or ‘blue’ does not provide enough detail to allow comparison.
● One of the reasons for making the test semi-quantitative is to allow easier comparison, which may be less subjective and therefore more accurate. You will be expected to know the steps needed in the procedure and the expected results.