PAG5: COLORIMETER

Describe the method for using a colorimeter. (8)

1. Switch the colorimeter on and allow it to stabilise for 5 minutes.

2. Select the red filter (for Benedict's test - use a complementary colour to starting solution) on the colorimeter. If using Benedict's test, centrifuge the solution or allow it to sit to precipitate out the copper solid.

3. Set the colorimeter to zero using a cuvette ¾ filled with distilled water.

4. Ensure the cuvette is placed in the colorimeter so the light passes through the clear sides.

5. Make sure the slides are clean and there are no bubbles in the solution.

6. Using a pipette, fill the cuvette ¾ with the sample.

7. Place in the colorimeter and read the absorbance of light.

8. Less light is absorbed by a paler solution, so there is greater transmission for a paler solution.

How can a calibration curve be used to measure the concentration of reducing sugars using a colorimeter? (4)

1. Use standard solutions of reducing sugars with known concentrations.

2. Carry out a Benedict's test on each sample, ensuring the Benedict's solution is in excess.

3. Use the colorimeter to measure the percentage transmission for each sample.

4. Plot a graph of transmission against reducing sugar concentration.

What is the relationship between transmission and reducing sugar concentration when using a colorimeter?

Higher transmission and lower absorbance indicate more sugar present (greater concentration)

Why does absorbance not have units? (1)

Absorbance is a ratio comparing light passing through a solution to light passing into the solution

What key experiments in OCR A A level Biology is colorimeter used for? (4)

- Measuring reducing sugar concentration

- Using Benedict's test followed by a colorimeter to measure absorbance/transmission.

- Investigating membrane permeability

- Measuring the absorbance of pigment released from beetroot cells at different temperatures or ethanol concentrations.