Colorimetry

Free Colorimetry revision notes for OCR A Level Biology – covering specification point 2.1.2(r).

Colorimetry

Colorimetry is a quantitative method used to determine the concentration of a coloured solution by measuring how much light is absorbed (or transmitted) through a solution.

A colorimeter measures how much light (of a specific wavelength) passes through a solution placed in a cuvette.

A blank (usually distilled water) is used to calibrate the colorimeter to zero absorbance (or 100% transmission) for comparison with the solution being tested.

Colorimetry for reducing sugars

Procedure:

1. Add excess Benedict’s reagent to your sample and heat it to 80^oC.

The more reducing sugar present, the more precipitate that forms, and the fewer copper(II) ions remain in solution.

2. Centrifuge the mixture to remove the precipitate.
3. Collect the supernatant (the clear liquid) and place into a cuvette.
4. Calibrate a colorimeter with distilled water (for comparison).
5. Use a colorimeter to measure the absorbance of the supernatant.

Use a red filter (blue Benedict’s solution absorbs red light).

6. Compare absorbance readings to those from solutions of known concentration to create a calibration curve by plotting absorbance vs concentration. 

Less absorbance = higher concentration of reducing sugar.

Then use the graph to find the concentration of unknown samples by interpolation.