P.04 Practical: Colorimeter

Overview of Using a Colorimeter

• Colorimeter Function:
  • Measures the amount of light absorbed by a coloured liquid.
  • Absorbance: Directly related to concentration—the deeper the colour, the more light absorbed.
• Selecting Light Colour:
  • Use a complementary light colour to the colour of the sample for accurate absorbance readings.
  • Example: To measure red pigment concentration, use green light, as red solutions reflect red light and absorb green and blue light.
• Cuvettes:
  • Special clear tubes used to hold liquid samples within the colorimeter.
  • Ensures consistent light path length and uniform sample handling.

Calibration Process for Colorimeter

1. Prepare the Blank:

• Fill a cuvette with a set volume of clear liquid (same as the solvent in your samples but without any pigment).
• Insert the blank into the colorimeter and set absorbance to 0. This provides a baseline for measurements.

1. Measure Standards (Known Concentrations):

• Place each standard solution (known concentrations of pigment) in an identical cuvette.
• Insert into the colorimeter and record absorbance for each concentration.
• Between each measurement, recheck with the blank to ensure the absorbance remains at 0. If not, recalibrate by starting from Step 1.

1. Create Calibration Curve:

• Plot absorbance values against known concentrations on a graph to create a calibration curve.
• This curve is used to determine the concentration of unknown samples by comparing absorbance readings.

Measuring Unknown Sample Concentrations

1. Measure Unknown Sample Absorbance:

• Place each unknown sample in a cuvette and measure its absorbance using the colorimeter.

1. Determine Concentration Using Calibration Curve:

• Use the calibration curve to match each sample’s absorbance with the corresponding concentration value on the curve.

Key Terms

Practical Tips

• Accuracy:
  • Ensure that each cuvette is clean and handle it by the edges to avoid fingerprints, which can affect absorbance readings.
  • Consistent Volume:
  • Use the same volume for each sample and blank to ensure accurate comparisons.
• Frequent Calibration:
  • Check the blank after every sample reading to maintain accuracy and recalibrate if necessary.