Colorimeter Use in AS & A Level Biology & Chemistry

Credit: Microbes Notes

Practical Handbook Page (Cambridge-aligned)

1. Purpose of this Page

This page supports teachers in selecting, justifying, and assessing the use of a colorimeter in AS & A Level practical work. It aligns with Cambridge International Biology (9700) and Chemistry (9701) practical skill frameworks.

2. Why Use a Colorimeter at AS & A Level?

• Measures absorbance of coloured solutions

• Allows quantitative comparison of concentration and reaction rates

• Appropriate for school laboratories

• Develops key skills: measurement, data handling, evaluation

Note: A spectrophotometer offers higher precision, but a colorimeter is fully acceptable for most AS investigations and many A Level contexts.

3. Practical Skill Codes (Cambridge Framework)

Biology (9700)

• P – Planning

• M – Manipulation & measurement

• O – Observation

• A – Analysis

• E – Evaluation

Chemistry (9701)

• P – Planning

• M – Manipulation & measurement

• A – Analysis & evaluation

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Biology (9700) – Investigations Using a Colorimeter

Topic Area	Typical Investigation	Skill Codes	How the Colorimeter is Used
Enzymes	Amylase + starch (iodine test)	M, O, A, E	Measures decrease in blue-black colour to calculate rate of reaction
Enzymes	Effect of temperature / pH on catalase	P, M, A	Compares absorbance to determine relative enzyme activity
Photosynthesis	DCPIP reduction by chloroplasts	M, O, A	Measures decrease in blue colour as DCPIP is reduced
Respiration	Methylene blue / resazurin reduction	M, A	Tracks colour change linked to respiration rate
Cell membranes	Beetroot pigment leakage	M, O, A, E	Measures absorbance of leaked pigment
Biological molecules	Quantitative Benedict’s test	M, A	Estimates glucose concentration from colour intensity
Microbiology	Yeast / bacterial growth	M, A, E	Measures turbidity (cloudiness) of culture

Biology – Exam-ready sentence

A colorimeter is used to measure absorbance of a coloured solution, allowing the rate of an enzyme-controlled reaction or the concentration of a substance to be determined.

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Chemistry (9701) – Investigations Using a Colorimeter

Topic Area	Typical Investigation	Skill Codes	How the Colorimeter is Used
Reaction kinetics	Iodine–thiosulfate reaction	M, A	Measures change in absorbance with time
Transition metals	Cu²⁺ / Fe³⁺ solutions	M, A	Determines concentration from absorbance
Equilibria	FeSCN²⁺ complex formation	P, M, A, E	Measures equilibrium concentration
Beer–Lambert law	Calibration curve	P, M, A	Plots absorbance vs concentration
Redox reactions	KMnO₄ / dichromate systems	M, A	Tracks colour change during redox
Environmental chemistry	Nitrates / phosphates in water	M, A, E	Measures colour intensity after test reaction

Chemistry – Exam-ready sentence

A colorimeter can be used to measure the absorbance of coloured solutions, enabling concentration changes and reaction rates to be determined quantitatively.

6. Choosing Between a Colorimeter and a Spectrophotometer

Feature	Colorimeter	Spectrophotometer
Light source	Filtered white light	Monochromatic light
Precision	Moderate	High
Cost & setup	Low, simple	Higher, complex
Suitability	AS practicals, school labs	Advanced A Level, research
Exam acceptance	Fully acceptable	Fully acceptable

7. Assessment Guidance for Teachers

When assessing practical work involving a colorimeter, look for:

• M – Correct zeroing, filter choice, consistent cuvette use

• A – Correct processing of absorbance data (tables, graphs)

• E – Recognition of limitations (filter accuracy, stray light, human error)

8. Common Student Errors

• Not zeroing the colorimeter

• Using dirty or scratched cuvettes

• Inconsistent orientation of cuvettes

• Confusing absorbance with transmittance

9. Safety & Good Laboratory Practice

• Wear eye protection when handling chemicals

• Dispose of reagents according to school protocol

• Handle electrical equipment with dry hands

10. Quick Reference Summary

• Use a colorimeter when the investigation involves a coloured solution.

• It supports key Cambridge skills: M, A, E.

• It is ideal for enzyme kinetics, photosynthesis, equilibria, and Beer–Lambert law.

End of Handbook Page