Colorimetry measures how much light of a chosen wavelength a coloured solution absorbs. The absorbance is used to find the concentration of the coloured substance.

How does path length affect absorbance?

A longer path through the solution means more light is absorbed. Cuvettes of the same width are used so that path length stays constant.

Why is a filter of the complementary colour used in a colorimeter?

A solution absorbs most strongly the colour opposite its own. A blue solution absorbs orange-red light, so a red or orange filter gives the greatest sensitivity.

Where is colorimetry used in industry and healthcare? Give three examples.

A mining chemist measures copper in ore. A pharmaceutical chemist checks the aspirin content of tablets, and a clinical chemist measures substances such as glucose in blood.

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Analytical Chemistry Techniques

Principles of colorimetry

3 things to know
A2 9

Overview

How colorimetry measures concentration, the Beer–Lambert relationship, complementary filters, preparing standards by dilution, reading a calibration curve with a worked value, and its limitations.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is colorimetry?

Key term

Why is colorimetry an accurate way to find concentration?

Key term

How is absorbance related to concentration?

Questions from this subtopic

Each one with the model answer underneath.

1
What is colorimetry?
Model answer
Colorimetry measures how much light of a chosen wavelength a coloured solution absorbs. The absorbance is used to find the concentration of the coloured substance.
2
Why is colorimetry an accurate way to find concentration?
Model answer
Absorbance is measured electronically rather than judged by eye. The instrument detects small differences in colour intensity that the eye would miss.
3
How is absorbance related to concentration?
Model answer
Absorbance is directly proportional to concentration for dilute solutions, as the Beer–Lambert law states. A doubled concentration gives double the absorbance.
4
How does path length affect absorbance?
Model answer
A longer path through the solution means more light is absorbed. Cuvettes of the same width are used so that path length stays constant.
5
Why is a filter of the complementary colour used in a colorimeter?
Model answer
A solution absorbs most strongly the colour opposite its own. A blue solution absorbs orange-red light, so a red or orange filter gives the greatest sensitivity.
6
Where is colorimetry used in industry and healthcare? Give three examples.
Model answer
A mining chemist measures copper in ore. A pharmaceutical chemist checks the aspirin content of tablets, and a clinical chemist measures substances such as glucose in blood.

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