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The deeper the colour of a solution, the more of the coloured substance it contains. A colorimeter measures that depth of colour precisely, as the amount of light the solution absorbs. With a set of standard solutions and a calibration graph, it becomes a surprisingly accurate way to measure concentration. It’s one of the most versatile practicals in chemistry and biology, used for everything from copper ions to food dyes to rates of reaction.
How a colorimeter works
A colorimeter has four main parts:
- Light source: a bulb or LED.
- Filter (or an LED of a specific colour): selects a narrow range of wavelengths.
- Cuvette: a small, square container (usually with a 1 cm path length) holding the sample.
- Detector: a photocell that measures how much light passes through.
The instrument compares the light passing through the sample with the light passing through a blank and displays either absorbance or percentage transmission.
Absorbance is proportional to concentration, as described by the Beer–Lambert law:
A = εcl
Choosing the right filter
A solution looks coloured because it absorbs some wavelengths of white light and transmits the rest. The colour you see is made of the wavelengths that get through. So the colour absorbed most strongly is roughly the complementary colour to the one you see.
To get the biggest absorbance change (and the most sensitive measurement), use a filter of the complementary colour to the solution:
| Colour of solution | Colour absorbed most | Filter to use |
|---|---|---|
| Blue (e.g. copper(II) sulfate) | orange / red | red or orange |
| Yellow (e.g. potassium chromate) | violet / blue | blue |
| Red (e.g. iron thiocyanate complex) | blue-green | blue-green or green |
| Purple (e.g. potassium permanganate) | green / yellow-green | green |
| Green (e.g. nickel(II) sulfate) | red / violet | red |
| Orange (e.g. potassium dichromate) | blue | blue |
A simple way to check: try each filter with your most concentrated standard and choose the one that gives the highest absorbance.
Aim
To find the concentration of copper(II) sulfate in an unknown solution using a colorimeter and a calibration curve.
Equipment and chemicals
- Colorimeter with a set of filters (or an LED colorimeter)
- Cuvettes and cuvette rack
- 0.100 mol/dm³ copper(II) sulfate stock solution
- Distilled water
- Burettes or graduated pipettes for accurate volumes
- Boiling tubes or small volumetric flasks
- Unknown copper(II) sulfate solution
- Tissue for wiping cuvettes
- Eye protection
Method
1. Make the standards
Prepare 10.0 cm³ of each standard by mixing stock solution and distilled water accurately (for example, from two burettes):
| Standard | Stock (cm³) | Water (cm³) | Concentration (mol/dm³) |
|---|---|---|---|
| Blank | 0.0 | 10.0 | 0.000 |
| 1 | 2.0 | 8.0 | 0.020 |
| 2 | 4.0 | 6.0 | 0.040 |
| 3 | 6.0 | 4.0 | 0.060 |
| 4 | 8.0 | 2.0 | 0.080 |
| 5 | 10.0 | 0.0 | 0.100 |
Mix each thoroughly.
2. Set up the colorimeter
- Switch on and let the lamp warm up.
- Select the red filter (copper(II) sulfate is blue).
- Fill a cuvette about three-quarters full with distilled water (the blank). Handle cuvettes only by the ribbed or frosted sides, and make sure the clear sides face the light path.
- Wipe the outside, insert it, and set the absorbance to zero.
3. Measure the standards
- Rinse a cuvette with a little of standard 1, then fill it.
- Wipe, insert, and record the absorbance.
- Repeat for each standard, working from least to most concentrated.
- Re-check the blank reads zero every few measurements.
4. Measure the unknown
Rinse and fill a cuvette with the unknown solution and record its absorbance. Repeat at least twice and take the mean.
Sample results
| Concentration (mol/dm³) | Absorbance |
|---|---|
| 0.000 | 0.00 |
| 0.020 | 0.10 |
| 0.040 | 0.21 |
| 0.060 | 0.31 |
| 0.080 | 0.41 |
| 0.100 | 0.52 |
| Unknown (mean) | 0.26 |
Analysis
- Plot absorbance (y-axis) against concentration (x-axis).
- Draw a straight line of best fit through the points and the origin.
- From the unknown’s absorbance (0.26), draw a horizontal line across to the calibration line, then down to the x-axis.
Using the line (gradient ≈ 5.2 dm³/mol for these data):
c = 0.26 ÷ 5.2 = 0.050 mol/dm³
The unknown lies within the range of the standards, so the result is reliable. For more on calibration graphs, see making and using a calibration curve.
Why copper(II) sulfate absorbances are small
Hydrated copper(II) ions absorb light only weakly (a small molar absorptivity), so fairly concentrated solutions are needed to get useful absorbance readings. For measuring low copper concentrations, such as in water samples, chemists add ammonia to form the deep blue [Cu(NH₃)₄(H₂O)₂]²⁺ complex, which absorbs much more strongly and gives far greater sensitivity.
Using colorimetry to follow a reaction rate
Colorimeters can also record absorbance over time, which makes them ideal for rates of reaction:
- Iodine clock and iodination of propanone: the colour of iodine fades as it reacts.
- Decolourisation of potassium permanganate by ethanedioic acid.
- Crystal violet with sodium hydroxide: the purple dye fades.
Because absorbance is proportional to concentration, a graph of absorbance against time is effectively a concentration–time graph, from which rates and orders of reaction can be found.
Evaluation and sources of error
| Source of error | Effect | Improvement |
|---|---|---|
| Fingerprints or scratches on cuvettes | extra absorbance or scattering | handle by frosted sides; wipe before inserting |
| Air bubbles in the cuvette | scattered light, false readings | tap gently to remove bubbles |
| Inaccurate dilutions | standards not at stated concentrations | use burettes or graduated pipettes, not measuring cylinders |
| Instrument drift | readings shift over time | re-zero with the blank frequently |
| Cuvette not always facing the same way | small inconsistencies | mark one side and always insert the same way |
| Unknown outside the range of standards | unreliable extrapolation | dilute the unknown or extend the standards |
Colorimeters vs spectrophotometers
A colorimeter uses filters or coloured LEDs that each pass a band of wavelengths, often 30–50 nm wide. A UV–visible spectrophotometer uses a diffraction grating to select a very narrow band (just a few nanometres) at any chosen wavelength, including ultraviolet. Spectrophotometers are more accurate and can scan a full absorption spectrum to find λmax, but they’re more expensive. For many school and field measurements, a simple, inexpensive colorimeter gives perfectly good results.
Safety
- Wear eye protection throughout the practical.
- Copper(II) sulfate is harmful if swallowed, irritating to eyes and skin, and toxic to aquatic life. Dispose of solutions in the designated waste container, not down the sink.
- Mop up spills and wash hands after the practical.
Key takeaways
- A colorimeter measures how much light of a chosen colour a solution absorbs.
- Use a filter of the complementary colour to the solution for maximum sensitivity.
- Zero with a blank, measure a series of standards, and plot absorbance against concentration.
- Read the unknown’s concentration from the calibration line, staying within the range of standards.
- Colorimetry can also follow reaction rates, because absorbance is proportional to concentration.
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