Analysing Food Dyes with a Colorimeter

Brightly coloured drinks and sweets owe their colour to small amounts of synthetic dyes. Food regulators set limits on how much of each dye can be used, and food labs check those limits routinely. In this practical, you’ll measure the concentration of a blue dye in a coloured drink using a colorimeter, the same principle a food lab would use, with a spectrophotometer.

It’s a classic quantitative practical that brings together dilutions, the Beer–Lambert law, calibration graphs and evaluation of errors.

The science

A coloured solution absorbs some wavelengths of visible light and transmits others. The amount of light absorbed, the absorbance, is proportional to the concentration of the coloured substance (at a fixed path length and wavelength):

A = εcl (the Beer–Lambert law)

Rather than relying on a value of ε from a book, you’ll make a calibration graph using solutions of known concentration, then read the unknown’s concentration from the graph. See colorimetry and calibration curves.

Choosing the dye and drink

This practical is described for Brilliant Blue FCF (E133, often called Blue 1), a common blue food dye. Choose a drink whose ingredients list only this one blue dye, so nothing else absorbs strongly at the chosen wavelength. If using a different dye, the same method works; only the wavelength and stock concentration change.

You’ll need a stock solution of the dye with a known concentration, either supplied by your teacher or made by dissolving a weighed amount of pure dye.

Equipment

• colorimeter (with filters or wavelength setting) or a simple spectrophotometer
• cuvettes (and a cuvette holder or rack)
• stock dye solution, e.g. 10.0 mg/L Brilliant Blue FCF
• distilled water
• burettes or graduated pipettes (for making dilutions accurately)
• 10 cm³ volumetric flasks or labelled test tubes
• the drink sample
• tissues, a wash bottle

Safety

• Food dyes at these concentrations are low hazard, but don’t drink anything used in the lab.
• Wipe spills; dyes stain skin and clothes.
• Follow your lab’s electrical safety rules for the colorimeter.

Method

1. Choose the wavelength

Brilliant Blue absorbs most strongly in the orange-red region, near 630 nm. On a colorimeter, choose the red or orange filter. (A blue solution looks blue because it transmits blue light; it absorbs the complementary colour. See spectroscopy misconceptions.)

If you have a spectrophotometer, scan a standard across the visible range to find λmax, or measure absorbance at several wavelengths and choose the highest.

2. Prepare standards

Make a dilution series from the 10.0 mg/L stock, each to a total of 10.0 cm³:

Standard	Stock (cm³)	Water (cm³)	Concentration (mg/L)
1	0.0	10.0	0.0 (blank)
2	2.0	8.0	2.0
3	4.0	6.0	4.0
4	6.0	4.0	6.0
5	8.0	2.0	8.0
6	10.0	0.0	10.0

Concentration = stock concentration × (volume of stock ÷ total volume). For example, standard 3: 10.0 × 4.0 ÷ 10.0 = 4.0 mg/L. Use burettes or graduated pipettes for accuracy, and mix each thoroughly. See molarity and dilution.

3. Zero the colorimeter

Fill a cuvette with distilled water (the blank), place it in the colorimeter and set the absorbance to zero. This removes any absorbance from the water and the cuvette itself.

4. Measure the standards

For each standard:

1. Rinse the cuvette with a little of the solution, then fill it about three-quarters full.
2. Wipe the clear faces with a tissue and hold the cuvette only by the frosted sides.
3. Insert it the same way round each time and record the absorbance.

Measure from the most dilute to the most concentrated to reduce carry-over.

5. Measure the drink

If the drink is fizzy, let it go flat first (bubbles scatter light). Measure its absorbance directly. If it’s higher than your most concentrated standard, dilute it by a known factor (e.g. 5.0 cm³ of drink made up to 10.0 cm³ with water, a dilution factor of 2) and measure again.

Repeat each measurement at least twice.

Sample results

Concentration (mg/L)	Absorbance (1)	Absorbance (2)	Mean absorbance
0.0	0.000	0.000	0.000
2.0	0.265	0.269	0.267
4.0	0.531	0.527	0.529
6.0	0.792	0.798	0.795
8.0	1.061	1.055	1.058
10.0	1.325	1.321	1.323

Drink, diluted 5.0 cm³ to 10.0 cm³: absorbance 0.612 and 0.616, mean 0.614.

Analysis

1. Plot mean absorbance (y-axis) against concentration (x-axis) and draw the best-fit straight line through the origin. See graphs in chemistry.
2. Gradient ≈ 1.323 ÷ 10.0 = 0.132 per mg/L (from the best-fit line).
3. Diluted drink concentration = 0.614 ÷ 0.132 = 4.65 mg/L (or read directly from the graph).
4. Original drink concentration = 4.65 × 2 (dilution factor) = 9.3 mg/L.

To convert to mol/L, divide by the molar mass of the dye (for the disodium salt of Brilliant Blue FCF, about 793 g/mol): 9.3 × 10⁻³ g/L ÷ 793 g/mol ≈ 1.2 × 10⁻⁵ mol/L.

Evaluation

Is the calibration linear?

The points lie close to a straight line through the origin, so the Beer–Lambert law holds over this range. At much higher absorbances (above about 1.5), graphs often curve, which is why the drink was diluted into the middle of the range.

Sources of error

Error	Type	Effect	Improvement
Fingerprints or scratches on cuvette	random or systematic	absorbance too high	wipe faces; handle by frosted sides; use unscratched cuvettes
Inaccurate dilutions	systematic	wrong standard concentrations, shifted line	use burettes or volumetric pipettes and flasks
Bubbles in drink	random	absorbance too high and unstable	degas the drink; tap cuvette
Other dyes absorbing at 630 nm	systematic	concentration overestimated	check the label; choose a drink with a single dye
Colorimeter drift	random	inconsistent readings	re-zero with the blank regularly
Wide filter bandwidth	systematic	slight non-linearity	use a spectrophotometer set to λmax

See experimental errors and Beer–Lambert practice problems.

Questions

1. Why is the colorimeter zeroed with distilled water?
2. Why was the red/orange filter chosen for a blue dye?
3. Why was the drink diluted before measurement?
4. Why should the line pass through the origin?
5. A drink contains a green colour made from blue and yellow dyes. How might this affect your result, and why?

Extensions

• Measure the dye in several brands and compare.
• Extract dye from sweet coatings by dissolving them in a known volume of water, then find the mass of dye per sweet.
• Combine with paper chromatography to identify which dyes are present first. See solving a “crime” with ink chromatography.
• Investigate the fading of dye with bleach over time, measuring absorbance every minute: a rates experiment.

Key takeaways

• Absorbance is proportional to concentration, so a calibration graph turns absorbance into concentration.
• Choose the wavelength (filter) of the colour complementary to the solution’s colour.
• Make accurate standards by dilution, zero with a blank, and handle cuvettes carefully.
• Dilute samples into the calibration range and multiply by the dilution factor.
• Evaluate linearity and sources of error, especially from other absorbing substances.