Food Dye and Complementary Colors

Abstract

Solutions with known concentrations of the dye FD&C Green No. 3 are used to make a standard curve. The curve is then used to determine the concentration of FD&C Green No. 3 in Listerine mouth wash. Information for other food dyes is provided to expand the procedure to those dyes.

Introduction

There are seven dyes that can be used in food and beverages in the United States. Their names and properties are shown in the table below.

Table 1. Properties of food dyes. The peak absorption is the wavelength of light the dye absorbs best. Red dye # 40, for example, absorbs green light with a wavelength near 504 nm (and lets red light pass through). The light filter used for each dye has a maximum transmission near the dye's maximum absorption. A colored LED with a peak emission near the dye's maximum absorption can be used instead of a colored filter with a white light.

The color column below shows the complementary colors. To measure the concentration of a colored chemical species, use light of the complementary color, because this is the color that is absorbed best. To measure the concentration of a yellow chemical, for example, use blue or blue violet light. The precise color of the light will depend on the peak wavelength absorbed by the colored chemical.

Figure 2. Color column showing complementary colors on opposite sides of the column. Use the complementary color of light to measure a colored ingredient. For example, to measure the concentration of a blue dye, use orange light.

Each of the colors shown in Figure 2 are associated with a range of wavelengths. It is useful to know the wavelengths that are absorbed in order to choose the light filter or the LED that is appropriate for the procedure. LEDs have well-characterized wavelengths, whereas filters tend to have broader spectral bandwidths.

Table 1. Colors of solutions to be tested are listed in the left column. The color of the light absorbed for each solution is listed in the middle. The wavelength of the absorbed light is listed on the right. This wavelength is also the optimum wavelength for measuring the concentration. (Colors and wavelengths from Collection of Interesting General Chemistry Experiments, A.J. Elias, 2002)

The goal of this project is to use a light box to measure the concentration of green dye # 3 in mouth wash. A standard curve will be made from a set of green dye #3 solutions with known concentrations. The concentration of the unknown is then determined using the curve. In general, this procedure can be adapted to measure the concentration of any colored solute.

Materials and Equipment

• Light box,
• Cuvettes, 12
• Masking tape
• FD&C green dye #3, powder. Can be purchased as part of a food dye kit at Rainbow Colors, http://www.rainbowcolorsct.com/Dyes.htm.
• Scale, accurate to 0.01 grams
• Liquid measuring cups
• Beral pipets
• Graduated cylinder, 250 ml
• Plastic cups, as needed
• Spoon, plastic
• Distilled water
• Test tubes, 15 ml. 12
• Listerine, or other food product with FD&C green #3 as unknown.
• Stopwatch, for chemical kinetics experiment
• Bleach, for chemical kinetics experiment
• Safety goggles
• Latex gloves
• Plastic wrap
• Permanent marker
• Lab notebook
• Graph paper

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Safety issues

The chemicals used for this test are irritants. Use gloves and goggles when working with chemicals.

Procedure

Make a stock solution.

The table below shows examples of stock solutions made starting with 0.5 gm of powder. A stock solution can be used to make standard solutions from 1 to 100 micromolar.

At high concentrations of the dye (say over 50 micromolar, but "high" depends on the dye and the instrument) the assay will not be linear - large changes in concentration will not cause equally large changes in the output from the photometer. The unknown should be diluted so that it falls midway in the linear range of the standards (say around 10 micromolar).

1. Weigh out 0.50 gm of FD&C green dye powder.
2. Dissolve the dye in 20 ml of distilled water.

   1. This stock is 45.3 mM
3. Pipet 22.1 microliters of the 45.3 mM stock into 10.0 ml of distilled water.

   1. This is a 100 micromolar solution

Dilution series of standard solutions

Dilute the unknown sample, if necessary.

Objective:

Measure concentration of dye in popular mouthwash.

Terms and definitions

Solute - A chemical that is dissolved in a solution.

Molar absorption coefficient, ε - A measurement of how strongly a chemical species absorbs light at a given wavelength. ε is the Greek letter epsilon.

M - molar concentration

Blank - A sample with no solute. It is used to correct for light that is absorbed or scattered by the solvent, the sample tube, etc.

Standard solutions - Solutions with known concentrations of the solute. Used to make a standard curve.

Standard curve - A graph showing the concentration of standard solutions on the x-axis and the absorbance on the y-axis. Used to determine the concentration of the unknown.