Red #40 Food Dye in a Cherry Cough Drop

The Amount of Red #40 Food Dye in a Cherry Cough Drop by a Beer’s Law Experiment

The food dye Red #40 is a safe and convenient subject for a Beer’s Law exercise. Cherry cough drops are an inexpensive source of the dye for classroom use. This article provides the method for an experimental measurement of the amount of dye in a cough drop. The Beer’s Law equation including an authoritative value of the absorptivity of Red #40 is given.

A visible wavelength spectrophotometer is a good choice for a first introduction to quantitative instrumental chemical analysis. Chemistry students can quickly learn to use this instrument. After doing such an experiment, they should be able to demonstrate an understanding of the basics of quantitative spectrophotometry: the percent transmission of light; the definition of absorbance; the spectrum convention that an absorbance band is displayed as peaking upwards from the baseline of zero absorbance; and the use of Beer’s Law at a single wavelength for the estimation of the concentration of an analyte.

The azo dye Red #40 is an excellent choice of subject for a first experiment. First produced by Allied Chemical and known also as Allura Red AC, this water-soluble sodium salt absorbs intensely in the blue region of the visible spectrum. It is widely used as an FDA/Health Canada approved food dye additive. It is the only approved food dye that is sometimes used alone in consumer products. Information about the water-soluble sodium salt form of this substance may be found in Wikipedia (1).

*FD&C indicates a dye allowed for use in human foods, drug coatings, and cosmetics.

Single wavelength spectrophotometry at the peak wavelength of absorbance (501 nm – 504 nm) is used in the food industries for quality control, process control, and analysis of unknowns containing Red #40. The spectral scans in this document were obtained using a NovaSpec Plus Diode Array Spectrophotometer, at visible wavelengths in a 1.0 cm cell. The venerable Spectronic 20 analog instrument in the image below and other of its kind were retired from use in the first-year laboratory of our programs at Mohawk College in 2005, replaced by Spectronic 20D digital instruments.

Why Choose FD&C Red #40 for Spectrophotometry Experiments

This substance is a very good choice for introductory spectrophotometry exercises:

1. Safety. This allowed food additive is non-toxic. The main hazard is staining of skin or clothing.
2. Disposal. It is safe to wash the small volumes of residues down any sink.
3. Low Cost and Wide Application. In many products Red #40 is the only dyestuff present. The amount present ranges from very low concentrations in some synthetic beverages and cough drops, to moderate amounts in products such as mouthwashes and cough syrups, to much higher amounts in a product such as Cherry Kool-Aid Jammers (2), and very much higher amounts in Cherry Kool-Aid Liquid Concentrate (3). The amount of dye required for a class experiment is extremely small.
4. Options. There are many types of experiments possible, from simple to complex.
5. Known Absorptivity. An authoritative value for the absorptivity of Red #40 dye at the wavelength of peak absorbance and the Beer’s Law equation is known, as is the range of concentrations at which Red #40 adheres to Beer’s Law.

 

Beer's Law Cough Drop Experiment Method

A cherry cough drop or hard candy is used as a sample. The ingredient list should include Red #40 or Allura Red, otherwise confirm this by a spectral scan. The analysis result can be used to calculate the mass of Red #40 in the cough drop. By pre-weighing the cough drop it is possible to also calculate the Red #40 content in parts per million mass to mass (mg/kg). You can adapt the instructions here to the available equipment. If your volumetric flasks are larger than 100 mL, use multiple tablets, or find a larger cherry hard candy for the experiment.

Experimental Method for Halls® Menthol-Lyptus Cherry Cough Drops

The experimental method is summarized here. Students will require more detailed instructions.

1. Open a package of Halls® Menthol-Lyptus Cherry Cough Drops (4). Use one cough drop.
2. (Optional: Determine the mass of the cough drop to the nearest 0.01 g.)
3. Place the cough drop into a clean small beaker. Add about 25 mL of distilled water.
4. Warm gently on a hot plate and carefully swirl the beaker until the cough drop is fully dissolved.
5. Transfer quantitatively to a clean 100 mL volumetric flask*. Fill to the mark with distilled water. Mix well by making 30 slow inversions.
6. Measure the absorbance of the diluted solution at 501 nm. An average of several repeats should be used and averaged.
7. Use the Beer’s Law equation or plot to determine the concentration of the diluted solution in mg/L units
8. Use the solution volume to calculate the amount of Red #40 present in the cough drop.
9. If the mass of the cough drop has been measured, calculate the concentration of the Red #40 in the drop in mg/kg units (ppm mass to mass).

*The precision of a volumetric flask is not required. You can use small screwcap bottles each calibrated with a fill line placed by adding 100 g of distilled water to the bottle. The important part of the method is to use 30 slow inversions to mix the diluted solution completely.

 

Absorptivity of Red #40 and the Beer’s Law Equation

The Beer’s Law equation (5) has the form:

A (absorbance) = (constant of proportionality) x (cell path length) x (concentration)

Absorbance (A) is a unitless quantity. The cell path length is by convention in cm units, and is usually given the symbol b. The path length is usually 1 cm for most analyses using solutions. The concentration of the absorbing substance will have whatever units are convenient (see below). The concentration is usually given the symbol c. The constant of proportionality may be given the symbol “a” called the absorptivity or given the symbol “ε” called the extinction coefficient. The ACS prefers the term absorptivity here (6).

Beer’s Law Equation: A = abc or A = εbc

An authoritative value of absorptivity is found in a 2016 document prepared under the auspices of the UN Food and Agricultural Organization and the World Health Organization (7). The absorptivity of Red #40 is given in the FAO/WHO document as 54.0 L / g x cm. The absorptivity value is found at the very end of the document, in a section titled ‘Method of Assay’. The analytical wavelength is 501 nm.

 

Units of Concentration and Units of Absorptivity

Units for the concentration of solutions and therefore the corresponding units for absorptivity are chosen according to the purpose of the analysis and the use of the resulting values. For applied uses, practical units based on mass per unit volume will be used. Examples are listed in the table.

Types of Use	Applied
Examples	industrial process and quality control; food, drug and cosmetic; environment; agriculture; wastewater; pollution; drinking water; etc.
Base Unit of Concentration	g / L
Base Unit of Absorptivity	L / g x cm

A good general rule is to tabulate values, prepare and display plots, and use plots and Beer’s Law equations for numbers that are between at least 0.1 and at most 100. If the useful Beer’s Law range of concentrations of the analyte is outside these limits using the base units (e.g. g/L), then it is best to shift the values into the required range using SI prefixes. There are exceptions to this rule, such as spectral scans of visible wavelengths, where wavelengths are by conventions given in nm units (e.g. 501 nm) rather than in pm units (e.g. 0.501 pm). Using Red #40 as a concrete example: Solve the Beer’s Law equation for an absorbance value of 0.5 in a cell of 1 cm path length. This will show what the upper limit of concentration will be for analytical measurement.

Base Unit g/L

Absorptivity = 54.0 L/g x cm

Beer’s Law Equation A = (54.0 L/g x cm) x (b cm) x (c g/L)

Value of c in a 1 cm cell for A = 0.5 concentration = 0.00926 g/L

Working Unit Shift concentrations into mg/L Absorptivity = 0.0540 L/mg x cm concentration at A = 0.5 is 9.26 mg/L

Working Beer’s Law Equation A = (0.0540 L/mg x cm) x (1 cm) x (c mg/L)

A practical example of this is shown in the calibration plot for Red #40 given below.

Additional Information

This article is excerpted with editing from the longer article “Beer’s Law Experiments with Red #40 Food Dye” that can be found in the supporting information.

An experiment reported in that article demonstrated that Red #40 adheres well to Beer’s Law up to an absorbance value of 0.5 at a concentration of just over 9 mg/L.

Solutions of higher concentration and absorbance may deviate from Beer’s Law behavior and should be avoided when making absorbance measurements.

A second experiment reported in that article demonstrated that KoolAid Jammers Cherry drink contains about 50 mg/L (ppm) of Red #40. This drink could serve as a low cost and convenient source of the dye for experiments with Red #40.

The downloaded file also contains:

• Additional suggested experiments.
• Background information to assist in devising an experiment which will produce student results that are valid and reproducible.
• Information for students and student problems.
• An appendix clarifying the three common uses of the term ppm (parts per million).

 

Practice Problem

Students will have to pay close attention to the distinction between ppm mass to mass and ppm mass to volume in this application.

a. Halls Menthol-Lyptus Cherry Cough Drops are coloured using Red #40 dye. One cough drop was fully dissolved by warming in 30 mL of distilled water. The solution was completely transferred to a 100 mL volumetric flask. The flask was filled to the mark and completely mixed by inverting slowly 30 times. The absorbance of the resulting solution in a 1 cm pathlength cell at 501 nm was 0.262. Use the Beer’s Law equation to estimate the concentration of Red #40 in the 100 mL of solution. Answer in mg/L units (ppm mass to volume).

b. Calculate the mass of Rd #40 contained in the 100 mL of solution and thus in the cough drop. Answer in mg units.

c. The mass of the cough drop was measured before dissolving as 3.75 g. Calculate the concentration of the dye in the drop. Answer in mg/kg units (ppm mass to mass).

Reference Sources

1. https://en.wikipedia.org/wiki/Allura_Red_AC.
2. https://www.kraftheinz.com/en-CA/koolaid/products/00066188070815-jammers.
3. https://www.kraftheinz.com/en-CA/koolaid/products/00068100001273-cherry-liquid-drink-mix.
4. https://gethalls.ca/en/relief/halls-cherry-flavour-cough-drops.
5. https://www.webassign.net/question_assets/ucscgencheml1/lab_9/manual.html.
6. Analytical Chem.,1990, 62, 91.
7. https://openknowledge.fao.org/server/api/core/bitstreams/1d90f15f-858c-49bb-b72f-b2789f6269e7/content  or https://www.fao.org/3/br639e/br639e.pdf. (see page 4, Methods of Assay).