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Analyze This! Make a Colorimeter to Measure the Concentration of Blue Dye in Various Liquids.

Difficulty
Time Required Long (2-4 weeks)
Prerequisites Basic knowledge of chemistry. Some familiarity with electronics would be helpful, but is not required.
Material Availability Specialty item: Cuvettes can be purchased online. See the Materials and Equipment list for details.
Cost High ($100 - $150)
Safety Working with bleach is hazardous. Protect your skin and eyes.

Abstract

Do you read the list of ingredients in foods and drinks before you buy them at the grocery store? If you do, you may have noticed that many of the items that are blue in color have the same dye, called FD&C blue 1. In this chemistry science fair project, you will build a simple colorimeter, a device that measures the concentration of colored chemicals in solutions. You will use the colorimeter to measure the concentration of blue dye #1 in sports drinks, and to track the rate at which the dye disappears when treated with bleach.

Objective

In this chemistry science fair project, you will make a simple electronic device that functions as a colorimeter. With it, you will measure the concentration of blue dye #1 in sports drinks, and track the rate at which dye disappears when treated with bleach.

Credits

David B. Whyte, PhD, Science Buddies

Cite This Page

MLA Style

Science Buddies Staff. "Analyze This! Make a Colorimeter to Measure the Concentration of Blue Dye in Various Liquids." Science Buddies. Science Buddies, 30 Sep. 2013. Web. 21 Sep. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p075.shtml>

APA Style

Science Buddies Staff. (2013, September 30). Analyze This! Make a Colorimeter to Measure the Concentration of Blue Dye in Various Liquids.. Retrieved September 21, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p075.shtml

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Last edit date: 2013-09-30

Introduction

One of the common challenges faced by chemists is sample analysis. The goal of sample analysis is to answer the questions: "What chemicals are in this sample", and "What are their concentrations?" If an ingredient is colored, then its concentration can be determined by light absorption. The concentration of a chemical in a solution is directly proportional to the amount of light it absorbs; if you double the concentration of the chemical, the solution absorbs twice as much light. This relationship is captured in the Beer-Lambert law:

Equation 1:

A = εCl
  • A = absorbance, which is unitless
  • ε = molar absorption coefficient, measured in liters/(mol x cm)
  • C = concentration, measured in moles per liter (mol/L)
  • l = path length of the light through the sample, measured in centimeters (cm)

The Beer-Lambert law says that the absorbancy of light by a chemical in a solution is equal to the product of the chemical's concentration, the path length of light in the container (l) in centimeters, and the molar absorption coefficient (formerly referred to as the extinction coefficient).

It makes sense that the amount of light absorbed is proportional to the concentration and to the length of the light beam's path through the solution. These are familiar concepts from everyday observations. But what is the molar absorption coefficient? The size of the molar absorption coefficient reflects how well the molecule absorbs light of a given wavelength. A blue solution absorbs red and yellow light better than it absorbs blue light, which is why it appears blue. For the same reason, a red solution absorbs blue and green light better than it absorbs red light. The color of the solution is determined by the color of light the molecules do not absorb, since this is the color that is transmitted to your eyes. The molar absorption coefficient is high at the wavelengths that are absorbed the most.

The color of visible light is related to its wavelength, as shown in Figure 1.

Electromagnetic spectrum
Figure 1. The electromagnetic spectrum and the wavelengths of visible light. The units are nanometers (nm). Radio waves, microwaves, and infrared radiation have longer wavelengths than visible light does. UV, X-rays, and gamma rays have shorter wavelengths than visible light does.

The molar absorption coefficient for a given molecule depends strongly on the color of the light. As mentioned above, if the molecule forms a blue color in solution, the solution absorbs red and yellow, or orange light. You can see this in the absorption spectrum for blue dye #1, a common food coloring, in Figure 2.

Absorbance spectrum of food dyes blue 1 and red 40
Figure 2. Absorption spectrum of the food dyes blue #1 and red #40. Note that the blue dye absorbs light strongly at a wavelength of about 620 nm, which is in the orange part of the visible spectrum (see Figure 1 for colors and wavelengths). Red dye #40 absorbs strongly at around 500 nm, roughly in the blue-green part of the spectrum. The efficiency with which the light is absorbed at a given wavelength is measured by the molar extinction coefficient. The blue dye has a very high molar extinction coefficient for orange light (that's why it's blue) and a lower value for other colors. Note that the red dye absorbs light with a wavelength of around 500 nm, which is in the blue-green part of the spectrum. (Thomasson, 1998.)

The absorption spectra, shown in Figure 2, for Blue 1 and Red 40 were generated using an instrument called a spectrophotometer. The spectrophotometer separates white light into a rainbow of colors and then passes the light through the sample. The amount of light that is transmitted is measured by a light detector on the exit side of the sample. For the dye blue 1, the detector measured two small peaks at around 320 nm and 420 nm and a large peak at around 620 nm, which corresponds to orange light. The solution appears blue because the red and yellow light has been absorbed by the dye.

For this chemistry science fair project, you will build a simple colorimeter, a device that measures the absorbance of light by a colored solution. The circuit has a light source and a light detector. The sample solutions will be placed between the light source and the detector, and you will measure the amount of light that passes through the sample. The light source will be a light-emitting diode (LED) and the detector will be a photoresistor. Light-emitting diodes are small, solid-state devices that produce light of various wavelengths with little energy lost as heat. The LED used in this science fair project produces white light. A photoresistor is an electronic component that "resists" the flow of electricity in a light-dependent fashion. It essentially blocks the flow of electricity in the dark, and becomes less "resistant" as the level of light is increased. To keep the device simple, the output will simply be the level of resistance from the photoresistor. The resistance of the photoresistor has the useful quality of being directly proportional to the absorbance, and thus, directly proportional to the concentration of the dye (Equation 1). Resistance is measured in ohms (Ω). As more light is absorbed, less light passes through the solution, so the resistance increases, and the number of ohms increases. You will use a digital multimeter to measure the resistance level of the photoresistor.

In this science fair project, the samples you will analyze contain FD&C blue dye #1 (see Figure 3). FD&C blue dye #1 is shortened to the term blue dye in the Experimental Procedure below. Commercial spectrophotometers use diffraction gratings to create a rainbow of colors for analyzing samples. To simplify the device used for this experiment, the procedure uses orange-colored water as a filter to produce the input light. The use of orange-colored light increases the device's sensitivity since the blue dye absorbs strongly in the orange part of the electromagnetic spectrum.

Chemical structure FD&amp;amp;amp;C blue dye #1
Figure 3. The chemical structure of FD&C blue dye #1. The alternating double bonds (the double lines in the structure) help capture orange light, making the solution appear blue. (Wikipedia, 2008.)

The light from the LED passes through the filter (orange water) and the sample (blue liquid) before it hits the detector. It also passes through the sides of the vessels holding the liquids. These sides need to be clear, flat, and 1 cm2. Cuvettes are designed precisely for this application.

One goal of this chemistry science fair project is to use the colorimeter to determine the concentration of blue dye in "unknown" samples, such as sports drinks. A second goal is to use the colorimeter to track the rate of loss of dye in a solution containing dye and bleach. The active ingredient in bleach is the hypochlorite molecule. Hypochlorite molecules break chemical bonds in the pigment molecules, resulting in the loss of color. The colorimeter will record the rate of this reaction, indicated by the loss of color over time. The study of reaction rates is called chemical kinetics.

Terms and Concepts

  • Sample analysis
  • Concentration
  • Light absorption
  • Solution
  • Proportional
  • Beer-Lambert law
  • Absorbance
  • Molar absorption coefficient
  • Nanometer (nm)
  • Absorption spectrum
  • Spectrophotometer
  • Colorimeter
  • Light-emitting diode (LED
  • Photoresistor
  • Solid state
  • Resistance, ohms (Ω)
  • Diffraction grating
  • Cuvette
  • Chemical kinetics
  • Chemical structure
  • Calibrate
  • Standards

Questions

  • What are the terms in the Beer-Lambert equation?
  • How does the absorbance, A, relate to the fraction of light transmitted, T/To, through the sample?
  • What are some ways chemists use colorimeters in their work?
  • What causes certain chemicals, such as food dyes, to be brightly colored?

Bibliography

Materials and Equipment

Most of the following supplies can be purchased at RadioShack.

For the Circuit:
  • Solderless breadboard socket, RadioShack Catalog # 276-003
  • Wires for solderless breadboard, RadioShack Jumper Wire Kit Catalog # 276-173
  • Insulated test/jumper leads, 24-inch, RadioShack Catalog # 278-1157
  • 9-V batteries (2)
  • Heavy-duty 9-V snap connectors, RadioShack Catalog # 270-324
  • Clear tape
  • Photoresistor (5-pack) RadioShack Catalog # 276-1657
  • Ultra-high-brightness LED, RadioShack Catalog # 276-0005
  • Resistor, 220-ohm, carbon film (5-pack), RadioShack Catalog # 271-011
  • Digital multimeter; we recommend an auto-ranging digital multimeter; available at most hardware or auto supply stores or Amazon.com.
  • Cuvettes; available from Amazon.com
For the Solutions:
  • Cups or glasses, each must hold 12 oz. (8)
  • Masking tape
  • Food coloring, including FD&C blue dye #1 (called simply "blue dye" in the procedure)
  • Liquid measuring cups
  • Measuring spoons
  • Spoon
  • Eyedropper
Testing the Solutions:
  • Cardboard box to block light, or a metal bread pan or a dark-colored ceramic bowl
  • Blue sports drinks, must contain FD&C blue dye #1 (at least 2 different types, 1 large container of each)
  • Stopwatch
  • Bleach
  • Safety goggles, available from Carolina Biological, item #: 646706B
  • Disposable gloves. Can be purchased at a local drug store or pharmacy, or through an online supplier like Carolina Biological. If you are allergic to latex, use vinyl or polyethylene gloves.
  • Plastic wrap
General:
  • Permanent marker, black, chisel-tip
  • Lab notebook
  • Graph paper

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Experimental Procedure

The following procedure can be broken into four parts:

  1. Make a simple colorimeter that converts the concentration of dye in a solution into electrical resistance, which you can read off a multimeter;
  2. Make a set of standard solutions, so that you know how to convert between the data you have (resistance) and the data you want (concentration);
  3. Determine the amount of dye in samples with unknown concentrations; and
  4. Track the rate of color loss in a bleach-treated solution. This last part of the procedure involves chemical kinetics, which studies the rates of chemical reactions.

Part 1: Making the Colorimeter

Beginning to Build the Circuit

  1. To start this science fair project, clear a space on which to build the light detector. Figure 4 shows a picture of the partially assembled circuit.
    1. To learn how to use a breadboard to build a circuit, see the Science Buddies page Use a Breadboard to Build and Test a Simple Circuit.
  2. Attach the battery to the snap connector.
  3. Tape the battery to the breadboard with the clear tape.
  4. Insert the black wire (the ground) into the ground bus. The ground bus is a row of holes that are in contact with the black wire from the battery.
  5. Insert the red wire from the battery (the power wire) into the power bus.
Picture of simple colorimeter circuit
Figure 4. Partially assembled circuit. The red wire is inserted into the power bus and the black wire is inserted into the ground bus. The white wire is a switch—the LED is powered when it is connected to the power bus. The LED and photoresistor are placed on the circuit in such a way that the four cuvettes can be placed on the circuit in a row. The empty cuvettes are placed upside-down over the LED and the photoresistor. The orange solution filters the light from the LED. The sample cuvette holds the solution (sports drink or standard made with food dye) containing blue dye.
Attaching the Photoresistor
  1. Insert the photoresistor leads in holes E28 and F28. Bend the wires at the top of the photoresistor so the flat surface faces sideways. The side with the squiggly line should point toward the battery. The photoresistor will function as the light detector.
  2. Insert a white jumper wire into the hole at A28.
  3. Insert a white jumper wire into the hole at J28.
    1. The white wires are in contact with the photoresistor wires.
    2. You can read the resistance across the photoresistor using the multimeter. If you need help using a multimeter, check out the Science Buddies Multimeter Tutorial.
    3. The resistance decreases when light hits the photoresistor.
    4. The concentration of dye is directly proportional to absorbance (Beer-Lambert law), and absorbance is directly proportional to resistance.
Making the LED Circuit
Circuit diagram for LED
Figure 5. The LED circuit. The resistor is used to set the proper voltage across the LED.
  1. Insert the longer lead of the LED in hole E15.
  2. Insert the shorter lead of the LED in hole F15.
    1. LEDs must be connected the correct way round. The cathode is the short lead and is connected to ground. The long lead is the anode and is connected to the power bus.
  3. Insert one lead of the 220-ohm resistor in hole H15.
  4. Insert the other lead from the 220-ohm resistor in any hole along the ground bus.
  5. Insert one end of a white jumper wire into the hole at 15B.
  6. Bend the wires near the top so that the LED points sideways, toward the photoresistor.
  7. Insert the other end of the white wire into the power bus.
  8. The LED should light up. If it does not light up, check your circuit.
    1. Make sure the LED is placed in the correct orientation and that the wires form a complete circuit.
  9. Unplug the wire leading to the LED.
    1. It is important not to run the battery down, as this will affect the readings when you are collecting data. Only power the LED when you are collecting data.
    2. For long-term use, connect a 9-V DC power supply.
Fitting the Cuvettes
  1. Place a cuvette upside-down over the LED.
  2. Place a cuvette upside-down over the photoresistor.
  3. Place two empty cuvettes between the LED and the photoresistor. See Figure 4, above.
  4. The four cuvettes should touch each other and form a straight line.
    1. Move the LED and resistor as needed.
    2. Use tape to hold the cuvettes over the LED and the photoresistor in place. But do not block the light path!
  5. The light from the LED should shine directly onto the photoresistor.
  6. The photoresistor should be near the side of the cuvette facing the LED. Bend the wires on the LED and photoresistor, if needed.
Hooking Up the Multimeter and Testing the Circuit
  1. Turn on the multimeter.
  2. Set the multimeter to read resistance (ohms, Ω).
  3. Attach the multimeter to the white wires connected to the photoresistor.
    1. Use the wires with the alligator clips, if needed.
  4. Turn on the LED.
  5. Cover the device (but not the multimeter) with the box to block ambient light.
  6. Read the resistance across the photoresistor.
  7. Record the resistance across the photoresistor in your lab notebook.
    1. Note the units of the resistance. A "k" indicates kilo ohms and an "M" indicates mega ohms.
  8. Remove the box.
  9. Turn off the LED.
  10. Cover the circuit with the box again.
  11. In the dark, the resistance should be in the mega-ohm range.
  12. Stray light will cause problems with the data.
    1. Perform the readings in a dimly lit room if stray light is a problem.
    2. As an option to decrease stray light, use a black permanent marker to shield the photoresistor from light from the sides and back of the cuvette.
  13. Record the resistance in your lab notebook.
  14. Remove the box.
  15. Turn off the multimeter.

Part 2: Making the Solutions

Now that you have set up the circuit, the next step is to make the solutions to calibrate it. You will make a series of dilutions of blue dye, with known concentration, to use as standards to test unknown samples. See Figure 6. Each dilution is made by mixing dye with water that is progressively diluted by half. It is essential to use clean (no dye) utensils and cups to get an accurate set of standards. Be careful with the food dye, as it will stain.

Making the Blue Dye Solutions
  1. Set out eight clean cups to make the dilution series.
  2. Label them 1–8 with masking tape and the permanent marker.
  3. The solutions will be diluted as follows:
    • 1 (most concentrated)
    • 1/2
    • 1/4
    • 1/8
    • 1/16
    • 1/32
    • 1/64
    • Water only
  4. Pour 8 oz. of water into the first cup (#1).
  5. Pour 4 oz. of water into the remaining cups (#2–#8)
  6. Mix 1/8 teaspoon (tsp.) of blue dye with the 8 oz. of water in cup #1.
    1. The concentration of blue dye in the commercial package is approximately 0.026 mol. After dilution (1/8 tsp in 1 cup = 1:384 ), the concentration is 68 micromolar (µM) .
    2. This is the "strongest" solution. The other solutions will be repeatedly diluted by half.
  7. Stir the contents of cup #1 with a clean spoon.
Picture of blue dye dilution series
Figure 6. Dilutions series of FD&C blue dye #1.
  1. Pour 4 oz. from cup #1 into cup #2, using a 1/2-cup liquid measuring cup.
  2. Mix the contents of cup #2 with a clean spoon.
  3. Thoroughly rinse the 1/2-cup measuring cup.
  4. Mix 4 oz. from cup #2 with the water in cup #3 and stir with a clean spoon.
  5. Repeat the two-fold dilutions for cups 4–7.
  6. Cup 8 is the "blank," so should be water only.
  7. Transfer the standard solutions into eight clean cuvettes. Use the eyedropper or pour carefully.
  8. Make a data table in your lab notebook, showing the dilutions and the concentrations of blue dye in the solutions (#1 = 68 µM, #2 = 34 µM, etc..).
Making the Orange Solutions

Now it's time to make the orange filter solution. This solution will be placed in front of the cuvettes of blue dye to increase the sensitivity of the device.

  1. Pour 1/2 cup of water into a clean cup.
  2. Add two drops of red dye.
  3. Add two drops of yellow dye.
  4. Mix the solution with a clean spoon.
  5. Using the clean eyedropper, transfer the orange solution into a cuvette.

Determining the Amount of Dye in Samples with Unknown Concentrations

You are now ready to take readings from the device. Make sure the battery is fresh and keep the amount of time the light is on to a minimum to avoid running the battery down.

Testing the Samples of Known Concentrations of Blue Dye
  1. Attach the leads from the multimeter to the wires in contact with the photoresistor. Use the wires with the alligator clips.
  2. Turn on the multimeter and set it to read "resistance."
  3. Place the cuvette with the orange dye next to the LED.
  4. Place the cuvette with water next to the photoresistor.
  5. Plug in the wire to turn on the LED light.
  6. Cover the device with a box to block out all surrounding light.
  7. Read the resistance on the multimeter.
  8. Record your data in your lab notebook. The cuvette with just water is the "blank."
  9. Remove the cuvette with water.
  10. Place the cuvette with 68-µM dye in the colorimeter.
Simple colorimeter set-up
Figure 7. The setup for the colorimeter. The cuvette on the left covers the LED. The cuvette on the right covers the photoresistor. The orange solution serves as a light filter. The test sample contains an unknown concentration of FD&C blue dye #1.
  1. Cover the whole device (but not the multimeter!) with a box to block out all surrounding light again.
  2. Record the resistance on the multimeter in your lab notebook.
  3. Repeat 1–10 of this section with the entire set of standards two more times. The resistance should be higher as the solutions get darker.
  4. Average all of your results.
Testing the Samples with Unknown Concentrations of Blue Dye
  1. Using the clean eyedropper, fill three clean cuvettes with one of the blue-colored beverages.
  2. Place one of the "unknowns" on the device.
  3. Cover with the box.
  4. Record the resistance on the multimeter in your lab notebook.
  5. Repeat steps 1–4 of this section two more times.
  6. Repeat steps 1–5 of this section with the second and third samples (and any other samples you selected) of the blue-colored beverages that contain unknown amounts of blue dye.
Graphing Your Results
  1. Make a graph of the standard dilution series and the unknown samples. Subtract the resistance that you measured for water from all of the readings you made for samples with dye.
    1. This step subtracts the light loss due to the plastic, the water, and other factors.
  2. Graph the concentration of the standard solutions in micromolar on the x-axis vs. the resistance on the y-axis.
    1. If you are using Microsoft Excel, use the "Scatterplot" chart. Excel also has tools for adding trend lines.
  3. Graph the unknown samples on the chart.
  4. Determine the concentration of blue dye in the unknown samples, based on where they are on the graph.

Tracking the Rate of Color Loss in a Bleach-treated Solution

In this section, you will use the colorimeter to track the loss of color in a dye solution treated with bleach. Wear safety goggles and gloves for this section, as bleach can be harmful to your skin. You may want to start this part with a fresh 9-V battery. You will have to mix the contents of the cuvettes, so use a piece of plastic wrap to cover the tops of the cuvettes while you mix the contents by turning it over several times. The setup is the same as shown in Figure 7.

Making and Testing the Solutions
  1. Make 8 oz. of blue dye solution with a concentration of 68 µM.
    1. Mix 1/8 tsp. of blue dye in 1 cup of water with a clean spoon.
  2. Fill nine clean cuvettes with the blue dye solution.
    1. You will test three concentrations of bleach (two drops first, then one drop, then four drops) in three separate trials.
    2. Leave about a 1/4 inch of space on top so you can add the bleach solution.
  3. Fill one clean cuvette with the same volume of water.
  4. Place the cuvette with the water in the colorimeter, next to the photoresistor, as in Figure 7. The orange dye is still used as a filter.
  5. Turn on the LED.
  6. Place the box over the colorimeter.
  7. Record the resistance in your lab notebook.
  8. Remove the cuvette with the water.
  9. Put on your safety goggles and gloves. Add two drops of bleach to the water with the clean eyedropper.
    1. Caution: Bleach is caustic and will cause burns if you get it in your skin!
    2. This step it to measure how much the color of the bleach affects the absorbance (it is a small effect).
  10. Place the plastic wrap over the cuvette to cover the top and invert the cuvette several times to mix the bleach and the water.
  11. Place the cuvette with the bleach water in the colorimeter.
  12. Cover the device with the box.
  13. Record the resistance in your lab notebook.
  14. Remove the cuvette with water and bleach.
  15. Place a cuvette with blue dye in the colorimeter.
  16. Cover the circuit with the box.
  17. Record the resistance of the cuvette with the dye in your lab notebook.
Starting the Chemical Reaction
  1. Start the stopwatch.
  2. Add two drops of bleach to the cuvette with dye.
  3. Mix the bleach and the dye.
    1. Pick up the cuvette with the bleach.
    2. Place the plastic wrap over the cuvette to cover the top.
    3. Invert the cuvette several times to mix the bleach and the water.
  4. Place the cuvette in the colorimeter.
  5. Cover the colorimeter with the box.
  6. Using the stopwatch, record the resistance every minute for the first 5 minutes, then every 2 minutes until the resistance stops changing.
  7. Repeat steps 4–17 of the previous section and steps 1–6 of this section two more times. For each trial, measure the water alone, the water plus bleach, the dye solution, and the dye plus bleach.
Graphing Your Results
  1. Subtract the value of the resistance for the bleach water from all of the measurements for the blue dye.
    1. This step subtracts the light loss due to the plastic, the water, the bleach, and other factors.
  2. Graph the resistance vs. time for the disappearance of the blue dye.
  3. Convert resistance to concentration, based on your standard curve.
  4. Graph the concentration of the dye vs. time.

Completing All Your Trials

  1. Complete the following with one drop of bleach, for a total of three trials:
    1. Repeat steps 4–17 of the "Making and Testing the Solutions"
    2. Repeat steps 1–7 of "Starting the Chemical Reaction"
    3. Repeat "Graphing Your Results."
    For each trial, measure the water alone, the water plus bleach, the dye solution, and the dye plus bleach.
  2. Complete the following with four drops of bleach, for a total of three trials:
    1. Repeat steps 4–17 of the "Making and Testing the Solutions"
    2. Repeat steps 1–7 of "Starting the Chemical Reaction"
    3. Repeat "Graphing Your Results."
    For each trial, measure the water alone, the water plus bleach, the dye solution, and the dye plus bleach.
  3. Compare the rates of change with different amounts of bleach.
  4. Show that the rate of change of the color follows an exponential curve of the form below:

Equation 2:

Dt = Do e-at
  • Dt = the concentration of dye at time, t (1 minute, 2 minutes, etc.)
  • Do = starting concentration of dye
  • e = a mathematical constant equal to approximately 2.718
  • a = a constant, determined by the experiment
  • t = time, measured in minutes

This equation states that the concentration of dye, D, at time, t, equals the concentration of dye at time 0, multiplied by e, raised to the negative product of the constant, a, and the time, t.

  1. Note that as t gets bigger (more time), the exponent of e gets larger, so (D)t gets smaller.
  2. You can make the graph a straight line by graphing the equation in this form:

Equation 3:

ln(Do/ Dt) = at

This equation says that the logarithm of the ratio of the concentration at time 0 and the concentration at time, t, equals the product of a and t.

  1. Graph your data using Equation 3. Note that the slope is equal to the value of "a."
  2. How does the value of "a" change when you add more bleach?

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Variations

  • Repeat the procedure, but replace the orange filter with water. Do you see a difference in the sensitivity?
  • Repeat the procedure to determine the concentration of red food coloring in various drinks using a blue-green dye filter.
  • Try to analyze the concentration of red and blue dye in a purple sport drink. Use colored water filters to isolate absorption due to the red dye from that due to the blue dye, and vice versa.
  • Assuming that the molar absorption coefficient is 100,000 L/(mol × cm), calculate how the absorbance (A) is related to the resistance.
  • Develop an assay for bleach concentration based on the rate of dye clearing.

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