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Adsorption: Dyeing Fabrics with Kool-Aid

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Time Required Average (6-10 days)
Prerequisites Knowledge of basic chemistry is helpful as well as a good understanding of mathematical operations such as algebra and simple linear regression.
Material Availability This project requires special electronic parts. A kit is available at the Science Buddies Store. Estimated time required includes time for shipping the kit. For data analysis, a spreadsheet program, such as Google SheetsTM or Microsoft® Excel®, is helpful.
Cost High ($100 - $150)
Safety This project requires working with boiling water. Make sure to take precautions to prevent burns from the hot liquid.


Have you ever wondered how your clothes get their color? Dyeing textiles is a very complicated process and involves a lot of chemistry. Not only are the properties of the dye and fabric important, but the dyeing conditions also have to be exactly right to get optimal color adsorption. Curious about how it works? In this science project, you will color wool with Kool-Aid® and explore the chemistry of dyeing.


Investigate the adsorption process of Kool-Aid onto wool fabrics.


Svenja Lohner, PhD, Science Buddies

This project is based on an introductory adsorption laboratory experiment from Dr. Polly R. Piergiovanni at Lafayette College.

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  • Kool-Aid® is a registered trademark of Kraft Foods, Inc.

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MLA Style

Lohner, Svenja. "Adsorption: Dyeing Fabrics with Kool-Aid" Science Buddies. Science Buddies, 28 July 2017. Web. 25 Sep. 2017 <https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p106/chemistry/kool-aid-dye-adsorption>

APA Style

Lohner, S. (2017, July 28). Adsorption: Dyeing Fabrics with Kool-Aid. Retrieved September 25, 2017 from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p106/chemistry/kool-aid-dye-adsorption

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Last edit date: 2017-07-28


What happens when you put clothes into a dye bath? As expected, they come out colored. However, the intensity and degree of color can be very different, depending on the type of dye and fabric you use. Not every dye works with every type of fiber. So how does the color stick to the clothes? First, the dye has to get in contact with the fiber and needs to get equally distributed into the fabric. In the second step, the dye has to be retained by the fiber surface. The adhesion of dye to the fiber surface is the most important step in dyeing, and is called adsorption. During this process, the dye (also called the adsorbate) forms a film on the surface of the textile, or adsorbent. The interaction between the dye and the fiber depends on their chemical structure and can be driven by chemical adhesion (chemisorption), which includes ionic or covalent bonding and the formation of hydrogen bonds, or physical adhesion (physisorption) including van der Waals' forces or electrostatic attractions.

Let us look into that in more detail using the materials for this experiment as an example. Your adsorbent will be wool fabric, which is a natural protein fiber with lots of amino groups (-NH2). The adsorbate that you will use is Kool-Aid, or more specifically, the food color that is one of its ingredients, called FD&C Red 40. Due to its chemical structure, shown in Figure 1, Red 40 is classified as an acid dye.

Red 40 is the salt of a sulfuric acid, meaning it contains SO<sub>3</sub><sup>-</sup> groups, which makes it an acid dye.
Figure 1. Red 40 is the salt of a sulfuric acid, meaning it contains SO3- groups, which makes it an acid dye. (Image credit: by Jü, via Wikimedia Commons).

So what is the dyeing mechanism for wool and Kool-Aid? Acid dyes only work in an acidic environment and you will use vinegar to acidify your dyeing bath. The acid protonates (which means it adds a proton to) the amino groups of the wool fiber, so they become cationic (positively charged). Red 40, on the other hand, is anionic (negatively charged) in a solution. During dyeing, the Red 40 will adsorb onto the wool fiber by forming an ionic bond, as shown in Figure 2.

Dyeing mechanism of wool with Red 40
Figure 2. Dyeing mechanism of wool fabrics with food color Red 40 from Kool-Aid.

Knowing the chemistry of how a piece of cloth becomes dyed is very important to the textile industry. A clothing manufacturer wants to get a good color, while using the least amount of dye, because dyes cost money! In this project, you will evaluate the dyeing process by investigating how much dye can be adsorbed by a piece of wool, depending on the dye concentration in a solution, and by looking at the adsorption equilibrium of Red 40, which is distributed between the dyeing solution and the textile fiber. The dyeing distribution is usually expressed by so-called adsorption isotherms that relate the concentration of dye adsorbed on the fibers to the equilibrium concentration of dye in a solution. The Langmuir isotherm is the simplest adsorption model and is based on the assumption that during adsorption a continuous monolayer of adsorbate molecules is formed that is surrounded by a homogeneous solid surface. It is most often used for the adsorption of solutes from a solution and is mathematically expressed as shown in Equation 1:

Equation 1: where
  • Q represents the maximum adsorbate that can be adsorbed onto the surface [mg dye/mg wool],
  • qe is the amount of dye adsorbed at equilibrium [mg dye/mg wool],
  • Ce is the concentration of dye in solution at equilibrium [mg dye/mL], and
  • b is the isotherm constant [mL/mg dye].

The equation can be rearranged, as shown in Equation 2:

Equation 2:

which means that plotting 1/qe against 1/Ce will result in a straight line that allows you to determine Q and b from its slope and intercept. To get all the necessary data for the adsorption isotherms, you will monitor the concentration of Red 40 in your dyeing bath (Ct) over time, which allows you to also calculate the amount of dye adsorbed by the wool (qt) using Equation 3 for each time point. Once the adsorption equilibrium is reached, qt becomes qe and Ct becomes Ce.

Equation 3: where
  • qt is the amount of dye adsorbed at time point t [mg dye/mg wool],
  • C0 is the initial dye concentration in solution [mg/mL],
  • Ct is the dye concentration in solution at time point t [mg/mL],
  • V is the volume of dye solution [mL], and
  • w is the mass of the fabric [mg].

To measure the dye concentration in a solution, you will build a simplified version of a special device, called a spectrophotometer. The color of your sample is related to the wavelength of light it reflects, as shown in Figure 3.

spectre horizontal
Figure 3. The spectrum and wavelengths of visible light. The units are nanometers (nm). (Image credit: by Maulucioni, via Wikimedia Commons).

A spectrophotometer measures the light absorption of your sample at a specific wavelength. It does this by separating white light into a rainbow of colors using diffractive gratings 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. The result is an absorption spectrum, such as the one shown for the food dyes Red 40 and Blue 1 in Figure 4. For Red 40, the detector measures one small peak at around 320 nm and a large peak at around 500 nm, which corresponds to blue-green light. The solution appears red because the blue and green light has been absorbed by the dye.

absorption spectra graph
Figure 4. Absorption spectra of the food dyes Red 40 and Blue 1. Note that the red dye absorbs light strongly at a wavelength of about 500 nm, which is in the blue-green part of the visible spectrum (see Figure 3 for colors and wavelengths). Blue 1 absorbs strongly at around 620 nm, roughly in the orange part of the spectrum. (Thomasson, 1998.)

Your simplified spectrophotometer will consist of a circuit that has a light source (a white light-emitting diode (LED)) and a light detector (a photoresistor). Your dye solutions will be placed between the light source and the detector, and you will measure the amount of light that passes through the sample, as shown in Figure 5. To keep your setup simple, you will not use diffractive gratings to split the light from your light source into separate wavelengths. Instead you will use an absorption filter, or wavelength selector (blue-green-colored water) to produce the desired input wavelength that corresponds to the absorbance maximum of Red 40 (500 nm). Your measurement output will be the level of resistance from your detector. The photoresistor has a high resistance in the dark, and its resistance decreases as light levels increase. The resistance of your photoresistor is related to the absorbance of your sample and is measured with a digital multimeter in ohms (Ω). As more light is absorbed by your sample, less light passes through the solution, so the resistance increases.

Setup of your own simplified spectrophotometer device for measuring the concentration of food dye Red 40 in your dyeing solution.
Figure 5. Setup (not to scale) of your own simplified spectrophotometer device for measuring the concentration of food dye Red 40 in your dyeing solution. The light from the LED passes through the filter (blue-green water) and the sample (red 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 cm wide. Cuvettes are designed precisely for this application.

But how do we get from the light absorbance of a dye sample to its concentration? This question is answered by the Beer-Lambert law (Equation 4), which states that the concentration of a chemical, such as Red 40, in a sample solution is directly proportional to the amount of light it absorbs; if you double the concentration of the chemical or dye, the solution absorbs twice as much light.

Equation 4:
  • A is the absorbance, which is unitless,
  • ε is the molar absorption coefficient [L/(mol x cm)],
  • c is the concentration [mol/L], and
  • l is the path length of the light through the sample [cm].

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 red solution, such as Red 40, absorbs blue and green light better than it absorbs red light, which is why we perceive it as red. For the same reason, a blue solution absorbs yellow and red light better than it absorbs blue 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, as shown in Figure 4.

This was a lot of information to process. Take your time to understand the principles of adsorption and absorption and then hit the laboratory or kitchen and dye some wool!

Terms and Concepts

  • Adsorption
  • Adsorbate
  • Adsorbent
  • Chemisorption
  • Physisorption
  • FD&C Red 40
  • Acid dye
  • Adsorption equilibrium
  • Adsorption isotherm
  • Langmuir isotherm
  • Spectrophotometer
  • Wavelength
  • Light absorption
  • Light transmission
  • Absorption spectrum
  • Light-emitting diode (LED)
  • Photoresistor
  • Absorption filter
  • Wavelength selector
  • Resistance
  • Ohm
  • Beer-Lambert law
  • Molar absorption coefficient
  • Breadboard
  • Standard
  • Calibration curve


  • What is the difference between adsorption and absorption?
  • How are different dyes classified and which ones work best for natural versus synthetic fibers?
  • How can you best optimize a specific adsorption process and which variables are most important?
  • What are the different adsorption isotherm models and how do they differ from each other?
  • Where else do adsorption processes occur besides dyeing, and why are they important?
  • How does a commercial spectrophotometer work and for what can it be used?


You can find more information about textile fibers and dyes in these references:

If you want to know out more about adsorption and adsorption isotherms, you can use the following references:

For help with the multimeter and breadboard circuit, use these resources:

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Materials and Equipment Product Kit Available

These specialty items can be purchased from the Science Buddies Store:
  • DIY Sensors Kit (1). From this kit, you will need:
    • Solderless breadboard (1)
    • Jumper wires (3)
    • Alligator clip leads (2)
    • 9 V battery (1)
    • 9 V battery snap connector (1)
    • Photoresistor
    • High-brightness white LED (1)
    • 220 Ω resistor (1)
    • Digital multimeter

You will also need to gather these items (not included in this kit):

  • Plastic cups, 16 oz (8)
  • Mason jars, 12 oz (7); available from Amazon
  • Liquid food color, blue and green
  • 2-L bottle or bowl, empty
  • Spoon
  • Kool-Aid drink mix singles (15.6 g per pack), cherry flavor, unsweetened (4)
  • Tap water
  • Distilled white vinegar (1 L)
  • Stove top or oven
  • Oven mitts
  • Pot big enough to fit the 7 Mason jars
  • Thermometer for liquids; available from Amazon
  • Scissors
  • Cuvettes (3.5 mL); available from Amazon
  • Timer
  • 100 % wool (8 pieces or strings). You can either use wool felt pieces (2.5 cm x 5 cm) or wool yarn (about 26 inches each). Depending on the material you use, it can take shorter or longer to reach the adsorption equilibrium. Wool felt and yarn that has been tested, include:
    • Wool felt; available from Amazon [6–7 h to reach adsorption equilibrium]
    • Wool felt; available from JoAnn [4–5 h to reach adsorption equilibrium]
    • Wool yarn; available at Amazon [6–7 h to reach adsorption equilibrium]
  • Measuring cup, metric
  • Small cardboard box to cover the spectrophotometer
  • Transfer pipettes or medical droppers (8); available from Amazon
  • Permanent marker
  • Aluminum foil
  • Digital scale with 0.1 g increments; a digital scale that would be suitable is the Fast Weigh MS-500-BLK Digital Pocket Scale, available from Amazon
  • Clear tape
  • Lab notebook
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Experimental Procedure

Building Your Spectrophotometer

In this section, you will assemble a circuit on a breadboard. If you have never used a breadboard before, you should refer to the Science Buddies reference How to Use a Breadboard before you proceed. You can follow a step-by-step slideshow that will show you how to put components in the breadboard one at a time. Alternatively, Table 1 lists each component and its location on the breadboard. Important: Read these notes before you proceed.

  • Resistors are marked with colored bands. These colors do matter. Make sure you pick the right resistors for each step, according to the markings.
  • It matters which direction some of the components are facing. Make sure you read the slideshow captions for any special notes about inserting each part.
  • This section only shows you how to assemble the circuit. For a detailed explanation of how the circuit works, see the Help section.
slideshow 1 colorimeter blank breadboard Attach the 9 V battery to the snap connector and tape the battery to the breadboard with clear tape. Connect the black wire from the battery to the ground (-) bus on the right side. Connect the red wire from the battery to the power (+) bus on the left side. Connect the photoresistor from E28 to F28. Bend the wires at the top of the photoresistor so that the flat surface faces sideways towards the battery. nsert a black jumper wire into hole A28. The other end will be connected to the multimeter. Insert a red jumper wire into hole J28. The other end will be connected to the multimeter. Insert the longer lead of the LED into hole E15 and the shorter lead in hole F15. Bend the LED so it points sideways towards the photoresistor. Connect a 220 resistor (red, red, brown, gold) from H15 to the ground (-) bus on the right side. Connect a red jumper wire from B15 to the power (+) bus on the left side to switch on the LED. slideshow 11 colorimeter

slideshow 0 colorimeter holder
Click through the slideshow for step-by-step breadboard diagrams of the circuit.

Part name PictureBreadboard Symbol Location
9 V battery 9V battery breadboard 9v battery symbol Red wire to (+) bus
Black wire to (-) bus
Photoresistor photoresistor breadboard photoresistor symbol E28, F28
White LED white LED from Jameco breadboard LED symbol Long lead to E15
Short lead to F15
Jumper wires (3) black jumper wire breadboard jumper wire black symbol A28 to multimeter
J28 to multimeter
B15 to (+) bus
220 Ω resistor 220 resistor picture 220 resistor breadboard symbol H15 to (-) bus
Table 1. Components for the spectrophotometer circuit (Image credits: Jameco and Fritzing).

Testing the Spectrophotometer

After you have finished building your circuit, testing the spectrophotometer is necessary to ensure that all the electronic components are connected correctly and your device works as expected. Note: Stray light will cause problems and may result in fluctuating data. Perform the readings in a dimly lit room if stray light is a problem and make sure to always place the cuvettes into your device the same way. It also helps to make sure that the cardboard box is always placed on top of the breadboard the same way, meaning that the same side should be facing toward you at all times.

  1. Place one empty cuvette, upside-down, over the LED; and another empty, upside-down cuvette over the photoresistor. If the cuvettes are not clear on all sides but have two grooved or frosted sides, make sure that you put the clear side facing towards the LED as well as the photoresistor. Bend the LED and photoresistor as needed to fit underneath the cuvettes.
  2. Place two empty cuvettes between the LED and the photoresistor. Again, make sure that you always face the clear sides of the cuvette towards the LED and the photoresistor. The four cuvettes should touch each other and form a straight line. You can use clear tape to hold the cuvettes over the LED and the photoresistor in place. But do not block the light path!
  3. The light from the LED should now shine directly onto the photoresistor, as shown in Figure 6. Bend the wires on the LED and photoresistor for adjustment, if needed.
LED photoresistor alignment
Figure 6. Make sure that the LED and the photoresistor are properly aligned. Note that in this picture, the cuvettes are not placed on top of the photoresistor and LED, yet.
  1. Set up the multimeter to measure the resistance of the photoresistor.
    1. Plug the black multimeter probe into the port labeled COM.
    2. Plug the red multimeter probe into the port labeled VΩMA.
    3. Turn the dial setting to 200 ohms (the green "200" at the bottom of the dial near the Ω symbol).
    4. Use alligator clips to attach the multimeter probes to the jumper wires connected to the photoresistor coming from A28 and J28.
  2. Turn on the LED by connecting the jumper wire from B15 to the power (+) bus.
  3. Cover the circuit (but not the multimeter) with the cardboard box to block ambient light. Make sure that the same side of the cardboard box is facing you every time.
  4. Read the resistance across the photoresistor and record it in your lab notebook.
    1. Note the units of the resistance. A "k" indicates kilo-ohms (kΩ) and an "M" indicates mega-ohms (MΩ).
    2. If your multimeter screen displays a "1 .", that means the resistance is too high for the dial setting. Turn the dial up to the next highest range (for example, from 200 to 2000) and check again.
    3. If this is your first time using a multimeter, refer to the Science Buddies resource How to Use a Multimeter, specifically the section How do I Measure Resistance?, to learn more.
  5. Remove the box and turn off the LED by removing the jumper wire from the power (+) bus.
  6. Cover the circuit with the box again. In the dark, the resistance should be in the mega-ohm range. Remember that you may need to adjust the dial setting to get a measurement. Record the resistance in your lab notebook.
  7. Remove the box and turn off the multimeter to conserve battery power.

Calibrating the Spectrophotometer

Now that you have set up the spectrophotometer, the next step is to make the standard solutions to calibrate it. You will make a series of dilutions of Red 40 dye, with known concentration, as shown in Figure 7, and measure them with your spectrophotometer to create a calibration curve. Each dilution is made by consecutively diluting your solution by half. It is essential to use dye-free utensils and cups to get an accurate set of standards.

Calibration solutions for the spectrophotometer.
Figure 7. Standard solutions of Red 40 for calibration of the spectrophotometer.
  1. Take eight clean cups and label them #1–8.
  2. In the 2 L bottle or bowl, prepare a 4:1 water-vinegar solution by mixing 1600 mL of tap water with 400 mL of distilled white vinegar.
  3. Use the measuring cup to pour 400 mL of the water/vinegar mixture into the first cup (#1). Add 200 mL into cups #2–8 each.
  4. Add 1 pack (15.6 g) of unsweetened cherry Kool-Aid singles drink mix to cup #1 and stir with a clean spoon until everything is dissolved. Note: One pack (15.6 g) of unsweetened cherry Kool-Aid singles drink mix contains 47 mg of Red 40 food dye. This means the Red 40 concentration for standard #1 is about 0.12 mg/mL.
  5. Use the measuring cup to pour 200 mL from cup #1 into cup #2 and mix with a clean spoon.
  6. Thoroughly rinse the measuring cup and the spoon and mix 200 mL from cup #2 with the water in cup #3.
  7. Repeat the two-fold dilutions for cups #4–7. Cup #8 will be your "blank" and should not contain any dye.
  8. Label eight clean cuvettes #1–8 (at the very top of the cuvette so you do not block the light path with your writing) and transfer the seven standard solutions and the blank into the matching cuvette using a fresh or rinsed transfer pipette or medical dropper for each one. Note: The cuvettes hold approximately 3 mL of solution.
  9. Prepare your blue-green absorption filter, by adding 120 mL (or 1/2 cup) of water-vinegar mixture into an extra cup. Add two drops of blue and two drops of green liquid food color dye and mix the solution well with a clean spoon.
  10. Transfer the blue-green solution into a clean cuvette and place the cuvette next to the LED so that the clear sides face the LED and the photoresistor..
  11. Set the multimeter to read resistance again. Remember that you might have to adjust the range as you take different readings.
  12. First, place your blank sample without dye in between the blue-green cuvette and the photoresistor, as shown in Figure 8 on the left. Again, the clear sides of the cuvettes should face towards the LED and the photoresistor.
Setup of the spectrophotometer during measurement.
Figure 8. Setup of the spectrophotometer for measuring the blank (left) and standard (right) solutions. Note, that in these pictures, the LED is still switched off and the setup is not covered with a cardboard box yet
  1. Plug in the jumper wire from B15 into the power (+) bus to turn on the LED and cover the breadboard with a small cardboard box. Read the resistance on the multimeter and record the data in your lab notebook.
  2. Remove the blank cuvette and replace it with the cuvette containing the next standard solution, starting with the lowest concentration. Cover the breadboard again with the cardboard box and write down the resistance for this solution. Continue the measurements for each of your seven standards.
  3. Repeat steps 12–14 with the entire set of standards, including the blank, two more times.
  4. Make a data table in your lab notebook showing the dilutions and concentrations of Red 40 dye in all your standards (#1 = 0.12 mg/mL, #2 = 0.06 mg/mL, etc.) together with all three recorded resistance measurements for each solution. The resistance should be higher as the solutions get darker.

Starting and Monitoring the Dyeing Process

You will prepare seven different dye bath solutions with different concentrations of Red 40 to compare the concentrations of adsorbed dye and the color of the wool at the adsorption equilibrium.

  1. In the 2 L bottle or bowl, prepare more of the water-vinegar solution by mixing 1200 mL of tap water with 300 mL distilled white vinegar.
  2. Take seven clean Mason jars and label them #1–7.
  3. Use the measuring cup to pour 300 mL of the water-vinegar solution into jar #1, and 150 mL into jars #2–7 each.
  4. Add three packs (15.6 g) of unsweetened cherry Kool-Aid singles drink mix to jar #1 and stir with a clean spoon until everything is dissolved.
  5. Use the measuring cup to pour 150 mL from jar #1 into jar #2 and mix with a clean spoon.
  6. Thoroughly rinse and dry the measuring cup and the spoon and mix 150 mL from jar #2 with the water in jar #3.
  7. Repeat the two-fold dilutions for jars #4–7. Discard 150 mL of dye solution from jar #7 so you have 150 mL in each jar. Jars #1–7 will be your different dyeing bath solutions.
  8. Transfer about 3 mL of each of these solutions into a fresh (and labeled) cuvette using a fresh or rinsed transfer pipettes for each one, and measure them on the spectrophotometer as you did with the standards before (described starting at step 12 in the section Calibrating the Spectrophotometer). Make sure to label the cuvettes at the very top and take three measurements for each solution. Note: If the resistance of your solution exceeds the maximum resistance of your calibration curve, dilute your sample and measure again. You can do a 1:2 dilution in a fresh cuvette (1.5 mL water-vinegar solution + 1.5 mL dye solution) or a 1:6 (2.5 mL water-vinegar solution + 0.5 mL dye solution). Write down the resistance values for each of your samples in your lab notebook. These values will be your initial dye concentrations at time point 0 (C0).
  9. Cover the top of each jar with aluminum foil to prevent evaporation in the water bath.
  10. Fill up an empty pot that fits all seven jars one-third full with tap water and place it on the stovetop to heat the water up until it boils. This will be your water bath to keep your dyeing solutions at a constant temperature. Note: While working around boiling water, make sure to take every precaution to not burn yourself!
  11. Once the water boils, place all your dyeing solutions (jars #1–7) into the water bath, as shown in Figure 9. Keep the water bath boiling and let them heat up for about 15 minutes. You might need to keep the burner on a high setting to heat the dyeing solutions up. Pick one jar and monitor the temperature of the solution with a thermometer throughout the experiment. Note: If the jars are too large to fit into the pot, you can also transfer all the solutions into smaller Mason jars. Make sure that the jars you choose can tolerate the heat of the boiling water bath.
Dyeing solutions in the boiling water bath.
Figure 9. Put all the dyeing solutions into the boiling water bath on the stove and monitor their temperatures over time.
  1. Take the wool felt and cut eight pieces of the same size (about 2.5 cm x 5 cm). If you use wool yarn, cut 8 strings the length of about 26  inches.
  2. Weigh each piece or string and record their masses in your lab notebook.
  3. Once your dyeing solutions reach about 94–99°C, add one wool felt piece or string to each of the jars. You will keep one for color comparison. Keep track of which one you added (concerning their masses) to which jar. Take a clean spoon and submerge the wool felt or string into the solution to make sure it has good contact with the dye from all sides. Ideally, it should be sitting on the bottom of your jar.
  4. Start your timer and set it to 1 h. During that hour, keep the water bath boiling and make sure to monitor the temperature of your dyeing bath solutions (check after half an hour). It should always stay in between 94–99°C. When necessary, add boiling water (from a kettle) to the water bath to increase the water level again.
  5. After 1 h take a sample from each dye solution using a clean pipette and transfer it into a labeled and clean cuvette. Measure each sample and the blank sample three times on the spectrophotometer, as described starting at step 12 in the section Calibrating the Spectrophotometer. Write down the measured resistance for each solution in your lab notebook. Remember to do dilutions for each sample that exceeds the resistance values of your calibration curve.
  6. Set your timer again to 1 h and repeat sampling and measuring your dyeing solutions and blank every hour until you do not see a change in resistance for each sample anymore. Depending on the wool material you use, this can take between 4–7 hours (see the Materials and Equipment section for more details). Make sure to also monitor the temperature of your dyeing solutions every half hour.
  7. After about 4–7 hours, all your samples should have reached or should be close to the adsorption equilibrium, which means that their resistance should have reached a plateau. Take all the jars out of the water bath using a towel or oven mitts and inspect the solutions. Write down your observations in the lab notebook. You should notice that all the solutions became less red over time, some even might have become clear, as shown in Figure 10. At the same time, the wool felt or yarn should get more red as it adsorbed more Red 40.
Dyeing solutions and wool felt at adsorption equilibrium.
Figure 10. Dyeing solutions at adsorption equilibrium. Note, how some of the solutions became clear and the wool felt turned into different shades of red during the dyeing process.
  1. Take the wool felt pieces or strings out of the jars with a spoon—carefully so as not to burn yourself—and rinse them briefly under tap water. Let them dry overnight and assess their color the next day.

Analyzing Your Data

  1. Open a computer spreadsheet program, such as Google Sheets or Microsoft Excel, and enter the resistance data for your calibration curve. Calculate the average for your three resistance readings for each sample. Subtract the resistance that you measured for the blank from all of the readings you made for samples with dye. This step subtracts the light loss due to the plastic, the water-vinegar, and other factors.
  2. Graph the average resistance of your three readings on the y-axis versus the concentration of the standard solutions in mg/mL on the x-axis.
  3. Add a trend line to your data and display its equation and its correlation factor R2.
  4. For each time point, enter your resistance data into the spreadsheet and calculate the average of your three readings for each sample. Again, subtract the resistance that you measured for the blank from all of the readings you made for samples with dye.
  5. Using your calibration curve, determine the concentration of Red 40 dye in each of your sample solutions (C0 and Ct). Remember to account for your dilution factor if a sample had to be diluted.
  6. Now, calculate the concentration of dye that has been adsorbed by the wool felt or yarn. You can do that by subtracting the measured concentration of Red 40 in the solution for a given time point from the initial concentration of Red 40 at time point 0 (Cadsorbed = C0 - Ct).
  7. Make a graph that shows the concentration of adsorbed dye on the wool felt or yarn over time, plotting time (in hours) on the x-axis and the concentration of adsorbed dye (in mg/mL) on the y-axis. How do the graphs look for each dye concentration? Do you see that the adsorbed dye concentrations level off after some time? Once adsorption gets to a plateau, the adsorption equilibrium is reached.
  8. From the data of your last time point, or when you reached the adsorption equilibrium, you can determine your adsorption isotherms. To do this, you have to calculate qe for each of your dye solutions (cup #1–7) according to Equation 3 in the Introduction. Look back into your notes to find the mass of your wool felt pieces or strings (in mg). The volume of your dye bath should be 150 mL.
  9. Graph the adsorption isotherm of your dyeing process at about 95°C by plotting Ce, the concentration of Red 40 dye in each jar at equilibrium, on the x-axis (in mg/mL) and qe for each Red 40 concentration on the y-axis. Note: Look in the Bibliography to find out if your curve follows the expected trend. From where your curve levels off, you can determine the adsorption capacity, or the maximum amount of Red 40 dye that can be adsorbed into the wool felt or yarn.
  10. Now you can fit your data to the Langmuir adsorption isotherm model and determine the Langmuir constants Q and b, as shown in the Introduction in Equation 2. In your spreadsheet, calculate 1/Ce (mL/mg dye) and 1/qe (mg wool/mg dye) and plot 1/qe on the y-axis versus 1/Ce on the x-axis. You should get a straight line. Add a trend line to your data and display its equation and correlation coefficient R2. Determine the Langmuir constants Q and b from the intercept and slope of the trend line, with 1/Q being the intercept and 1/(Q b) being the slope.
  11. How close is your calculated Q, which indicates the adsorption capacity or the maximum Red 40 that can be adsorbed onto the wool surface, to the experimental value that you determined from your adsorption isotherm graph from step 9?
  12. Finally, look at your dried wool felt pieces or strings from each of your dye bath solutions. Do their colors represent the results that you got from your dyeing data? How does the color of the wool felt or yarn that has been dyed with the highest concentration of Red 40 compare to the one with the lowest one? How do you think the knowledge of adsorption isotherms for a given dye/fiber combination is important for dying processes in the textile industry?


For troubleshooting tips, please read our FAQ: Adsorption: Dyeing Fabrics with Kool-Aid.

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  • How does pH affect the dyeing process? As mentioned in the Introduction, acid dyes need an acidic pH to work. But how acidic does the dyeing solution have to be to get good color adsorption? Does lowering the pH even more increase the adsorption capacity? Do this experiment, and vary the pH of your dyeing baths by adding more acid or a base to the solutions.
  • What other factors can influence the dyeing results? During the adsorption process, the Red 40 dye and the wool fibers are forming an ionic bond, which gives the wool its color. Can you disturb the chemisorption by adding another salt to the dyeing bath that competes with the Red 40 for bonding locations at the wool fibers? Compare your dyeing results in dyeing baths with and without extra salt addition.
  • As the term "isotherm" infers, the adsorption isotherms are only valid for a certain constant temperature. How do you think the adsorption isotherm will change if you change the temperature of your dyeing water baths? Will the adsorption equilibrium be very different? Does it take less time or more to reach the adsorption equilibrium with higher or lower temperatures? You can find out by repeating the dyeing experiment at different temperatures. Make sure you monitor your experiment long enough to reach adsorption equilibrium and choose your sample timing accordingly.
  • Wool is one natural fiber that works well with acid dyes like Red 40. What other fibers can be used? Will silk, another natural protein fiber, work just as well? What about cotton or acrylic? Set up dyeing water baths with different fabrics and compare the results.
  • Does the adsorption isotherm look the same for a different color; for example, the food dye Blue 1? Repeat this experiment, but this time use a different flavor of Kool-Aid that contains the food color Blue 1 instead of Red 40. Remember to change your adsorption filter for the spectrophotometer from a blue-green color into an orange color (2 drops of yellow and 2 drops of red food dye in 1/2 cup of water-vinegar solution).
  • Can you use the data you got from this experiment for a different adsorption isotherm model, such as the Freundlich isotherm? Look up the equation for this adsorption isotherm and use your data to make similar graphs as you did for the Langmuir isotherm. Which isotherm model fits your data better?

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Frequently Asked Questions (FAQ)

If you are having trouble with this project, please read the FAQ below. You may find the answer to your question.
Q: How does the circuit work? What is the circuit diagram?
A: he circuit consists of two parts. One part is a simple series circuit with a battery, LED and a current-limiting resistor. The resistor is necessary to prevent too much current from flowing and burning out the LED. The LED emits light when current flows through it. This light shines on the photoresistor, a type of light-sensitive resistor. Its resistance decreases with increasing amounts of light, and this resistance is measured by a digital multimeter (DMM). Figure 11 shows a schematic of the circuit. You can learn more about the LED and the photoresistor from their respective datasheets.
Circuit diagram for photoresistor and LED circuit
Figure 11. Circuit diagram for the circuit used in this project.

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