Abstract

Objective

Determine how changing various parameters, such as the concentration of starting ingredients, changes the dynamics of the BR reaction, and capture information about color changes for computer analysis.

Introduction

The Briggs-Rauscher (BR) chemical reaction makes for a dramatic demonstration when it is done in front of a chemistry class. A few chemicals are mixed together in a clear beaker with water and hydrogen peroxide. The clear solution suddenly turns an amber color, then dark blue, and then clear again. The cycle repeats 10 or more times. If the liquid is stirred, the color changes uniformly throughout the solution. If the liquid is not stirred, you can see the colors developing in one part of the solution and then migrating through the rest of the solution. The rate at which the colors change depends on the precise concentrations of the reactants and on the temperature of the solution.

Note: This science project is an extension of another Science Buddies science project, entitled: Minds of Their Own: A Chemical Reaction that Changes, then Changes Back!. To find out more information about the chemistry of the BR reaction, you should read the Introduction of that project. The goal of this science project, however, is to build a device to capture data for the changes that are occurring in the BR reaction for computer analysis.

To capture the data, you need a sensor. In this case, the sensor is a photoresistor. It measures color changes in the solution. More precisely, it measures how much light is absorbed by the solution as the colors change. The output from the sensor is resistance. Resistance is a measure of how well something blocks the flow of electricity. A photoresistor is a specific kind of resistor; its resistance depends on how much light strikes it. As more light hits the photoresistor, the resistance becomes lower. Resistance is measured in units called ohms. A typical photoresistor might have a dark resistance of 10 mega-ohms. That is, when there is no light striking it, its resistance is 10 million ohms. When the same photoresistor is exposed to light, from a lamp or the Sun or other light source, its resistance might decrease to less than 100 ohms (the precise numbers for the dark and light resistance depend on many factors. The numbers used here are just for illustration). Photoresistors are inexpensive and rugged. They also provide a linear response to light levels. If the amount of light is doubled, the resistance decreases by a factor of two (the actual response depends on factors such as the wavelength of light, etc., but you don't need to worry about that level of detail for this science project).

The output from the sensor has to be detected by something. For this science project, the detector is a device called a Picoboard. The Picoboard is an electronic sensing device that reads various kinds of inputs, including resistance. You will use it to measure the resistance from a photoresistor. The goal is to capture the resistance of a photoresistor on a computer. The Picoboard has electronic components that convert resistance (the input) into a signal (the output) that can be communicated to a computer. The signal from the Picoboard is sent to the computer through a USB cable. The Picoboard comes with four resistance inputs. For this science project, you will just use one of them. The resistance input consists simply of two wires. The Picoboard measures the resistance across these two wires. If the wires are not connected to anything, there is no way for electricity to flow, so the resistance is infinite. If the wires are connected by a metal object, such as a coin, electricity can flow easily and the resistance is very low. Metals have low resistance, so are good conductors of electricity.

The final thing you'll need is a program on the computer that can communicate with the Picoboard. This program has to be able to read the data that is being sent to it by the Picoboard, and display it in a way that you, the experimenter, can see. The program you will use for this is a downloadable program that was created at the Massachusetts Institute of Technology (MIT), called Scratch. Scratch is fun and easy to learn, and it will allow you to control the Picoboard, collect data from sensors, and store your data on the computer for later analysis.

The experimental setup will be simple. A photoresistor will be attached across the two wires that lead to the resistance input on the Picoboard. The photoresistor will be placed under a clear plastic cup with the chemicals for the BR reaction in an aqueous solution. The changes in color will cause changes in the resistance of the photoresistor (because of more or less light passing through), which will be detected by the Picoboard and transmitted to the Scratch program. The Scratch program will record the resistance value several times per second and store the values in a file. To analyze the data, it will be copied from the Scratch file into a graphing program, such as Microsoft® Excel®.

This is an advanced project that will require a lot of independent problem solving. However, the chemical reaction setup, the Picoboard detector hardware, and the Scratch data-collection software are all designed to be very user-friendly. The goal is to collect, store, and analyze data for changes in the reaction at a rate of several times per second. This will allow you to achieve a far better picture of the changes that are occurring in the BR reaction than is possible just by watching it. If you are interested in further exploration, such as running various tests at once, check out the Variations section at the bottom of the page.

Terms, Concepts, and Questions to Start Background Research

• Briggs-Rauscher (BR) chemical reaction
• Hydrogen peroxide
• Rate
• Concentration
• Reactant
• Sensor
• Photoresistor
• Light absorption
• Resistance
• Ohm
• Mega-ohm
• Picoboard
• Conductor
• Scratch

Questions

• What are some kinds of sensors around your house? Hint: What electronic devices sense touch, temperature, light, sound, etc.?
• Based on your research, how does a photoresistor work?
• Based on your research, is the output from a photoresistor analog or digital? What form does the signal have to be (analog or digital) for the computer to process it?
• Who first described an oscillating chemical reaction?
• How would you expect the oscillations to change if the temperature were increased?

Bibliography

• Massachusetts Institute of Technology. The Unofficial Sensor Board FAQ. Retrieved August 22, 2010, from http://scratch.mit.edu/forums/viewtopic.php?id=6403
• Massachusetts Institute of Technology. Sensor Boards. Retrieved August 22, 2010, from http://info.scratch.mit.edu/Sensor_Boards

Materials and Equipment

• The Fascinating Oscillating Reaction Kit, Educational Innovations, Inc.; available from www.teachersource.com. The kit comes in two sizes:
• Item CK-475 has one set of chemicals and provides sufficient chemicals for the basic procedure.
• Item CK-480 is a larger classroom kit that has more chemicals.
• The kit contains the following items:
• Malonic acid
• Manganese sulfate
• Sodium iodate
• Sodium thiosulfate
• Sulfamic acid
• Starch solution
• Wooden rack
• Pipet
• Stirrers
• Spoon
• Small scoops
• Cups
• Instructions
• Hydrogen peroxide (3%) is required and not supplied in the kit; available at any drug store
• Permanent marker
• Distilled water; available at grocery or drug stores
• Stopwatch
• Lab notebook
• Photoresistor(s); available from RadioShack at www.radioshack.com, part #: 276-1657
• Picoboard; available online at www.picocricket.com/whichpicobard.html
• Scratch; available for free download from scratch.mit.edu
• Piece of cardboard, 4–5 inches (in.) square
• Rubber band
• Computer with USB port and graphing software; you must have permission to and be able to download Scratch to the computer.
• Spreadsheet program, such as Microsoft Excel
• Newspaper to cover the work surface
• Stirrer or spoon
• Clear tape
• Optional: Video camera
• Helper

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Experimental Procedure

Preparing Your Experimental Setup

1. Download Scratch. See the Science Buddies Scratch User Guide: Installing & Getting Started with Scratch for more information. Be sure you have permission from the computer owner to download this program.
2. Once you have Scratch installed on your computer, download the program Light Capture - SB from the Scratch website.
1. This is the program used to collect data.
2. The program must be downloaded to run this science project properly. It will run from the online site, but it won't be able to communicate with the light sensor otherwise.
3. Plug the Picoboard into the USB port on your computer.
1. Read the Picoboard instructions for troubleshooting tips.
2. Consult our Scratch User Guide: Connecting & Using a Picoboard with Scratch if you'd like additional information about the Picoboard and how to use it.
4. Attach the photoresistor to the Picoboard by plugging the wire into jack A on the Picoboard. The program only senses input from jack A. Then attach the two alligator clips onto the two leads from the photoresistor.
5. Tape a photoresistor to the piece of cardboard with tape.
1. The side with the squiggly line should face up.
2. Do not cover the face of the photoresistor.
6. Start the "Light Capture - SB" program, as follows.
1. Click on the green flag to start the program.
2. Click on the red stop sign to halt the program.
3. Click on the down arrow to reset the program.
7. The program should start collecting data. See Figure 1, below.

Figure 1. Picoboard, photoresistor, and Scratch program for this Briggs-Rauscher science project. The Scratch screen is visible on the laptop screen. To measure color changes in the reaction, a clear cup with the reactants is placed over the photoresistor and held in place with the rubber band. As the reaction proceeds, resistance changes in the photoresistor are detected by the Picoboard and transmitted to the computer, where a Scratch program stores and graphs the data.

8. Try varying the level of light to see if the graph changes.
1. The graph in Figure 1 was made by covering the photoresistor with a piece of cloth several times for 27 3-second intervals.
9. If it is not working, read the Picoboard instructions for troubleshooting tips.
10. The light level and time are recorded in the small boxes on the screen. You can copy the light level and time data by right-clicking with your mouse on the boxes and selecting "export." The data can then be pasted into a spreadsheet program, such as Microsoft Excel.

Running the Briggs-Rauscher Reaction

1. Attach a clear plastic cup to the cardboard, over the photoresistor, using the rubber band.
2. Follow the directions that came with the Briggs-Rauscher kit to set up solutions A and B for the reaction. The reaction starts when the two solutions are mixed. Do not mix them just yet.
3. Place the cardboard/photoresistor/cup assembly on a work surface covered with newspaper.
4. The surface should be well-lit. Avoid sunlight or other variables that can change the brightness around the reaction.
5. Reset the Scratch program.
6. Start the Scratch program.
7. Mix solutions A and B in the cup with a clean stirrer or spoon. Note the time in your lab notebook.
8. As the reaction proceeds, you should see color changes in the solution and an oscillating curve of the light level on the computer screen. Record your observations of the reaction in your lab notebook. As an option, record the reaction with a video camera.
9. Allow the reaction to go to completion (about 10 minutes).
10. Dispose of the chemical down a sink as you run cold water.

Collecting the Data

1. Right-click on the box labeled "light level."
2. Select "export."
3. Choose a folder in which to save the data.
4. Repeat for the "Timer" data.
5. Choose names for your data files that will allow you to keep track of the data. Record the names and contents of each data file in your lab notebook.
6. When you hit "reset" on the Scratch program, the Scratch data will be erased.

Varying the Concentration of Malonic Acid and Determining How this Affects the Reaction Cycles

1. The timing of the color changes you saw above can be influenced by altering the relative amounts of the reactants. If you are interested in trying this part of the procedure, you can perform the Science Buddies project entitled Minds of Their Own: A Chemical Reaction that Changes, then Changes Back!.

Figure 2. Varying the malonic acid level in the Briggs-Rauscher reaction affects the rate of the periodic changes. The reaction that produced the yellow line had twice as much malonic acid as the reaction that produced the blue line. The overall time was about 7 minutes.

2. You can also develop your own procedures for varying the concentrations of the starting chemicals.

Analyzing Your Data

1. Copy the light level and timer data into a spreadsheet for a graphing application, such as Microsoft Excel.
2. Graph the light level (y-axis) vs. the time (x-axis). Remember to label the axes and to indicate the units. If you are using Excel, graph the data in an "XY scatter" chart.
3. Compare the graphs for various conditions. For example:
1. Time between low points on the graph.
1. Make another graph of malonic acid concentration vs. cycle time.
2. Total number of cycles.
3. Shape of curve (for example, changing the length of the amber phase affects the shape of the curve).
4. Describe why varying the malonic acid resulted in the observed changes in the reaction.

Variations

• The Picoboard has four inputs for resistance. As a variation on this science project, the Scratch program can be modified to collect data from four simultaneous Briggs-Rauscher reactions, perhaps to investigate varying concentrations of reactants or the effect of temperature on the reaction. If you pursue this avenue of research, you will have to devise a way to start all four reactions at the same time.
• Vary the concentration of other chemicals.
• Vary the temperature. Determine the relationship between temperature and oscillation rate.
• There is also a periodic electrochemical change in the reaction that can be measured with a multimeter set to DC volts. Use two pencils sharpened at both ends as electrodes to study this chemical process. How can you capture this data?
• Devise ways to improve the procedure. For example, make an experimental setup with more control over the light level.
• What if you change the color of the light?
• Try running two (or more) reactions simultaneously. You will need to edit the Scratch program to do this. You can start with this Scratch program.
• Because the reactions generate free radicals, the BR reaction can be used to test for antioxidants. Do some research on this subject and devise a procedure to measure antioxidant properties of various chemicals. Ask your chemistry teacher for help obtaining chemicals that should provide a strong positive signal in the test.
• Edit the Scratch program to vary the number of data points taken per unit time.

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

David B. Whyte, PhD, Science Buddies

• Microsoft® is a registered trademark of Microsoft Corporation.
• Excel® is a registered trademark of Microsoft Corporation.

Last edit date: 2010-09-03 12:00:00