Using an iSight Camera and a Sony HandyCam with BTV-Pro on a Macintosh to Study Chemical Waves in the Belousov-Zhabotinsky Reaction, Liesegang Rings and other Spatial Phenomena.
John A. Pojman, Ph.D., Jace Ponder and Giselle Schnaulbelt
Department of Chemistry & Biochemistry
The University of Southern Mississippi
An iSight is an inexpensive camera to capture digital video of chemical reactions. With BTVpro shareware ($40) and the freeware ImageJ, it is possible to sophisticated analysis of spatial phenomena in the chemistry. Using a Sony Handycam, it is possible to study even more complicated systems.
An iSight camera is cheap and easy to use. We use them in our Physical Chemistry lab to study chemical waves in the Belousov-Zhabotinsky (BZ) reaction.
Macintosh running OS X with iChat installed.
(You can use a firewire camcorder with BTVpro under System 9 but not an iSight.)
Firewire-enabled digital camcorder such as a Sony HandyCam
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For a recipe for chemical waves in the BZ reaction, we refer to (Epstein and Pojman, 1998). The solutions were added to a petri dish containing ultrafine silica gel (CAB-O-SIL) to prevent convection. The petri dish was placed on a Tundra Light-Box to provide backlighting.
The iSight camera autofocuses, which can be a problem. When the camera was placed over the petri dish, it keeps trying to find something to focus on. By place a clear plastic ruler under the dish, the camera can focus on the markings on the ruler. This also provides calibration.
All movies are Quicktime.
All Movies are in Quicktime. To download the QuickTime plugin.
To view a movie, click on the image.
BZ wave movie -- one frame per second. The ruler is in cm.
For more information on performing this lab, see (Pojman, 1994) or download a pdf of the lab we use.
BZ wave movie showing the difficulty the iSight can have focusing. Movie is 10X real time.
The iSight can work with iMovie, and real-time movies of the BZ waves or any phenomena. But it is not possible to directly record a time-lapse movie. It is possible to create time-lapse movie with QuickTime Pro but if you are recording a slow phenomenon, you will record gigabytes of movie and then have to postprocess them. BTVPro solves the problem. You can set it to start capturing the movie after a delay and then at any frequency you want. It also can superimpose a real-time clock on the image.
Another great feature is that BTVPro overides the autosleep of many camcorders so you can use them as a video camera even with a tape running. This is valuable if you want to make movies of experiments in which the focus will change during the experiment.
For the "Mystic Garden" experiment, the iSight had trouble focusing as the image changed.
Time Lapse movie of "Mystic Garden" experiment using iSight Camera. Total time is 12 hours. Notice that the right side is in focus but the left side is not.
Time Lapse movie of "Mystic Garden" experiment using a Sony HandyCam.Camera. Total time is 12 hours.
This is a great experiment for time-lapse video. Crystals of varios salts are added to a sodium silicate solution. The crystals grow up because of the interaction with gravity but directional growth occurs in weightlessness (Jones, 1998). The iSight did not work well for this experiment because the autofocus kept changing focus as new crystals grew.
The Silicate garden over 20 hours after addition of the crystals,
using a Sony HandCam.
The Diffusion Equation describes how the concentration at position x changes
as a function of time.
The equation says that the rate of change of the concentration of the ith chemical species at position x is the diffusion coefficient, Di (cm2 s–1), that is multiplied times the rate of change of the gradient of the concentration at the position. (This second derivative with respect to position is called the one-dimensional Laplacian and is a measure of how sharply the concentration changes with position.)
We can look at some solutions to this equation that describe simple diffusion.
Let’s assume we have a tube in which a solute is present in half of the container. The concentration is taken initially as 1. The concentration profile at any time can be computed by:
where D is the diffusion coefficient, with units of cm2 s–1. Notice how slow is diffusion. After 10,000 seconds, the edge of the concentration profile has only advanced 0.5 cm.
Figure 2. Calculated concentration profiles from the solution to the diffusion equation with D = 10–6 cm2 s–1 at different times.
A pH indicator, bromophenol blue, is shown diffusing into an agar gel over a 24 hours period. The scale is in cm.
let's see how much faster the dye will spread through the water if gel is not present.
Left: Diffusion in a gel in a cuvette. Each frame is 30 minutes. Right: Dye added to water
without gel (1 frame per second).
Both movies were made with an iSight.
The movie illustrates 24 hours of diffusion of the pH indicator bromophenol blue in an agar gel. Notice again how slow the dye diffuses into the gel. Even with this simple setup, it is possible to extract the diffusion coefficient. Figure 3 shows a curve fit to the grey level vs position for an image after 12 hours. ImageJ is a free Java program that allows quantitative data analysis include plotting grey level versus position along a selected line.
Figure 3. A grey scale image that was imported into ImageJ. Curve fit to the grey level along the vertical axis of the tube, after 12 hours. From the curve fit we can calculate a diffusion coefficient of 3 x 10–6 cm2 s–1, a reasonable value for a large organic molecule in water.
Liesegang Rings are periodic patterns that arise spontaneously in the counter-current diffusion of two salts that can form an insoluble precipitate. For example, imagine if lead nitrate, Pb(NO3)2, is dissolved in water containing agar, is poured in a test tube and allowed to set. Then an agar-containing solution of potassium iodide, KI, is poured on topof the lead nitrate gel. The potassium and iodide ions will diffuse in the bottom gel while lead ions and nitrate ions will diffuse up. (The gel is used to prevent convection.) When lead and iodide ions meet, they form lead iodide, PbI2an insoluble precipitate.Rings appear at greater and greater distances. Using time-lapse video we can measure the position of each new ring and the time at which it appeared. We then calculate the distance (in cm) of the ring from the interface of the two gels and call it x. By plotting x2 versus time in second we can obtain a straight line whose slope is the diffusion coefficient.
Figure 4. Left: Time lapsed movie of Liesigang rings -- total time was 11 hours. Right: Same movie but with contrast enhanced using ImageJ
Figure 5. A plot of the square of the position of a ring versus the time at which it appeared. A linear regression gives an estimate for the diffusion coefficient of 4 x 10–5 cm2 s–1, a reasonable value for an ionic compound in water.
For a pdf of the lab procedure, click here.
References and Further Reading
Crank, J. Mathematics of Diffusion; Clarendon: Oxford, 1975.
Cussler, E. L. Diffusion: Mass Transfer in Fluid Systems; Cambridge: London, 1997.
Einstein, A. Investigations on the Theory of the Brownian Movement; Dover: ISBN:
Epstein, I. R.; Pojman, J. A. An Introduction to Nonlinear Chemical Dynamics: Oscillations, Waves, Patterns and Chaos; Oxford University Press: New York, 1998.
Jones, D. E. H., Walter, Ulrich "The Silicate Garden Reaction in Microgravity: A Fluid Interfacial Instability,"Journal of Colloid and Interface Science.1998, 203, 286.
Pojman, J. A.; Craven, R.; Leard, D. "Oscillating Reactions and Traveling Waves in the Physical Chemistry Lab,"J. Chem. Ed. 1994, 71, 84-90.
Sharbaugh III, A. H.; Sharbaugh Jr., A. H. "An Experimental Study of the Liesegang Phenomenon and Crystal Growth in Silica Gels,"J. Chem. Ed. 1989, 66, 589-594.
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