HandsOn 26 - Liesegang Experiment


I. Pre-Lab


Figure 6.6: Multiple parallel Liesegang rings in silica gel.


Figure 6.7: Circular Liesegang rings in a petri dish of agar gel.

There are clear similarities between the Liesegang experiment and the diffusion tube experiment. In both experiments:

There are, however, some differences between the two diffusion experiments:

Q6.15: Can you think of other differences and similarities between the diffusion chamber and Liesegang experiments?

Q6.16: Can you account for the appearance of multiple parallel disks in Figures 6.6 and 6.7? Think of what happens as the copper sulfate diffuses into the gel? Will a precipitate of copper chromate appear? If so, what will that do to the local concentration of chromate ion? What will be the consequence for diffusion of chromate ion to or from nearby regions? What happens to the local concentration of the copper ions diffusing through the gel? What happens next?

Q6.17: Can you make a guess as to how long you may have to wait to observe a pattern? Hint: A generalization of the result obtained in Appendix 6-A to cases when randomness of steps is caused by collisions with other particles turns out to be
x2avg = 2Dt,
where the proportionality constant D is called the diffusion constant, and its value varies from material to material. The value of the diffusion constant for copper sulfate in the gel is roughly what it is in water, on the order of 10-6 cm2/sec. The preceding equation is usually called the diffusion equation.


II. Experiment

For this experiment, you will need the following materials:

In the following, it is assumed that the experiment will be done with approximately 8 ml of gel. For other sizes, scale the ingredients proportionately. Leave space at the top of the vial for the copper sulfate.

Q6.18: Compare your predicted pattern with what you observed. Is the symmetry what you expected? Refine your speculation about the source of the multiple rings.

Q6.19: Did the pattern take as long to develop as you expected? Does the diffusion equation above provide an approximate relation between the time t for a single ring to form?

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