6.2 - Periodic Precipitation: Liesegang Rings
26. Liesegang ExperimentIn the diffusion chamber experiment, diffusion is coupled with a chemical reaction. Ammonia and hydrogen chloride gases react to form the solid (dust) ammonium chloride in the form of a disk. This as an example of a pattern formation process called reaction-diffusion, a combination of diffusion and chemical reaction.
In the diffusion tube experiment the resulting pattern was simple: a disk of ammonium chloride dust. However, reaction-diffusion processes have been linked to more complicated patterns observed in stones called agates, the stripes of the zebra, and the development of embryos.
Here you will study a more complex reaction-diffusion pattern that results when ions diffuse through a gel rather than gases diffusing through air. A gel (for example, Jell-O) can be thought of as water loosely confined and kept from flowing by a network of transparent "bags'' consisting of crisscrossed long-chain molecules.
In the experiment described here, potassium chromate is dispersed uniformly through a gel in a small cylindrical container and copper sulfate crystals are placed on top of the gel. Copper ions go into solution and diffuse through the gel. When they meet chromate ions, they join to form insoluble copper chromate, which has a green color and thus becomes visible. Surprisingly, the result is not one disk, but a series of parallel disks. These are called Liesegang rings, after the German chemist Z. Q. Liesegang, who first reported them. His original experiments took place in a flat dish with the copper sulfate placed at the center, so the pattern was, in fact, rings.
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