Liesegang rings

Liesegang rings (/ˈliːzəɡɑːŋ/) are a phenomenon seen in many, if not most, chemical systems undergoing a precipitation reaction, under certain conditions of concentration and in the absence of convection.

The phenomenon was first noticed in 1855 by the German chemist Friedlieb Ferdinand Runge. He observed them in the course of experiments on the precipitation of reagents in blotting paper. In 1896 the German chemist Raphael E. Liesegang, noted the phenomenon when he dropped a solution of silver nitrate on to a thin layer of gel containing potassium dichromate. After a few hours, sharp concentric rings of insoluble silver dichromate formed. It has aroused the curiosity of chemists for many years. When formed in a test tube by diffusing one component from the top, layers or bands of precipitate form, rather than rings.

The reactions are most usually carried out in test-tubes into which a gel is formed that contains a dilute solution of one of the reactants.

If a hot solution of agar gel, also containing a dilute solution of potassium dichromate is poured in a test-tube, and after the gel solidifies, a more concentrated solution of silver nitrate is poured on top of the gel, the silver nitrate will begin to diffuse into the gel. It will then encounter the potassium dichromate and will form a continuous region of precipitate at the top of the tube.

After some hours, the continuous region of precipitation is followed by a clear region with no sensible precipitate, followed by a short region of precipitate further down the tube. This process continues down the tube forming several, perhaps a couple of dozen regions of clearing, then precipitation rings.

Over the decades huge number of precipitation reactions have been used to study the phenomenon, and it seems quite general. Chromates, metal hydroxides, carbonates, and sulfides, formed with lead, copper, silver, mercury and cobalt salts are sometimes favored by investigators, perhaps because of the pretty, colored precipitates formed.

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