Rhythmic Banding of Precipitates (liesegang’s Rings)

Rhythmic banding of precipitates was first discussed by Raphsel E. Liesegang (1)) hence the name “Liesegang’s rings.” He observed that when a solution of silver nitrate is placed on a gelatine gel containing dilute potassium bichromate, rhythmic banding of the precipitate occurs. If the solutions are in a dish or on a plane surface the precipitate is in concentric rings; if in a test-tube, superimposed layers of the precipitate are formed. Numerous explanations have been advanced, but none is generally acceptable. Ostwald’s explanation (2)) which postulated a metastable condition, was accepted until Liesegang, Bechold (3)) and Hatschek (4) proved it untenable. Bechold (3) advanced the theory that the band precipitate is soluble in the products of the reaction, and new bands are formed only when the concentration of the reaction products is so dilute that it will not interfere. It has been shown, however, that the effect of the by-products is negligible (5). Bradford (6, 7) in 1916 put forward a theory which would explain ring formation by adsorption. Since then he has published much work to support his views. Holmes (8) has also advanced a theory similar to Bradford’s, but attributes the formation to ionic activities, instead of adsorption effects of the colloidal particles of the precipitate. Bancroft (9)) quoting Hatschek, states that all theories fail to account for the specific effect of the gel. Bradford,’ however, accounts for this by saying that the value of N in von Weimarn’s formula is influenced by the gel. We do not think this explanation is adequate. The explanation given by Bradford and by