Liesegang Rings

LIESEGANG RINGS (Liesegang), layered structures that form during the deposition of insoluble precipitates in gels (see). When the interaction of substances diffusing in a gel leads to the formation of a precipitate, peculiar 'rhythmic structures' often arise, which were studied in detail by Liesegang. For example, if a plate of gelatin jelly containing a small amount (approximately 0.1%) of potassium bichromate is placed with a drop of concentrated solution (25-50%) of silver nitrate, then in the gel after some time, ring-like deposits of silver chromate appear, alternating with layers free of precipitate. As the distance from the center of diffusion increases, the distances between the ring-like layers, initially extremely small, successively increase. The same layered, rhythmic precipitations or L. r. are obtained when forming in gels calcium phosphate or calcium carbonate, insoluble salts of barium, lead, etc. - V. Ostwald gave the following explanation for this phenomenon. The soluble silver salt forms with the bichromate encountered during its diffusion a supersaturated solution of sparingly soluble silver chromate. In the absence of crystallization centers, supersaturation can be more or less significant; but as soon as it exceeds a certain limit (the so-called 'metastability limit'), crystallization occurs, and the precipitate captures all the excess of the insoluble salt formed from the adjacent layer of the gel, thereby compacting the precipitate. As a result, in the immediate vicinity of the precipitated precipitate, the concentration of chromic salt decreases. Silver ions must now diffuse further to again reach the supersaturation necessary for the deposition of a new layer of precipitate by combining with the counter-diffusing potassium bichromate. In reality, however, this simple scheme is greatly complicated. The gel itself and various impurities contained in it can have a strong influence on the precipitation of the crystalline precipitate and its further growth (for example, through the protective and stabilizing action of colloids). Therefore, in different gels, the ring formation of the precipitate occurs far from uniformly. Due to diffusion processes, extremely characteristic rhythmic, layered structures can arise in the gel under complete constancy of external conditions. L. rings are of great interest due to their often striking resemblance to various natural structures, in particular to many biological structures. It must be admitted, however, that in very many cases there is only a purely external similarity. For example, the annual rings in tree trunks or the annual layers of fish otoliths do not depend on rhythmically precipitating sediments, but on a completely different, external rhythm of periodically changing growth conditions. In other cases, however, for example in the deposition of mineral salts in the body, the phenomena described play an essential role. They apparently include the deposition of concentric layers of lime salts in teeth (Retzius bands), as well as around Haversian canals in mammalian bones. The structure of liver, bile, and kidney stones, amyloid prostatic bodies, corpora arenacea probably also depends on the L. r. forming in them. Finally, the possibility of their appearance must be taken into account when impregnating cells with various metallic precipitates, widely practiced in histological technique (methods of Golgi, Ramon y Cajal, etc.). In cases where tissue previously hardened with chrome salts is treated with silver salt, the histological technique exactly reproduces the conditions of the basic Liesegang experiment, and the resulting histological structures represent an undeniable artifact. Similar remarks apply to many of those microchemical methods that attempt to accurately determine the localization of soluble salts in tissues or cells by precipitating them. Under certain conditions, the precipitate may be deposited not in the place where the dissolved substance being investigated was originally located. - In recent times, Stempell pointed out that the regularity in the formation of L. r. is disturbed when they are exposed to mitogenetic rays (see). Siebert and Tokin showed, however, that in this case there is an effect of gaseous substances that disrupt the course of the labile process of rhythmic precipitation.