Liesegang rings? fascinating creations of nature
"Circle of the Devil"
Please look at a few photographs showing living organisms and samples of inanimate nature: a colony of bacteria on an agar medium, a mold growing on fruits, fungi on a city lawn and minerals - agate, malachite, sandstone. What do all items have in common? This is their structure, consisting of (more or less well-defined) concentric circles. Chemists call them Liesegang rings.
The name of these structures comes from the name of the discoverer? Raphael Edouard Liesegang, although he was not the first to describe them. This was done in 1855 by Friedlieb Ferdinand Runge, who was involved, among other things, in carrying out chemical reactions on filter paper. Created by a German chemist? Self-grown images? () can certainly be considered the first Liesegang rings obtained, and the method of their preparation is paper chromatography. However, the discovery was not noticed in the world of science? Runge did it half a century ahead of schedule (the Russian botanist Mikhail Semyonovich Tsvet, who worked in Warsaw at the beginning of the XNUMXth century, is a well-known inventor of chromatography). Well, this is not the first such case in the history of science; for even discoveries must "come on time."
Raphael Eduard Liesegang (1869-1947)? German chemist and entrepreneur in the photography industry. As a scientist, he studied the chemistry of colloids and photographic materials. He was famous for discovering structures known as Liesegang rings.
The fame of the discoverer was earned by R. E. Liesegang, who was helped by a combination of circumstances (also not for the first time in the history of science?). In 1896, he dropped a crystal of silver nitrate AgNO.3 on a glass plate coated with a solution of potassium dichromate (VI) K2Cr2O7 in gelatin (Liesegang was interested in photography, and dichromates are still used in the so-called noble techniques of classical photography, for example, in the technique of rubber and bromine). Concentric circles of brown precipitate of silver(VI)Ag chromate formed around a lapis lazuli crystal.2CrO4 interested the German chemist. The scientist began a systematic study of the observed phenomenon and therefore the rings were eventually named after him.
The reaction observed by Liesegang corresponded to the equation (written in abbreviated ionic form):
In a dichromate (or chromate) solution, an equilibrium is established between the anions
, depending on the reaction of the environment. Because silver(VI) chromate is less soluble than silver(VI) dichromate, it precipitates.
He made the first attempt to explain the observed phenomenon. Wilhelm Friedrich Ostwald (1853-1932), winner of the 1909 Nobel Prize in Chemistry. The German physical chemist stated that precipitation requires supersaturation of the solution in order to form crystallization nuclei. On the other hand, the formation of rings is associated with the phenomenon of diffusion of ions in a medium that prevents their movement (gelatin). The chemical compound from the water layer penetrates deep into the gelatin layer. The ions of the "trapped" reagent are used to form a precipitate. in gelatin, which leads to depletion of areas immediately adjacent to the sediment (ions diffuse in the direction of decreasing concentration).
Liesegang rings in vitro
Due to the impossibility of rapid equalization of concentrations by convection (mixing of solutions), does the reagent from the aqueous layer collide with another region with a sufficiently high concentration of ions contained in gelatin, only at a certain distance from the already formed layer? the phenomenon is repeated periodically. Therefore, Liesegang rings are formed as a result of the precipitation reaction carried out under conditions of difficult mixing of the reagents. Can you explain the layered structure of some minerals in a similar way? Diffusion of ions occurs in a dense medium of molten magma.
The ringed living world is also the result of limited resources. Circle of the devil? composed of mushrooms (from time immemorial it was considered a trace of the action of "evil spirits"), it arises in a simple way. Mycelium grows in all directions (under the ground, only fruiting bodies are visible on the surface). After a while, the soil becomes sterilized in the center? the mycelium dies off, remaining only on the periphery, forming a ring-shaped structure. The use of food resources in certain areas of the environment can also explain the ring structure of bacterial and mold colonies.
Experiments with Liesegang rings they can be carried out at home (an example of an experiment is described in the article; in addition, in the 8/2006 issue of Młodego Technika, Stefan Sienkowski presented Liesegang's original experiment). However, it is worth paying the attention of experimenters to several points. Theoretically, Liesegang rings can be formed in any precipitation reaction (most of them are not described in the literature, so we can become pioneers!), but not all of them lead to the desired effect and almost all possible combinations of reagents in gelatin and aqueous solution (suggested by the author, experience will be good).
mold on fruit
Remember that gelatin is a protein and is broken down by some reagents (then a gel layer is not formed). More pronounced rings should be obtained using test tubes as small as possible (sealed glass tubes can also be used). Patience is key, however, as some experiments are very time consuming (but it's worth the wait; well-formed rings are easy? Beautiful!).
Although the phenomenon of creativity Liesegang rings may seem to us only a chemical curiosity (they do not mention it in schools), it is very widespread in nature. Is the phenomenon mentioned in the article an example of a much broader phenomenon? chemical oscillatory reactions during which periodic changes in the concentration of the substrate occur. Liesegang rings they are the result of these fluctuations in space. Of interest are also reactions that demonstrate fluctuations in concentrations during the process, for example, periodic changes in the concentrations of glycolysis reagents, most likely, underlie the biological clock of living organisms.
See experience:
Chemistry on the web
?Abyss? The Internet contains many sites that may be of interest to a chemist. However, a growing problem is the overabundance of published data, sometimes also of dubious quality. Not? will quote here the brilliant predictions of Stanislav Lem, who more than 40 years ago in his book ?? proclaimed that the expansion of information resources simultaneously limits their availability.
Therefore, in the corner of chemistry there is a section in which addresses and descriptions of the most interesting "chemical" sites will be published. Related to today's article? addresses leading to sites describing Liesegang rings.
The original work of F. F. Runge in digital form (the PDF file itself is available for download at the shortened address: http://tinyurl.com/38of2mv):
http://edocs.ub.uni-frankfurt.de/volltexte/2007/3756/.
Website with address http://www.insilico.hu/liesegang/index.html is a real compendium of knowledge about Liesegang rings? the history of the discovery, theories of education and many photographs.
And finally, something special? film showing Ag precipitation ring formation2CrO4, the work of a Polish student, a peer of MT readers. Of course, posted on YouTube:
It is also worth using a search engine (especially a graphical one) by entering the appropriate keywords into it: “Liesegang rings”, “Liesegang bands” or simply “Liesegang rings”.
In a dichromate (or chromate) solution, an equilibrium is established between the anions
and, depending on the reaction of the environment. Because silver(VI) chromate is less soluble than silver(VI) dichromate, it precipitates.
The first attempt to explain the observed phenomenon was made by Wilhelm Friedrich Ostwald (1853-1932), winner of the Nobel Prize in Chemistry in 1909. The German physical chemist stated that precipitation requires supersaturation of the solution in order to form crystallization nuclei. On the other hand, the formation of rings is associated with the phenomenon of diffusion of ions in a medium that prevents their movement (gelatin). The chemical compound from the water layer penetrates deep into the gelatin layer. The ions of the "trapped" reagent are used to form a precipitate. in gelatin, which leads to depletion of areas immediately adjacent to the sediment (ions diffuse in the direction of decreasing concentration). Due to the impossibility of quick equalization of concentrations by convection (mixing of solutions), the reagent from the aqueous layer collides with another region with a sufficiently high concentration of ions contained in gelatin, only at a distance from the already formed layer? the phenomenon is repeated periodically. Thus, Liesegang rings are formed as a result of a precipitation reaction carried out under conditions of difficult mixing of the reagents. Can you explain the formation of the layered structure of some minerals in a similar way? Diffusion of ions occurs in a dense medium of molten magma.
The ringed living world is also the result of limited resources. Circle of the devil? composed of mushrooms (from time immemorial it was considered a trace of the action of "evil spirits"), it arises in a simple way. Mycelium grows in all directions (under the ground, only fruiting bodies are visible on the surface). After a while, the soil becomes sterilized in the center? the mycelium dies off, remaining only on the periphery, forming a ring-shaped structure. The use of food resources in certain areas of the environment can also explain the ring structure of bacterial and mold colonies.
Experiments with Liesegang rings can be carried out at home (an example of an experiment is described in the article; in addition, in the issue of Młodego Technika dated 8/2006, Stefan Sienkowski presented the original Liesegang experiment). However, it is worth paying the attention of experimenters to several points. Theoretically, Liesegang rings can be formed in any precipitation reaction (most of them are not described in the literature, so we can become pioneers!), but not all of them lead to the desired effect and almost all possible combinations of reagents in gelatin and aqueous solution (suggested by the author, experience will be good). Remember that gelatin is a protein and is broken down by some reagents (then a gel layer is not formed). More pronounced rings should be obtained using test tubes as small as possible (sealed glass tubes can also be used). Patience is key, however, as some experiments are very time consuming (but it's worth the wait; well-formed rings are easy? Beautiful!).
Although the formation of the Liesegang ring may seem like a chemical curiosity (it is not mentioned in schools), it is very widespread in nature. Is the phenomenon mentioned in the article an example of a much broader phenomenon? chemical oscillatory reactions during which periodic changes in the concentration of the substrate occur. Liesegang rings are the result of these fluctuations in space. Of interest are also reactions that demonstrate fluctuations in concentrations during the process, for example, periodic changes in the concentrations of glycolysis reagents, most likely, underlie the biological clock of living organisms.
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