Metal vegetation

In terms of chemistry, a metal vegetation is a type of dendrite which appears at the time of certain chemical reactions. The metal vegetations can present forms similar to those of the dendrites of solidification, due as for them for a purely physical purpose. The metal vegetations appear in particular during reactions of Oxydo-réduction in aqueous solution. The name of tree or shrub is often allotted to them.

Metal types of vegetations

Observations

First publications

Diderot allots the first publication of metal vegetations to the Greek chemist Rhodos Carasses (publication in the Journal of the scientists in 1677), itself not being the discoverer of the experiment only it describes.

Developments of the 18th century

Several experiments took place at the 17th century. The experiment described by Athanase Kircher consists of the setting in contact of a mercury drop with a money solution (experiment of the Arbre of Diane ). Lémery as for him uses money dissolved by the nitric acid and dilutes its products in water. Dilution slowing down the reaction, the growth of the tree takes 40 days thus. Lémery obtains, with this reaction slower, of the trees more ramified than the other chemists. The branches end in small balls.

These observations of metal vegetations are also reported by Pierre Lorrain, abbot of Vallemon, Curiositez of nature and art on the vegetation: or agriculture, and gardening in their perfection , Paris, Claude Cellier, 1705. In 1706, Lémery produces the Arbre of Mars .

One second observation of importance will be due in 1731 to Charles Marie of Condamine. This last dissolves like the precedent of the metal money by action of the nitric acid lays out the solution in a fine layer. it poses the head of a nail in the center of the solution and sees appearing, in a few hours, a money wire network on the basis of the nail, with horizontal because of the smoothness of the layer of liquid. This particular experimental configuration enables him to study the effect of the slope of the bottom of récipent on the shape of the tree obtained. It produces vegetations for a great number of metal combinations (combination between beforehand dissolved metal and that of the nail).

Modern observations

Nowadays, an experiment of tree of Diane is used to illustrate the courses of chemistry of the Secondary education. It is carried out while plunging a copper wire in a silver nitrate solution. While the money shrub is formed on the basis of the wire, the solution is coloured in blue, color taken by the ions coppers II hexahydratés.

Interpretations

See also: Amorce=Informations complementary, Dendrite (fractale)

The alchemists who were the first to observe the phenomenon compared it with a palingenesis, i.e., in the old direction, with a spontaneous Generation.

The interpretation given by Charles Marie of Condamine (reported in the Encyclopedia ) is that the solvent one (nitric acid) “gives up” metal dissolved to go to dissolve that of the metal nail, when the dissolution of this last is easier. Once the solvent “evaporated”, metal settles. This crystalline form is not, according to Charles Marie of Condamine, the same nature as the completely “released metal pieces of niter” and, by preventing freedom of movement of solvent, would be at the origin of the irregularity of the ramifications.

In more modern terms, interpretation does not call upon a Dissolution but with a reaction of oxydoreduction. In the experiment of the tree of Diane, the metal money is oxidized beforehand by the nitric acid. From this attack the formation results from a silver nitrate solution I. When the iron nail is plunged in the solution, of the ions money I are reduced out of silver to the surface of the nail, where iron metal is oxidized in ions iron II. A reaction of oxydoreduction supposes an exchange of electrons, therefore a conducting medium. The metal money being excellent a Conducting electric, the ions money can precipitate on the surface of silver, while the metal iron of the nail is oxidized in ions iron gradually. Crystalline metal thus grows on the basis of the center and in the shape of a needle.

More recently, the chemists studied the phenomena related to the transport of matter in order to explain the connected shape of tree of the metal vegetations. The models which allow this study are comparable with those used more largely the growth in dendrite S. Ainsi, a model of aggregation limited by the diffusion was proposed. He considers an initial particle to which other particles are incorporated which follow a Brownian Movement. This model makes it possible to produce tree structures which are connected with those observed in the experiments of metal vegetations. This formulation, which does not take account of the physical and chemical phenomena of surface, can be supplemented by phenomenologic approaches, like the models of electrochemical average field.

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