How does silver cause a chemical change?
PhotochemistryThe doctrine of the chemical effects of light developed hand in hand with physical chemistry, but did not take off until a new technique, photography, and numerous other processes connected with it, arose out of it.
Light is a wave movement of the ether, a special form of radiant energy, to which rays of electrical energy, heat rays, etc. be expected. - Since light is a kind of energy, the principle of the conservation of energy also applies to it, "that energy can change according to form, but not according to quantity". The light energy can therefore be converted into other forms of energy, e.g. when irradiating soot into heat, whereby light is absorbed. - However, light can sometimes also influence chemical processes, whereby light is also absorbed and used to initiate, accelerate or carry out a chemical process.
The thermal and photochemical effects of light run side by side and can be kept apart, e.g. will Fehlingcal solution (alkali copper tartrate solution) influenced by the spectrum in such a way that chemical reduction to copper oxide is brought about in the ultraviolet part, but heat effects in the red part. On the other hand, cyanine, a blue quinoline dye which is not lightfast, is vigorously bleached in the red and yellow parts of the spectrum where it absorbs light, and thermal absorption and photochemical effects fall on the same spectral region.
The absorption of light and its conversion into other forms of energy therefore depend on the wavelength of the light. - Light can only act on a body when it is absorbed by it. That gave first Th. V. Grotthus on and proved Draper for silver salts 1841, J. Herschel for petals that bleach in the light. These are therefore sensitive to colored rays, which are complementary to their own color, because they are absorbed by the colored flowers. - H.W. bird proved that not only the absorption of the light-sensitive substances themselves, but also the absorption of added substances, namely dyes, play an important role. Silver bromide, otherwise only sensitive to blue, violet and ultraviolet, and little to green, yellow and red, becomes sensitive to these rays through coloring with red, yellow and green absorbing dyes. It is now possible to make silver bromide (including chlorine silver) sensitive to any color that is otherwise ineffective on silver bromide. This is what orthochromatic photography is based on (Orthochromasia, s.d.). This also applies to other light-sensitive bodies birds principle plays a role.
In most photographic processes, light has an accelerating effect on such chemical reactions, which occur by themselves in a much longer period of time even without the effect of light (e.g. with chromate gelatine, ferric oxalate). Ostwald compares this type of light effect with catalysis, in which the speed of a reaction is influenced (increased) by the presence of a substance that does not take part in the reaction itself.
That which has been proven for the visible optically bright rays Law of decrease in light intensity with the square of the distance from the light source applies also to the chemically active rays. This sentence is from Bunsen and Roscoe for the chlorine gas photometer and more recently by Eder also for Silver bromide gelatin has been proven experimentally; also for the decomposition of carbonic acid by green plants under the influence of light is this »Distance law«Has been found approximately valid (Volkov and V. Tieghem). - If, for example, a certain area is five times the distance away from a light source when the light is incident perpendicularly, the light only acts on it with the 25th part of its original intensity. In the latter case, photographic processes will have to expose 25 times longer in order to achieve approximately the same photographic effect; however, there are deviations from this simple rule (Reciprocity rule) instead of; Even if the chemically active rays of light obey the law of distance and their intensity decreases with the square of the distance, the chemical effect of the relatively poorly bright rays often falls noticeably behind that which one would expect proportionally to the decreasing light intensity.
Almost all substances seem to be more or less sensitive to light, partly on their own, partly in contact with others (see also Sensitizers). Even the glass is highly sensitive to light, especially glasses containing manganese, which turn violet in the light (also under the influence of radium rays). - In numerous cases, the light brings about a chemical change that is immediately perceptible to the eye (blackening of silver salts, browning of bichromates on paper in the light, etc.), which one then does direct lighting effect is called; those created in this way photographic images is called direct photographs (direct photographic copying process). The visible change in the light-sensitive substance that occurs here is to be determined quantitatively by chemical analysis (transition from white chlorine silver to dark silver subchloride, reduction of the chromates to brown chromium dioxide, etc.). - Certain silver compounds (AgJ, AgBr, AgCl) and mercury compounds do not change in any directly visible way on very short exposure, even though they have been minimally chemically or physically changed. These effects of light are so small that they cannot be detected by quantitative chemical analysis; but it is a hidden light (latent light image) present, which is expressed in particular by the increased reducibility of the exposed silver compound against reducing agents. These reducing agents (iron salt, pyrogallol, hydroquinone, etc.) must be chosen so that they do not reduce the unexposed silver salt, but the exposed one strongly to metallic silver; then the latent light image  become clearly visible (»evoked«Or»developed«). The "development" of the latent light images can also take place through the action of vapors, which condense more strongly on the exposed areas than on the unexposed areas, whereupon the principle of Daguerreotype (Development with mercury vapor) is based. The development processes are of the utmost importance for applied photography.
Light can affect different types of chemical reactions. - It can allotropic Cause changes, e.g. permanent modification changes from yellow to red phosphorus, from sulfur, selenium, oxygen, etc. - The electrical conductivity of selenium is increased during exposure. This plays a big role in light telegraphy. Only certain modifications of selenium are sensitive to light. If the exposure stops, the old large conductive resistance is restored by itself. Temporary dissociation or ionization of the selenium molecules is likely to occur. - While very complicated processes occur when exposing selenium, some of which have not yet been fully established, this is easier with oxygen. - Ozone is created from oxygen not only by electrical discharge, but also by ultraviolet light, being a certain state of equilibrium between O2 and O3 entry. - If humid atmospheric air is penetrated by ultraviolet sunlight, ozone and so-called fog cores are formed, where free gas ions are likely to arise, which have become important to explain the formation of meteorological fog. Here the energy content of the oxygen is increased and the number of atoms in the molecule is increased from two to three, so there is, so to speak, one Polymerization instead of. The Photopolymerization has become particularly important for organic chemistry, for example cinnamic acid, styrene, many hydrocarbons are polymerized in the light, a solution of anthracene in anisole (boiling point 152 ° C), becomes cloudy in the light with the excretion of insoluble dianthracene:
A state of equilibrium gradually occurs, the Luther carefully examined. After the exposure has stopped, anthracene will gradually regress. The resulting photochemical equilibrium is different from the dark equilibrium. - So that the dark equilibrium remains unchanged, no constant expenditure of energy is required, provided that all external energy losses are avoided. It is different with photochemical equilibria. Here light has to shine continuously so that the state does not suffer any change. As soon as we inhibit the entry of light, our mixture gradually unstoppably returns to its original state. The maintenance of such a permanent state is causally linked to a permanent consumption of the incident light energy.
As Photoisomerization gives Ciamician the conversion of nitrobenzaldehyde into nitrosobenzoic acid at:
Such photoisomerizations are in greater numbers by the studies of Ciamician and silver among other things has been announced.
The light often effects reduction and oxidation. Potassium dichromate on paper or mixed with glue gives chromium dioxide when exposed to light, as there is a tendency that chromic acid is reduced to chromium dioxide or chromium chromate (K2Cr2O7= K2CrO4 + CrO2 + O), which is brown and tans gelatin, but gradually releases chromic acid when washed with water and forms basic chromium chromates. This process also takes place in the dark, but it takes months or weeks. - The light accelerates the process, as it were as a catalyst.
Lots of photochemical reactions are there Ion reactions; e.g. the decomposition of aqueous hydriodic acid, chlorine water, etc. in the light. Chlorinated water gives hydrochloric acid and oxygen as the main decomposition products:
or, written as an ion equation,
Under the influence of light there is a transfer of electrical charges in electrolytically dissociated bodies. - The great light sensitivity of the mixtures of ferric or mercuric salts with oxalic acid also belongs here. Mixtures of concentrated solutions of mercury chloride and oxalic acid give exposed precipitations of mercury chloride, while the oxalic acid is oxidized to carbonic acid. The photosensitivity of the ferrioxalates, which in the blueprint (s. Blueprints) and in Platinum print (s.d.) one, as important as it plays, is also one of this kind. - Furthermore, in many cases, light accelerates the oxidation of fats, resins, essential oils, base metals, etc. The oxygen becomes "active" in the presence of certain foreign substances, i.e. more reactive and has more oxidizing properties. - The increased oxidizing effect of oxygen under the influence of light is demonstrably linked in many cases to the presence of water; e.g. give In the presence of oxygen, ether and water abundant hydrogen peroxide, yes even pure wateracidified with a little sulfuric acid, forms some hydrogen peroxide in the presence of air in sunlight (Richardson). The formation of hydrogen peroxide in photochemical  oxidation processes seems to play a role as primary or secondary oxygen transfer, such as photo-oxidation in the disinfection of open waters by the hydrogen peroxide formed in light.
Also noteworthy is the oxidation of the leuco bases of dyes (Great, king) by exposure: if aqueous rhodamine solution is reduced with acetic acid and zinc dust to a colorless leuco base, shaken out with ether, the ethereal solution mixed with collodion and applied to paper and this is now exposed to electric arc light, the colorless leuco base is quickly reddened. It has complementary light. It is noteworthy that in the oldest photochemical reaction known in history, namely in the formation of the purple dye from the yellowish sap of the purple snail, light plays a similar role, in that the formation of the purple dye is favored by light.
The asphalt is changed in thin layers by oxidation in the light so that it loses its solubility in turpentine oil (use in photozinkotype and photolithography, see p. Asphalt photography).
Wood pulp tans in the light; in this way you can produce photographs on ground wood lids or ordinary writing paper (brown on a light background); here, violet and ultraviolet light have a particularly strong effect.
A 2 percent Iodoform-chloroform solution (Hardy & Willcockcal solution) excretes iodine in the light when oxygen is admitted; it reacts quite analogously to light, heat, X-rays and Becquerel rays. This reaction is from Leopold friend used in Vienna as a chemical measure for the intensity of X-ray radiation in X-ray therapy. In this process, photo-oxidation causes a dark color; Much more common than this, however, and the restitution of colors (see above, leuco bases) are fading phenomena (in the case of non-light-resistant dyes, they lead to Fading pictures [Neuhauß, Szczepanik, Worel] can be used photographically). Photochromies on silver chloride (paper images) are created according to the principle of the bleaching process (Wiener), while LippmannsPhotochromism comes about through interference phenomena (pseudo colors); s. photograph.
Let us now turn to the known photographic process, namely to the photochemical blackening process of chlorine silver, silver subchloride and, in the further course, even metallic silver being formed. The same is a photochemical cleavage, in which case the light counteracts the molecular forces:
Heat alone cannot bring about this process. The splitting off of chlorine from chlorine silver is carried out experimentally with electric arc light and verified using iodine potassium starch paper. Moist chlorine silver is placed in a larger powder jar and a potassium iodine starch paper dipped into the interior of the jar is attached between the stopper and the neck of the powder jar. When exposed to electrical light, the potassium iodine starch paper turns blue in a short time due to the chlorine released. Moist chlorine silver blackens faster than dry because water acts as a sensitizer. If the silver chlorine is exposed to light in closed tubes, chlorine will accumulate in the tube. As soon as a certain pressure is reached, the blackening process does not proceed any further (equilibrium state, which depends on the pressure of the chlorine and the intensity of the light). If the released chlorine were removed, the blackening would be promoted in the light. If silver chlorine is sealed in tubes with chlorine gas, weak light at atmospheric pressure cannot blacken it, but sunlight can, even if weakly; if the strong exposure stops, the silver chlorine becomes white again.
Bromide silver is different in sensitivity in different modifications. Compounds of granular silver bromide, which is in and of itself highly sensitive, with gelatin are extremely sensitive to light if they are subsequently caused. In the bromide-silver gelatine we have a sensitive reagent for light energy (0–02 S.M.K.). It does not blacken as intensely in the light as chlorine silver, but with a short exposure it produces what is known as a latent light image; it is said that the bromide silver becomes "active" against developers. The nature of the silver bromide that has become "active" through light or the latent silver bromide image is difficult to research because silver bromide reacts to various stimuli just as it does to light (analogy with iodoform), e.g. to silent electrical discharges, to mechanical pressure, to contact with wood , Metals (Photechie). The blackening caused by wood is said to be accompanied by the oxidation of resin or essential oils and the occlusion of the hydrogen peroxide produced in this way (Russell), which the latter, even in a dilution of well under a millionth percent, makes the bromide silver active against developers, respectively. originate from ozone, which under certain circumstances forms in the light (Blaas & Czermak). However, it cannot be ruled out that, as other researchers assume, some kind of radiation is present. So there are many stimuli to which the silver bromide gelatine reacts through increased reducibility in so-called photographic developers (strong reducing agents). In normal photographic latent light, it is very likely that a subbromide of silver is formed, i.e. the chemical change in silver bromide by splitting off small amounts of bromine. A simple fundamental experiment makes it possible to prove that bromide silver has undergone a chemical change in the latent light image.While unchanged silver bromide is simply dissolved (fixed) by fixing soda (sodium thiosulphate) and no viable residue remains, the latent light image resists fixing. As is well known, one can expose a bromide silver plate, then fix it, after which a glass blank layer remains with no visible trace of the image, and then develop it with Metol silver nitrate citric acid solution. So it is the image substance, the latent light image chemically modified bromide silver. Probably it is silver subbromide,  because it withstands the action of nitric acid much better than silver metal, but perhaps also a solid solution of silver in bromide silver. Silver bromide is primarily sensitive to blue, purple, and ultraviolet rays. Certain dyes sensitize the bromide silver to red, yellow and green. The effect of color sensitization is perceived as an optical resonance phenomenon which is transmitted from the light to the absorbing dye and from this to the bromide silver. Dyes, which by virtue of their extensive light absorption sensitize the silver bromide to almost the entire visible spectrum, are used for the production of panchromatic plates, e.g. isocyanines (ethyl red from Miethe, Pinachrome and pinacyanol from king). Such plates have gained enormous importance for orthochromatic photography and three-color printing.
Let us now draw some conclusions from the fields of photochemistry currently being explored: In photochemical processes, as in other chemical processes, there are complete and incomplete and reversible reactions, isomerization and polymerization. The reactions usually follow the laws of chemical mass action. Light of any type of radiation can influence chemical reactions, depending on the nature of the exposed body. Like other chemical reactions, photochemical processes can be endothermic, with the binding of heat, or exothermic, with the release of heat. The most important endothermic light reaction is the assimilation process of the carbonic acid of the air by the plants, whereby it is split into carbon and oxygen. The radiant energy of the sunlight is stored in the plants as they grow and released as heat when the wood is burned, for example. The blackening of the silver chlorine in the light is also endothermic, i.e. the light energy does chemical work and stores bound energy in the blackened silver chlorine. On the other hand, the combination of chlorine gas with hydrogen is exothermic in light, in that 22,000 calories are released, which leads to explosive phenomena. Even though light energy has been supplied and part of it has been used to carry out the process, the energy content of the hydrochloric acid formed is ultimately less than that of the individual components at the beginning of the process. Light often has a similar effect to heat in chemical processes, but in most cases the detailed course of the light reactions (the mechanism of the reaction) is different from that in the dark. The reaction speed and the chemical equilibrium are usually shifted during exposure. In numerous cases, the light forces various chemical processes with ease, which cannot be carried out or can only be carried out with difficulty if the light is excluded.
Photochemistry, the study of the chemical effects of light, has not only become of great economic and cultural importance through photography and the photomechanical reproduction processes, but through the chemical effects of light, inexhaustible sources of energy are also opened up in organic nature. which come from space to earth.
Literature:  Eder, Photochemie, 3rd ed., Halle 1906. -  Eder-Valenta, contributions to photochemistry and spectral analysis, Vienna and Halle 1904. -  Vogel, HW, photography of colored objects, Berlin 1885 . -  Vogel-König, Handbuch der Photographie, Vol. 1, 5th edition, Berlin 1907. -  Eder, The chemical effects of colored light, Vienna 1879. -  Namias, Theoretisch-Praktisches Handbook of photographic chemistry, Vol. 1, Halle 1907. -  Foam, photochemistry and photography, Part 1, Leipzig 1908. -  Lüppo-Cramer, Scientific work in the field of photography, Halle 1902. - [ 9] Lüppo-Cramer, Photographic Problems, Halle 1907. -  Luther, The chemical processes in photography, Halle 1899. -  Valenta, Photographische Chemie, 1st and 2nd part, Halle 1898/99.
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