Iodide silver

Silver iodide (SILL-ver EYE-oh-dide) is a light yellow crystalline or powdery material that darkens on exposure to light. The darkening occurs because silver ions (Ag+ silver atoms with a positive charge) are converted to neutral silver atoms (Ag°) that are dark gray in color. Silver iodide is used primarily in photography and in cloud-seeding experiments. 

Silver iodide. Turqouise atom is sliver and yellow atom is iodine, p l i s e r s resource 

The other major use for silver iodide is in cloud seeding. Cloud seeding is the process by which some foreign material—usually silver iodide or dry ice (solid carbon dioxide)—is dropped into a rain cloud. The crystals of silver iodide or carbon dioxide provide nuclei—tiny cores—on which water can condense to form water droplets. The process of cloud seeding was first developed in the 1940s by American chemist Vincent Schaefer (1927-1993). Schaefer used crushed 

Some concerns have been expressed about the environmental and health effects of using silver iodide for cloud seeding. However, only small amounts of silver iodide are released into the atmosphere. That which does fall to earth does not dissolve in water and so is unlikely to enter a community water supply. Tests have shown that the concen- 

Fukuta, Norihiko. Cloud Seeding Clears the Air. PhyAicA in Action (May 1998). Also available online at http //physicsweb.0rg/articles/world/11/5/3/1 (accessed on November 3, 2005). 

If Agl or AgCl is submerged in water, a certain number of molecules dissolve (review Ref. [84]). An equilibrium between the ions in solution and the crystal is established  

How is the surface potential quantitatively related to the concentration of the potential determining ions In equilibrium, the electrochemical potential of Ag+ ions at the crystal surface is equal to that in solution  

n°c Ag+)and p°L(Ag+) are the chemical standard potentials at the crystal surface and in solution, respectively, tpc and are the Galvani potentials in the crystal and in solution. In particular, Eq. (5.12) is valid at the point of zero charge  

A pzc is the difference of the Galvani potentials, which is caused solely by dipoles in the interface, not by free charges. If we subtract the two equations from each other we get 

It is assumed that ac Ag+) = aPQC(Ag+ ). The expression in brackets is the surface potential ipo- Thus we obtain the Nernst equation  

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The f potential of silver iodide can be varied over the range 75 mV, by varying the Ag or 1 concentration again demonstrating that varying the concentration of potential-determining ions can reverse the sign of the f potential. 

Reerink H and Overbeek J Th G 1954 The rate of coagulation as a measure of the stability of silver iodide sols Discuss. Faraday Soc. 18 74-84... 

Taking francium as an example, it was assumed that the minute traces of francium ion Fr could be separated from other ions in solution by co-precipitation with insoluble caesium chlorate (VII) (perchlorate) because francium lies next to caesium in Group lA. This assumption proved to be correct and francium was separated by this method. Similarly, separation of astatine as the astatide ion At was achieved by co-precipitation on silver iodide because silver astatide AgAt was also expected to be insoluble. 

Iodine as such finds few uses but a solution in alcohol and water, also containing potassium iodide ( tincture of iodine was commonly used as an antiseptic for cuts and wounds, but had rather an irritant action. Iodoform (triiodomethane), CHI3, is also an antiseptic, but newer compounds of iodine are now in use. Silver iodide, like silver bromide, is extensively used in the photographic industry. 

Addition of silver nitrate to a solution of an iodide in dilute nitric acid, yields a yellow precipitate of silver iodide practically insoluble in ammonia. 

The halide anion of quaternary ammonium iodides may be replaced by hydroxide by treatment with an aqueous slurry of silver oxide Silver iodide precipitates and a solu tion of the quaternary ammonium hydroxide is formed... 

Quaternary ammonium iodide Silver oxide Water Quaternary ammonium hydroxide Silver iodide... 

Iceland spar, see Calcium carbonate lodyrite, see Silver iodide... 

Photography. Photography (qv) represents one of the oldest industrial uses of iodide. The sensitive silver salt in rapid negative emulsions contains up to 7% or mote silver iodide [7783-96-2], Agl. Erom 1969 to 1985 estimates on iodine consumption for this purpose varied from 150 to 270 t/yr (66). Ttiphenylphosphonium iodide is also among the iodine derivatives used in photography. This derivative permits faster development and higher contrast photography. 

Silver Iodide. Silver iodide, Agl, precipitates as a yellow soHd when iodide ion is added to a solution of silver nitrate. It dissolves in the presence of excess iodide ion, forming an Agl2 complex however, silver iodide is only slightly soluble in ammonia and dissolves slowly in thiosulfate and cyanide solutions. 

Silver iodide exists in one of three crystal stmetures depending on the temperature, a phenomenon frequently referred to as trimorphism. Below 137°C, silver iodide is in the cold cubic, or y-form at 137—145.8°C, it exists in the green-yeUow colored hexagonal, or P-form above 145.8°C, the yellow cubic or a-form of silver iodide is the stable crystal stmcture. Silver iodide decomposes into its elements at 552°C. 

Although silver iodide is the least photosensitive of the three halides, it has the broadest wavelength sensitivity in the visible spectmm. This feature makes silver iodide particularly useful in the photographic industry. It resists reduction by metals, but is reduced quantitatively by zinc and iron in the presence of sulfuric acid. 

Cloud Seeding. In 1947, it was demonstrated that silver iodide could initiate ice crystal formation because, in the [ -crystalline form, it is isomorphic with ice crystals. As a result, cloud seeding with silver iodide has been used in weather modifications attempts such as increases and decreases in precipitation (rain or snow) and the dissipation of fog. Optimum conditions for cloud seeding are present when precipitation is possible but the nuclei for the crystalliza tion of water are lacking. 

Recovery Process. In past years iodine was recovered at Long Beach, California from oil field brine and from natural brines near Shreveport, Louisiana (36,37). The silver process was used. Silver nitrate reacts with sodium iodide to precipitate silver iodide. Added iron forms ferrous iodide and free silver. The ferrous iodide then reacts with chlorine gas to release free iodine. After 1966, the silver process was replaced with the blowing-out process similar to the bromine process. 

Fig. 9.2. The excellent crystallographic matching between silver iodide and ice makes silver iodide a very potent nucleating agent for ice crystals. When clouds at sub-zero temperatures are seeded with Agl dust, spectacular rainfall occurs.

The crystal structure of ice is hexagonal, with lattice constants of a = 0.452 nm and c = 0.736 nm. The inorganic compound silver iodide also has a hexagonal structure, with lattice constants (a = 0.458 nm, c = 0.749 nm) that are almost identical to those of ice. So if you put a crystal of silver iodide into supercooled water, it is almost as good as putting in a crystal of ice more ice can grow on it easily, at a low undercooling (Fig. 9.2). 

Pollution can cause opposite effects in relahon to precipitation. Addition of a few particles that act as ice nuclei can cause ice particles to grow at the expense of supercooled water droplets, producing particles large enough to fall as precipitation. An example of this is commercial cloud seeding with silver iodide particles released from aircraft to induce rain. If too many particles are added, none of them grow sufficiently to cause precipitation. Therefore, the effects of pollution on precipitation are complex. 

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