Silver iodide particles

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. 

Silver iodide particles in aqueous suspension are in equilibrium with a saturated solution of which the solubility product, aAg+ai, is about 10 16 at room temperature. With excess 1 ions, the silver iodide particles are negatively charged and with sufficient excess Ag+ ions, they are positively charged. The zero point of charge is not at pAg 8 but is displaced to pAg 5.5 (pi 10.5), because the smaller and more mobile Ag+ ions are held less strongly than-the 1 ions in the silver iodide crystal lattice. The silver and iodide ions are referred to as potential-determining ions, since their concentrations determine the electric potential at the particle surface. Silver iodide sols have been used extensively for testing electric double layer and colloid stability theories. 

In many colloidal systems, the double layer is created by the adsorption of potential-determining ions for example, the potential 0o the surface of a /Silver iodide particle depends on the concentration of silver (and iodide) ions in solution. Addition of inert electrolyte increases k and results in a corresponding increase of surface charge density caused by the adsorption of sufficient potential-determining silver (or iodide) ions to keep 0O approximately constant. In contrast, however, the charge density at an ionogenic surface remains constant on addition of inert electrolyte (provided that the extent of ionisation is unaffected) and 0O decreases. 

Ionic substances can acquire a surface charge by unequal dissolution of the ions of which they are composed. An example is silver iodide particles in an aqueous suspension, which are in equilibrium with a saturated solution. With excess T-ions, the silver iodide particles are negatively charged. With an excess of Ag+-ions, the particles will be positively charged. 

Ottewil et al. (1963) detected the maximum of floatability at the isoelectric point by varying the electrokinetic potential of silver iodide particles by adsorption of a cation-active surfactant. Furthermore, it has been established that the flotation rate is high within a narrow pH range and very low outside this range. In the former case the pH values correspond to very small potentials of the particle, i.e. in the vicinity of their isoelectric point, (Jaycock Ottewil 1963, Rubin Lackay 1968, Devivo Karger 1970). Addition of aluminium hydroxide extends the range of pH values which promotes flotation. 

Schaefer (1966) reported the activation of large numbers of ice nuclei on the addition of trace levels of iodine vapor to car exhaust (containing lead oxide nanoparticles) at temperatures from —3 to —20°C in the laboratory. The formation of lead iodide was concluded to have a seeding effect similar to that of silver iodide particles (Vonnegut, 1947), which had been used in an attempt to artificially modify cloud properties and enhance precipitation. Consequendy this method was proposed as a means to remove harmful aerosol formed in polluted urban areas, and also in artificial weather modification. However, the development of unleaded fuels, for which no similar ice nucleating ability was shown to occur in the presence of iodine (Hogan, 1967), provided a better long-term solution to this problem. 

Adsorption-desorption of lattice ions. Silver iodide particles in Ag" " or solutions are the typical example the crystal lattice ions can easily find their way into crystal sites and become part of the surface. They are called potential-determining ions (p.d.i.). 

Solid-state sensors for chloride, iodide, and fluoride are based on the solubility product of silver chloride or silver iodide particles in silicone rubber and a doped lanthanum fluoride single crystal, respectively. The fluoride-selective electrode was applied for the analysis of urine and bone tissue of people exposed to industrial sources as well as for control of therapeutic fluoride application for osteoporosis, whereas the chloride-selective sensor was applied to the analysis of sweat for the diagnosis of cystic fibrosis. In solid-state contact electrodes the solvent polymeric membrane is directly contacted to the solid field transducing element, although the reference electrode is separated from the ion-selective sensing pad. 

Gorbunov BZ, Kokutkina NA, Kutsenogii KP, Moroz EM (1979) Influence of the sizes of silver-iodide particles on their crystal-structure. Kristallografiya 24 334-337 (in Russian)... 

Graphon) [43]. Similar isotherms have been reported by Abe and Kuno [44,45] for the adsorption of polyoxyethylated nonylphenols onto carbon and calcium carbonate and by Elworthy and Guthrie [46] for the adsorption of a series of polyoxyethylene non-ionics onto the water-insoluble, antifungal antibiotic, griseofulvin. Other authors have examined the adsorption of polyoxyethylene ethers on paraffin wax [47] and silver iodide particles [48,49]. In general, the plateau region commences at a concentration at or near the CMC. 

Polymer adsorption is important in the flocculation and stabilization of colloidal sols and has been reviewed by Vincent et al. (1) and Tadros (2). Polyvinyl alcohol (PVA) has been used in these studies because of its practical application in textiles, adhesives, and coatings. The adsorption of PVA has been studied on silver iodide by Fleer (3) and Koopal (4), and on polystyrene (PS) latex particles by Garvey (5). The adsorption isotherms reported by these workers extend up to 600 ppm PVA. The adsorption at... 

Horak D, Cervinka M, Puza V (1998). Radiopaque poly(2-hydroxyethyl methacrylate) particles containing silver iodide complexes tested on cell culture. Biomaterials 19(14) 1303-7. 

D. A. Davies [7] suggested the use of explosive charges for rain-making. The charges, consisting of 15 g of blackpowder plus 1.5% of silver iodide, are sent by balloon into a cloud, where they are exploded by a time fuse. The particles of silver iodide thus released act as nuclei on which the water vapour in a raincloud coagulates, to fall as rain drops. 

The easiest model to treat theoretically is the sphere, and many colloidal systems do, in fact, contain spherical or nearly spherical particles. Emulsions, latexes, liquid aerosols, etc., contain spherical particles. Certain protein molecules are approximately spherical. The crystallite particles in dispersions such as gold and silver iodide sols are sufficiently symmetrical to behave like spheres. 

Silver iodide sol. Mix equal volumes of aqueous solutions (10 3 to 10 2 mol dm-3) of silver nitrate and potassium iodide. Separate the sol from larger particles by decantation or filtration. By arranging for the silver nitrate or the potassium iodide to be in very slight excess, positively or negatively charged particles, respectively, of silver iodide can be formed. 

To determine the concentration of 131I in drinking water by purification sequentially as silver iodide and palladium iodide, followed by counting beta particles. 

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