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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />7 <br /> <br />iodide [Hal] to the pointt~~~~~~~~ silver iodide is no longer soluble <br />---""'.....,.".,~- -""'-.'- - - ----.~- <br />__.__._n _ <br /> <br />and precipitates out' (Vonnegut. 1957). It is thus the solid <br /> <br />-~""'-~---.-.__. <br /> <br />precipitate that serves as the ice nucleus in the pre-to~d droplet, a <br /> <br />process known as conden~~!~~~:~reez~~. or immersion ioe nucleation. <br />~,",,,,-,.,-,~--...-----~-" _.-- <br /> <br />Later work by Tompkins. et al.. (1963). Burkardt. et al,. (1968), and <br />Chang (1984) supports this proposed mechanism. <br /> <br />The AgI-HaI aerosol was distiguished from aerosol produced by <br /> <br />combustion of acetone-AgI-ammonium iodide (BH4I) solutions by Vonnegut <br />(1949. 1957) where ~t vas c:~.:rectly I!~~__~~that ~.H..I is ooapletely <br /> <br />decomposed at high temperatures and so essentially pure AsI particles <br /> <br />p___~_.-'_.C,"-."""~-"C~'-~__n"""'-" . <br /> <br />are produced. This was later oontiraed by St.-Aaand. et ale (1911) and <br /> <br />_____.~,.""'-"~~-~ ._~~..~_._'._~_f~_~>, ~__ <br /> <br />DeMott. et ale (1983). <br /> <br />2.1.2 Chemical and Physical Properties of AgI-Bal Aerosol Particles <br /> <br />The solubility of AgI in either vater is low (eg. <br />-9 <br />3x10 g AgI!g H20). By adding an alkali iodide to the system. such as <br /> <br />BaI or II (potassium iodide). both ot which dissolve readily in vater or <br /> <br />acetone. the solubility of the AgI aDDearS to increase. For instance. <br /> <br />the amount of AgI which will dissolve in vater increases three orders of <br /> <br />magnitude when 0.01 g BaI per gram of vater is added (Davis. 1972). <br /> <br />This is not due to the common ion effect. as is otten supposed. The <br /> <br />common ion effect causes precipitation of the least soluble compound in <br /> <br />solution. Solubility is an equilibrium condition between a solid <br /> <br />compound and its ionic components (eg. <br /> <br />~ + - <br />AgI(s) -tAg + I ) Addition of <br /> <br />the ion in common. 1- ions from Hal in this case. would increase the I <br /> <br />concentration and cause a shift in the equilibrium away trom dissolution <br /> <br />and toward production of solid AgI precipitate (a shitt to the left in <br />+ - <br />the above equation). Instead. the Ag and I complexes with the alkali <br />