Iodine, dissociation atmosphere

We can, at this point, consider two well-known examples of dissociation of compounds At a pressure of one atmosphere, mercury forms an oxide below 300°C, but above this temperature the compound decomposes, while at lower oxygen pressures the decomposition temperature is lowered. Iodine atoms form iodine molecules at low temperatures, while at high temperatures the... 

Chemical Properties.—When heated above 100° C. arsenic triiodide dissociates slowly into its elements above its melting point this decomposition becomes more rapid.11 In air, the products are arsenic, arsenious oxide and iodine, and the action proceeds slowly even below 100° C. and is rapid at 200° C. at higher temperatures the triiodide burns with a pale blue flame.12 Heated in an atmosphere of nitrogen in... 

Solid iodine monochloride exists in two forms. The brownish-red tablets of the j8 form (m. 13.9°C.) are labile and pass readily into the ruby-red needles of the a form (m. 27.19°C.). Since iodine monochloride dissociates on boiling at atmospheric pressure, its boiling point is not definite. The literature gives values from 94.7° to 102°C. Calculations from vapor-pressure data give 33.4 cal. as the value of the entropy of vaporization at a vapor concentration of 0.00507 mol per liter. Iodine monochloride must therefore be an associated or polar liquid. 

It is believed that CH3I is produced by marine algae and released from seawater, and that this constitutes the main source of stable iodine in the atmosphere (Lovelock et al., 1973). Elemental iodine may be liberated from the sea surface by ultraviolet light (Miyake Tsunogai, 1963), or by the action of ozone (Garland Curtis, 1981), but I2 is dissociated very rapidly by photochemical action, and its mean residence time in daylight air is less than a minute. 

The vapor pressures of lead and polonium iodides have been investigated. The metals were heated in an atmosphere of iodine in a closed system. The pressure of the products was determined by a statistical method. At less than 80 atomic percent iodine, P0I4 forms in the condensed phase. At 473°K P0I4 dissociates into P0I2. Above 80 atomic percent iodine, the condensed phase exhibits the presence of Pole. The enthalpy of evaporation of Pole is 116kj mole. These experiments suggest that the separation of polonium from lead can be accomplished by their volatization in iodine vapors at elevated temperatures. 

All of these gases are subject to dissociation in the atmosphere, primarily via photolysis, with I2 having the shortest lifetime and therefore being the dominant source of reactive iodine, particularly in coastal locations. 

The chemical forms of gaseous iodine in the containment atmosphere that are of interest are molecular iodine and organic iodides whether these species reached the containment as a result of iodine release from the fuel, partitioning of volatile iodine species from water or as a result of combustion processes. Another volatile form of iodine, HI, is very hygroscopic and will rapidly dissociate in contact with water to form solvated T. There can be some conversion of molecular iodine, hig), into organic iodides, RI, in the radiation field of the containment atmosphere ... 

Detailed investigations of the reaction of Csl with boric acid in the condensed phase over the temperature range 400-1000 C under Ar, Ar-H2, and Ar—Ha-steam atmospheres were performed by Bowsher and Nichols (1985). The results showed that Csl decomposition in these reactions starts at temperatures above 400 °C and increases considerably beyond 700 °C, with the HI produced being partly converted to I2 (and/or iodine atoms) by thermal dissociation. Under such conditions, HI as well as I2 may react with the iron and nickel content of stainless steels under formation of the corresponding iodides (see below). Up to 960 °C, Csl and molten boric acid or boron oxide react in a diffusion-controlled reaction the rate of which is determined by the diffusion of the partners to the reaction zone. The reaction data measured in these experiments were consistent with Arrhenius law, showing an activation energy of 190 30kJ/mol. 

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