Iodine, thermodynamic data

Consider the following thermodynamic data, which concern the sublimation of iodine ... 

Collections of fundamental and thermodynamic data can be found in an earlier review [158] and in standard resources [13, 14]. However, due to the reactivity of iodine there are many less common or more reactive forms of iodine that have been less well characterized. For example, a blue 12 cation, a brown I3+, or a green I5+ cation are formed in concentrated sulfuric acid and 1+ is stabilized in donor environments such as pyridine [159]. So-called hypervalent iodine reagents have been developed as a versatile oxidation tool in organic synthesis and often iodine derivatives are employed as electron transfer catalysts. Some fundamental thermodynamic data and typical applications of iodine are summarized in Scheme 5. 

Less is known about the anhydrous bromine-containing oxysalts than about the corresponding compounds of chlorine and iodine. This is true both with respect to the total number of such salts, as well as the information available on each salt. Since 1974, when the last review of these compounds was published,1 the situation has changed only slightly, with the number of known bromites increasing from 2 to 3, bromates from 15 to 17, and perbromates from 3 to 8. There are still no thermodynamic data above 298 K. Consequently, this chapter is fairly brief, includes no high-temperature equilibrium calculations, and updates qualitative and semi-quantitative information using material published since 1972. 

Thermochemical cycles, and in particular the Sulphur-Iodine cycle, have been the subject of renewed intense interest in the last years. The accurate evaluation of their industrial potential is difficult, as it involves many aspects, from scientific questions such as the knowledge of thermodynamic data to safety, acceptability and economic assessments. 

Thermodynamic data for U and its adsorption reactions have already been discussed. Among other important radioisotopes, the behavior of iodine in is relatively simple and well understood. Iodine occurs as iodate (lOj) in highly oxidized waters and iodide (I ) under more reducing conditions, including in most groundwaters (cf. Pourbaix 1966). At 25°C and below pH = 5, the redox boundary defining equal concentrations of these species for = 10 M is given by... 

Uruska et al. [Ur 79a, b, 80] have determined the formation constants of DA molecular complexes (e.g. pyridine and pyridine derivates, with iodine) in a series of organic solvents using spectrometric and calorimetric measurements. The equilibrium constants and other thermodynamic data were found to correlate with solvent properties. 

Carlson, R.L. and Drago, R.S. (1963) Thermodynamic data for the formation of molecular complexes between phenyl-substituted amides and iodine. J. Am. Chem. Soc., 85,505-508. 

Giera, J., Sobczyk, L., Lux, R and Paetzold, R. (1980)Correlation between the dipole moments and thermodynamical data of iodine complexes with organic oxides, sulfides, and selenides. 7. Phys. Chem., 84, 2602-2605. 

The release of radioactive iodines from BWR circuits, first into the steam phase and then into the turbine hall, has also been considered thermodynamically (75). A re-analysis of some experimental data of Styrikovich et al (97), suggested that iodates were not, as had been tentatively proposed, likely to be present. Styrikovich s prediction of HIO as a principal species under BWR conditions was confirmed, but it was concluded that his experiments had not measured its steam/water partition coefficient. In view of the meagre experimental evidence, however, more work on this system is desirable. 

The data for sulphur, bromine and iodine compounds in Table 20.1 are different from the data in the original paper. In Table 20.1 the elements in their stable form at 25 °C and 1 bar are taken as reference states, as is usual in thermodynamics in the original publication S2(g), Br2(g) and 12(g) were taken as reference state. 

Other experimental and theoretical methods have been developed for the determination of the heat of sublimation of solid iodine these too are suitable for undergraduate laboratory experiments or variations on this experiment. Henderson and Robarts have employed a photometer incorporating a He-Ne gas laser, the beam from which (attenuated by a CUSO4 solution) has a wavelength of 632.8 nm, in a hot band near the long-wavelength toe of the absorption band shown in Fig. 3. Stafford has proposed a thermodynamic treatment in which a free-energy function ifef), related to entropy, is used in calculations based on the third law of thermodynamics. In this method either heat capacity data or spectroscopic data are used, and as in the present statistical mechanical treatment, the heat of sublimation can be obtained from a measurement of the vapor pressure at only one temperature. 

Seawater Iodine Data. Iodine is a bioactive element that exists predominantly as I" and I03" in seawater, with a total iodine concentration of about 470 nM at 35%. Thermodynamically, in fully oxygenated seawater (pH 8.05, pe 12.5), iodine should exist entirely as IO, (1, 2, 7). However, I" concentrations in surface seawater reach 50-150 nM through biological processing of I03 (2, 8) (Figure 1A). 

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