Polyiodide

Polyiodides are specific polyhalides containing species such as I3", I7", Iq". Polyhalides often dissociate on heating the solid complexes. 

Iodine is only slightly soluble in water and no hydrates form upon dissolution. The solubiHty increases with temperature, as shown in Table 2 (36). Iodine is soluble in aqueous iodide solutions owing to the formation of polyiodide ions. For example, an equiHbrium solution of soHd iodine and KI H2O at 25°C is highly concentrated and contains 67.8% iodine, 25.6% potassium iodide, and 6.6% water. However, if large cations such as cesium, substituted ammonium, and iodonium are present, the increased solubiHty may be limited, owing to precipitation of sparingly soluble polyiodides. Iodine is also more... 

Solutions of alkah metal and ammonium iodides in Hquid iodine are good conductors of electricity, comparable to fused salts and aqueous solutions of strong acids. The Hquid is therefore a polar solvent of considerable ionising power, whereas its own electrical conductivity suggests that it is appreciably ionized, probably into I" and I (triodide). Iodine resembles water in this respect. The metal iodides and polyiodides are bases, whereas the iodine haHdes are acids. 

Poly(imides) — see 3-Pyrroline-2,5-diones Poly(imidines) — see 3-Pyrrolin-2-ones Polyiodides... 

Compounds of Tl have many similarities to those of the alkali metals TIOH is very soluble and is a strong base TI2CO3 is also soluble and resembles the corresponding Na and K compounds Tl forms colourless, well-crystallized salts of many oxoacids, and these tend to be anhydrous like those of the similarly sized Rb and Cs Tl salts of weak acids have a basic reaction in aqueous solution as a result of hydrolysis Tl forms polysulfldes (e.g. TI2S3) and polyiodides, etc. In other respects Tl resembles the more highly polarizing ion Ag+, e.g. in the colour and insolubility of its chromate, sulfide, arsenate and halides (except F), though it does not form ammine complexes in aqueous solution and its azide is not explosive. 

The newest process to be developed oxidizes the brine with CI2 and then treats the solution with an ion-exchange resin the iodine is adsorbed in the form of polyiodide which can be eluted with alkali followed by NaCl to regenerate the column. About 65% of the iodine consumed in the world comes from brines. 

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

Gold(III) iodide has not been definitely characterized in the solid state substances with this formula in the solid state are probably gold(I) polyiodides Au+If AuI3 has also been detected in the gas phase (mass spectra). 

In screening electrolyte redox systems for use in PEC the primary factor is redox kinetics, provided the thermodynamics is not prohibitive, while consideration of properties such as toxicity and optical transparency is important. Facile redox kinetics provided by fast one-electron outer-sphere redox systems might be well suited to regenerative applications and this is indeed the case for well-behaved couples that have yielded satisfactory results for a variety of semiconductors, especially with organic solvents (e.g., [21]). On the other hand, many efficient systems reported in the literature entail a more complicated behaviour, e.g., the above-mentioned polychalcogenide and polyiodide redox couples actually represent sluggish redox systems involving specific interactions with the semiconductor... 

Structures of some polyiodides. The I2 building units are in bold face. Bond lengths in pm. For comparison molecule I—I 268 pm, VAN DER Waals distance L 1396 pm... 

C42H7o035 Na+ I 3 8 H20 Cyclomaltoheptaose-sodium polyiodide, octahydrate (CYDXTF)131... 

C42H70O35Na+I--8H2O Cyclomaltoheptaose-sodium polyiodide, octahydrate CYDXTF 43 334... 

The reaction between starch and iodine (or iodine-iodide mixtures) to form an inclusion compound was first reported in 1814 by Colin and de Claubry 131) and has since become familiar to all chemists through its applications in analytical chemistry. Its deep blue colour (kmax 620 nm) has been known for years to result from a linear arrangement of polyiodide within a canal formed by a helical coil of amylose. The helical amylose structure will trap other molecules 132,1331 and other hosts will stabilise polyatomic iodide guests134> 135). 

This helical arrangement of amylose, known as the V-form, may be precipitated from certain solutions (e.g. in butanol or DMSO) of amylose. Either hydrated (Vh-) or anhydrous (Va-) amylose absorbs I2 vapour to produce the blue compound with the necessary I" being produced in situ. Alternatively the compound may be formed from iodine-iodide mixtures in solution which allows the V-form to be produced and stabilised as the polyiodide compound 141 The compound was reported 142) to have the orthorhombic space group 7>212121. 

More recent crystallographic work has been directed at inclusion compounds of V-amylose (including more detailed examination of the polyiodide compound) and at the structures of Va-amylose and Vh-amylose. Despite the terminology used, both forms have significant water content and may therefore be classed as inclusion compounds in their own right. 

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