Potassium iodide , crystal structure

Figure 3.97 The spatial confinement inside a nanotube gives rise to crystal structure completely different from those observed for the bulk phase (a) scheme showing the structure of a potassium iodide crystal inside...

Colorless crystals or white powder monoclinic structure density 3.90 g/cm3 stable at ordinary temperatures melts at 560°C with partial decomposition, releasing oxygen moderately soluble in cold water 4.74 g/lOOmL at 0°C greater solubility in boiling water 32.3 g/lOOmL at 100°C soluble in potassium iodide solution insoluble in alcohol and liquid ammonia... 

Copper (I) iodide is a dense, pure white solid, crystallizing with a zinc-blende structure below 300°. It is less sensitive to light than either the chloride or bromide, although passage of air over the solid at room temperature in daylight for 3 hours results in the liberation of a small amount of iodine. It melts at 588°, boils at 1,293°, and unlike the other copper halides, is not associated in the vapor state. Being extremely insoluble (0.00042 g./l. at 25°), it is not perceptibly decomposed by water. It is insoluble in dilute acids, but dissolves in aqueous solutions of ammonia, potassium iodide, potassium cyanide, and sodium thiosulfate. It is decomposed by concentrated sulfuric and nitric acids. 

Analogical molecular structure is possessed by other simple compounds such as potassium iodide, ammonium chloride, sodium hydroxide, barium nitrate, ammonium acetate and so on. In all these compounds there is a transfer of one or several electrons from one element to the other, positive and negative ions being thus formed, that are hold together in a crystal by electrostatic attraction. A bond of this kind is no genuine chemical bond in tbe correct meaning of the term, but is just,a result of Coulomb forces of attraction between opposite charges. 

It has been known for almost 200 years that starch gives a deep blue color when a solution of potassium iodide and iodine is added [47]. More than a century later it was suggested that the complex consisted of a helical polysaccharide, with triiodide in the center of the helix [48]. Using flow dichroism, it was demonstrated that the triiodide was stacked in a linear structure, as required for the helical model [49]. Another study of the optical properties of crystals of the amylose-triiodide complex showed it to be consistent with a helical structure [50] and X-ray diffraction showed the triiodide complex gave the dimensions of a unit-cell of a helix with six glucose residues per turn [51]. This confirmed a helical structure for the amyiose complex with triiodide that predated the helical models proposed by Pauling for polypeptides [52] and the double helical model for DNA by Watson and Crick [53] by 10 years. 

Examining the inclusion of crystalline compounds Uke salts or metal oxides, it is a frequent observation that the two-dimensional confinement of the space of crystallization causes the formation of structures that differ sometimes completely from those found in the bulk phase. Antimony(III) oxide and potassium iodide may serve as examples here. For the Sb203-filaments inside nanotubes a valentinite structure with a lattice distortion inflicted by the outer restriction is observed (Figure 3.79d). NormaUy, this phase is found at high pressures. At standard conditions, the Sb203 adopts as cubic senarmonite structure with discrete Sb40,5-units. 

Amylose. A component (20-30%) of starch surrounded by amylopectin. A. is a linear a-l,4-glucan, Mr 50000-200000 (see figure at starch). Crystalline A. occurs in various polymorphic forms (A, B, C, and V-A.), that differ in conformation and crystal packing. A. is soluble in water and gives the characteristic blue color with iodine-potassium iodide solution (Lugol s solution) (formation of inclusion compounds, traces of iodide ions are necessary for occurrence of the blue color, formation of I5 ions I -1 I I -1). Because of its predominately unbranched structure, A. can be degraded to oligosaccharides both by a- and by /S-amylase. The screw-like (helical) conformation also allows the formation of inclusion compounds with alcohols. 

The structure of the enniatin B complex with potassium iodide has been studied by X-ray crystallography Unfortunately, from this investigation it could not be concluded with certainty whether the metal ion is entrapped in the central cavity or, instead, occupies a site between two adjacent ligand molecules. An arrangement of the latter type has been observed in the crystal structure of the 1 1 complex between RbNCS and the synthetic LDLLDL isomer of enniatin B In this case, Rb" ions are coordinated by five carbonyl oxygens (three of the upper and two of the lower depsipeptide molecules) and the nitrogen atom of the isocyanate anion, thus forming infinite sandwiches. 

In other crystals an octahedral metal atom is attached to six non-metal atoms, each of which forms one, two, or three, rather than four, bonds with other atoms. The interatomic distance in such a crystal should be equal to the sum of the octahedral radius of the metal atom and the normal-valence radius (Table VI) of the non-metal atom. This is found to be true for many crystals with the potassium chlorostannate (H 61) and cadmium iodide (C 6) structures (Table XIB). Data are included in Table XIC for crystals in which a tetrahedral atom is bonded to a non-metal atom with two or three covalent bonds. The values of dcalc are obtained by adding the tetrahedral radius for the former to the normal-valence radius for the latter atom. 

Tellurium bis[bis(2-hydroxyethyl)dithiocarbamate] and a thirty-fold molar excess of potassium bromide, iodide, or thiocyanate reacted in acetone acidified with acetic acid with replacement of one dithiocarbamate group per two molecules of tellurium dithiocarbamate by halide or thiocyanate. The deep-red crystals are stable as solids but decompose with deposition of tellurium when dissolved in methanol. The single-crystal X-ray structural analysis of the thiocyanato derivative revealed the presence of two chemically different tellurium atoms in the molecule that are in short contact1. 

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