Tri-iodide ion

In most direct titrations with iodine (iodimetry) a solution of iodine in potassium iodide is employed, and the reactive species is therefore the tri-iodide ion 13. Strictly speaking, all equations involving reactions of iodine should be written with 13 rather than with I2, e.g. 

For the sake of simplicity, however, the equations in this book will usually be written in terms of molecular iodine rather than the tri-iodide ion. 

Discussion. In addition to a small solubility (0.335 g of iodine dissolves in 1 L of water at 25 °C), aqueous solutions of iodine have an appreciable vapour pressure of iodine, and therefore decrease slightly in concentration on account of volatilisation when handled. Both difficulties are overcome by dissolving the iodine in an aqueous solution of potassium iodide. Iodine dissolves readily in aqueous potassium iodide the more concentrated the solution, the greater is the solubility of the iodine. The increased solubility is due to the formation of a tri-iodide ion ... 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

The cleavage of phenylmercuric iodide by iodine in the presence of excess iodide ion (to suppress free-radical reactions) at 25 °C in aqueous dioxan was reported to be first-order in both aromatic and tri-iodide ion, and faster than the reaction of alkylmercuric iodides724. A further study, together with bromodemercuration, both reactions being generally represented by... 

In the amperometric titration for the determination of total residual chlorine in seawater, tri-iodide ions are generated by the reaction between hypochlorite and/or hypobromite with excess iodide pH 4 (reactions (4.3) and (4.4)). The pH is buffered by adding a pH 4 acetic acid-sodium acetate buffer to the sample. 

Goldman et al. [4], on the other hand, suggest that the order of the addition of the reagents for generating tri-iodide ions is crucial for obtaining accurate results. If the acidic buffer is added first, at pH 4, molecular bromine may be formed (reaction (4.8)) ... 

Flow injection analysis has been used for the automated determination of hydrogen sulfide in seawater [20]. A low-sensitivity flow injection analysis manifold for concentrations up to 200 imol/l hydrogen sulfide had a detection limit of 0.12 xmol/l. Sulfide standards were calibrated by colorimetric measurement of the excess tri-iodide ion remaining after reaction of sulfide with iodine. The coefficient of variation was less than 1% at concentrations greater than 10 imol/l. The method was fast, accurate, sensitive enough for most natural waters, and could be used both for discrete and continuous analysis. 

Another example is the increase in the aquation of the iodidopentacyano system in the presence of the tri-iodide ion. Again, this is what I would call an off-site or OSR reaction. 

The tri-iodide ion is responsible for the solubility of elemental iodine in an aqueous solution of potassium iodide and gives rise to the deep yellow-brown colour of the solution, unlike the violet colour displayed by the iodine molecule in the vapour phase or in a solution of ethanol ... 

Valence bond theory resorts to using the two forms of the tri-iodide ion as canonical forms contributing to the resonance hybrid ... 

The exceptions to the octet rule described in the previous section, the xenon compounds and the tri-iodide ion, are dealt with by the VSEPR and valence bond theories by assuming that the lowest energy available d orbitals participate in the bonding. This occurs for all main group compounds in which the central atom forms more than four formal covalent bonds, and is collectively known as hypervalence, resulting from the expansion of the valence shell This is referred to in later sections of the book, and the molecular orbital approach is compared with the valence bond theory to show that d orbital participation is unnecessary in some cases. It is essential to note that d orbital participation in bonding of the central atom is dependent upon the symmetry properties of individual compounds and the d orbitals. 

The material obtained by incorporating more iodine gives a more complex spectrum, but stoichiometric (TTF)I3 shows two eight-line subspectra (Table 9), and the spectrum is very similar to those of salts of the symmetrical tri-iodide ion. In particular, the intensity ratio of the two subspectra (2.0 1.25) agrees well with previous data. In a linear I—I—F ion, an increase in the relative intensity of the spectrum of the central iodine atom is explained on the basis that, being bound to two other atoms, it suffers less recoil. The data therefore indicate complete single ionization of TTF (TTF)+(I3 ). 

This reaction may be used to estimate Cu2+ in solution for, it may be remembered, free iodine (or, more conveniently) the tri-iodide ion, I7, may be easily titrated with sodium thiosulfate. Cuprous oxide, Cu20... 

Note that the product formed by reaction with pyridine (reaction c) contains the tri-iodide ion, also present in solutions containing both iodine and potassium iodide thus, solutions of iodine in pyridine and iodine in aqueous KI have about the same color. 

Potassium iodide precipitates copper(I) iodide, which is white, but the solution is intensely brown because of the formation of tri-iodide ions (iodine) ... 

Adding an excess of sodium thiosulphate to the solution, tri-iodide ions are reduced to colourless iodide ions and die white colour of the precipitate becomes visible. The reduction with thiosulphate yields tetrathionate ions ... 

If the Sb5+ ions are in excess, iodine crystals precipitate out and float on the surface of the solution. When heated the characteristic violet vapour of iodine appears. If the reagent is added in excess, brown tri-iodide ions are formed which screen the yellow colour of hexaiodoantimonate(III) ions ... 

Sometimes, however, the solubility of a substance in a particular solvent is greatly changed by the presence of other solutes. For example, iodine is very much more soluble in a solution containing iodide ion than it is in pure tvater. The reason for the increase in solubility is that iodine, T, combines with iodide ion, I, to form the complex tri-iodide ion, Ig " ... 

In both cases the [I2 ] exponent was larger for polymerizations in the solvent of lower dielectric constant where the counter-ion, (deliberately omitted in the kinetic schemes), was, therefore, envisaged as tri-iodide ion, I3, corresponding to n = 2. In ethylene dichloride n = 1 and the gegenion was assumed to be unassociated, i.e. I . Visible spectrophotometry was used to measure [X], (=[I2]free " [I2M]) and, from a plot of [I2]total/[X] versus [M][X] /(1 + K[M])", the ratio was obtained. Data for K were obtained independently by ultraviolet spectrophotometry using well characterized methods [66, 67]. Thus having determined fej/fet, substitution of the experimental data, Rp, [M] and [I2] TOTAL) the final rate expression enabled values of fep to be calculated. Iodine is also conveniently estimated by iodometric titration... 

A sputtered platinum counter-electrode on a TCO substrate may have its electrocatalytic activity, in the reduction of the tri-iodide ions, enhanced by creating Pt colloids from an alcoholic solution of H2PtCl6. 

Oxidative microcoulometry has become a widely accepted technique for the determination of low concentrations of sulfur in petroleum and petroleum products (ASTM D-3120). The method involves combustion of the sample in an oxygen-rich atmosphere followed by microcoulometric generation of tri-iodide ion to consume the resultant sulfur dioxide. This distinguishes the technique from reductive microcoulometry, which converts sulfur in the sample to hydrogen sulfide that is titrated with coulometrically generated silver ion. 

An equilibrium is set up between iodine and tri-iodide ions, and if iodine molecules are removed from solution by a reaction, (ri-iodide ions dissociate to form more iodine molecules. A solution of iodine in potassium iodide can ifaus be titrated as though it were a solution of iodine m water. 

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