Tri-iodide

Boron tri-iodide, Blj (BClj plus HI at red heat or I2 plus NaBH ), m.p. 43°C, b.p. 2iO°C. It has very similar properties to boron trichloride. 

Iodine is a dark-coloured solid which has a glittering crystalline appearance. It is easily sublimed to form a bluish vapour in vacuo. but in air, the vapour is brownish-violet. Since it has a small vapour pressure at ordinary temperatures, iodine slowly sublimes if left in an open vessel for the same reason, iodine is best weighed in a stoppered bottle containing some potassium iodide solution, in which the iodine dissolves to form potassium tri-iodide. The vapour of iodine is composed of I2 molecules up to about 1000 K above this temperature, dissociation into iodine atoms becomes appreciable. 

This is a modification of Method 5, iodine being added to a mixture of red phosphorus and ethanol phosphorus tri- iodide is thus formed m situ, and readily reacts with the ethanol, giving ethyl iodide and phosphorous acid (p. 96). 

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 ... 

The resulting solution has a much lower vapour pressure than a solution of iodine in pure water, and consequently the loss by volatilisation is considerably diminished. Nevertheless, the vapour pressure is still appreciable so that precautions should always be taken to keep vessels containing iodine closed except during the actual titrations. When an iodide solution of iodine is titrated with a reductant, the free iodine reacts with the reducing agent, this displaces the equilibrium to the left, and eventually all the tri-iodide is decomposed the solution therefore behaves as though it were a solution of free iodine. 

A large excess of potassium iodide should be avoided, since the complex salt is not so readily hydrolysed as the tri-iodide. This is an excellent method, because the oxyiodide is precipitated in a form which is very convenient for filtration and weighing. 

Procedure. The cold bismuth nitrate solution, containing 0.1-0.15 g of Bi (Note 1), must be slightly acid with nitric acid (Note 2), and occupy a volume of about 20 mL. Add finely powdered solid potassium iodide, slowly and with stirring, until the supernatant liquid above the black precipitate of bismuth tri-iodide is just coloured yellow (due to K[BiI4]). Dilute to 200mL with boiling water, and boil for a few minutes. The black tri-iodide is converted into... 

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... 

Almost all of the rare-earth metal/rare-earth metal tri-iodide systems, R/RI3, contain binary phases with the rare-earth element in an oxidation state lower than -1-3 ( reduced rare-earth metal iodides) [3, 7, 10-13]. More common is the oxidation state -i-2. Elements that form di-iodides RI2 are illustrated in Fig. 4.1. 

Praseodymium tri-iodide, Prl3, as the starting material for reduction reactions, might be easily produced by the oxidation of praseodymium metal with elemental iodine [17]. With catalytic amounts of hydrogen dissolved in praseodymium metal powder, the reaction temperature can be as low as 230 °C [18]. Sublimation in high vacuum in tantalum tubes yields pure Prl3. 

When iodine and povidone are combined, a chemical reaction takes place forming a complex between the two entities. Some of the iodine becomes organically linked to povidone though the major portion of the complexed iodine is in the form of tri-iodide. Dilution of this iodophor results in a weakening of the iodine linkage to the carrier polymer with concomitant increases in elemental iodine in solution and antimicrobial activity. 

Using the tri-iodide/iodide redox couple and the sensitizers (22) and (56), several groups have reported up to 8-10% solar cell efficiency where the potential mismatch between the sensitizer and the redox couple is around 0.5 V vs. SCE. If one develops a suitable redox couple that decreases the potential difference between the sensitizer and the redox couple, then the cell efficiency could increase by 30%, i.e., from the present value of 10% up to 13%. Towards this goal, Oskam et al. have employed pseudohalogens in place of the triiodide/iodide redox couples, where the equilibrium potential is 0.43 V more positive than that of the iodide/iodide redox couple.17 Yamada and co-workers have used cobalt tris-phenanthroline complexes as electron relays (based on the CoII/m couple) in dye-sensitized solar cells.95... 

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)) ... 

Figure 4.1. Absorbance of tri-iodide at 353 nm formed from a fixed concentration of iodate in the presence of 30 xmol/l nitrite upon the addition of various volumes of 1% (w/v) sulphamic acid N denotes the case where neither nitrite nor sulphamic acid was present in the solution. From [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. 

Preparation.—Two new synthetic routes to simple iodo-phosphines have appeared this year. Thus phosphorus tri-iodide (8) is produced in fairly good yield when the iodides of lanthanum, strontium, or europium are treated with the corresponding phosphates.14 Tetraiododiphosphine (9) is formed in 75—80% yield, by the reaction... 

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. 

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