Methods Involving Iodine

When a reducing analyte is titrated directly with iodine (to produce 1 ), the method is called iodimetry. In iodometry, an oxidizing analyte is added to excess I to produce iodine, which is then titrated with standard thiosulfate solution. 

Molecular iodine is only slightly soluble in water (1.3 X 10 3 M at 20°C), but its solubility is enhanced by complexation with iodide. 

A typical 0.05 M solution of If for titrations is prepared by dissolving 0.12 mol of Kl plus 0.05 mol of I2 in 1 L of water. When we speak of using iodine as a titrant, we almost always mean that we are using a solution of I2 plus excess I.  

As described in Section 16-2, starch is used as an indicator for iodine. In a solution with no other colored species, it is possible to see the color of 5 pM If. With starch, the limit of detection is extended by about a factor of 10. 

In iodimetry (titration with If), starch can be added at the beginning of the titration. The first drop of excess If after the equivalence point causes the solution to turn dark blue. In iodometry (titration of If), If is present throughout the reaction up to the equivalence point. Starch should not be added to such a reaction until immediately before the equivalence point (as detected visually, by fading of the If Color Plate 10). Otherwise some iodine tends to remain bound to starch particles after the equivalence point is reached. 

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Iodometry is a titrimetric method involving iodine. It is an indirect method because the product of the reaction of ST with I" (I2) is titrated. 

Assay methods involving iodine can be categorized under the following heads namely ... 

In this chapter, we begin our description of the principal titrimetric methods involving a redox reaction. Such methods are sometimes studied in the part of analytical chemistry named oxidoreductimetry. The first tidimetric methods we shall study are direct and indirect iodometries, two methods that are very closely related. They constitute an important part of the methods involving iodine and iodide ions as redox reagents. 

Ana.lytica.1 Methods. Various analytical methods involve titration with oxidants, eg, hexacyanoferrate (ferricyanide), which oxidize dithionites to sulfite. lodimetric titration to sulfate in the presence of formaldehyde enables dithionite to be distinguished from sulfite because aldehyde adducts of sulfite are not oxidized by iodine. Reductive bleaching of dyes can be used to determine dithionite, the extent of reduction being deterrnined photometrically. Methods for determining mixtures of dithionite, sulfite, and thiosulfates have been reviewed (365). Analysis of dithionite particularly for thiosulfate, a frequent and undesirable impurity, can be done easily by Hquid chromatography (366). 

Rates of enolization can be measured in several wt s. One method involves determining the rate of halogenation of the ketone. In the presence of a sufficient concentration of bromine or iodine, halogenation is much faster than enolu ation or its reverse and can therefore serve to measure the rate of enolization ... 

Puacz et al. (1995) developed a catalytic method, based on the iodine-azide reaction, for the determination of hydrogen sulfide in human whole blood. The method involves the generation of hydrogen sulfide in an evolution-absorption apparatus. In addition, the method allows for the determination of sulfide in blood without interference from other sulfur compounds in blood. A detection limit of 4 g/dm3 and a percent recovery of 98-102% were achieved. Although the accuracy and precision of the catalytic method are comparable to those of the ion-selective electrode method, the catalytic method is simpler, faster, and would be advantageous in serial analysis. 

The explosion hazard associated with the usual laboratory preparation from white phosphorus and alkali may be avoided by an alternative method involving oxidation of phosphine with an aqueous iodine solution [1], The commercial 50% solution reacts violently with oxidants. On heating, it decomposes rapidly above 100°C evolving phosphine, which is liable to explode with air. It is recommended it... 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

The first method involves an assumption. Any time the value of the equilibrium constant is very large or very small, an assumption is possible. In this case, the constant is very small. A small equilibrium constant indicates that only a small number of iodine molecules will break apart to become iodine atoms. This means that x will be small. A small x value means that 0.500 - x will be close to 0.500 M. As an assumption, we will assume that 0.500 - x is 0.500 M. 

Theory Iodine in aqueous solution acts as an oxidizing agent which forms the basis of assay methods involving direct titration with iodine. Thus, we have ... 

Still another related method consists in reacting dienes with dimethyl disulphide in the presence of iodine, to produce the corresponding bis(o ,/J-dmiethylmercapto) derivative241-243. Again, characteristic fragmentation is obtained in the standard El spectra. However, this method involves complications if the double bonds are separated by less than four methylene groups due to the formation of cyclic thioethers. 

Five types of methods involving fundamental organohypervalent iodine reactions that can be used in heterocyclic synthesis from an open-chain precursor are presented in the following sections. 

Because of the polyfunctional nature of carbohydrates, protective-group strategy plays an important role in synthetic methodology involving this class of compounds. In the present Chapter, results are described from a study of the utility of N-trimethylsilyl- and N-tert-butyldimethylsilyl-phthalimide for the selective silylation of primary hydroxyl groups in carbohydrates. Also described, is a new, facile method for cleavage of acetals and dithioacetals in carbohydrate derivatives the method involves treatment of the derivatives with a dilute solution of iodine in methanol. 

Miscellaneous Methods and Applications Involving Iodine Redox... 

The use of hypervalent iodine reagents in carbon-carbon bond forming reactions is summarized with particular emphasis on applications in organic synthesis. The most important recent methods involve the radical decarboxylative alkylation of organic substrates with [bis(acyloxy)iodo]arenes, spirocyclization of para- and ortho-substituted phenols, the intramolecular oxidative coupling of phenol ethers, and the reactions of iodonium salts and ylides. A significant recent research activity is centered in the area of the transition metal-mediated coupling reactions of the alkenyl-, aryl-, and alkynyliodonium salts. 

The above classification highlights the common analytical methods. There is, however, a great deal of overlapping as far as the chemistry of the process is involved. For example, iodometric method involves an oxidation-reduction reaction between thiosulfate anion and iodine. It is, however, classified here under a separate heading because of its wide application in environmental analysis. 

A second method involves calculating the stabilities of 1,2- and 1,4-addition products (figure 7). Under conditions of low steric requirement, 1,2-addition will be preferred, because location of double bonds in pentagonal rings is unfavourable. However bulky reagents will sterically interact when cis to each other, so 1,4-addition is then preferred. Hydrogen and fluorine are thus predicted to favour 1,2-addition, whereas chlorine, bromine and iodine will favour 1,4-addition (Dixon et al. 1992). Preliminary results confirm these predictions ( 2 and 3). 

Bismuth(III) iodide has been prepared in the absence of solvents by the reaction of iodine with elemental bismuth1,2 or with bismuth (III) sulfide.3 Alternative methods involve precipitation of bismuth(III) iodide from aqueous solutions of bismuth salts by adding alkali-metal iodides,4 and the addition of bismuth (III) oxide to a solution of iodine and tin(II) chloride in saturated hydrogen chloride.5 In either case the initial product is purified by sublimation, usually in an atmosphere of carbon dioxide. The product obtained by precipitation requires several resublimations for complete purification.6... 

Alternative methods involved nucleophilic substitution at iodine in (dichloroiodo)benzene or BTI by the acyloxy group of silver or sodium carboxy-lates the latter permits the preparation of mixed [bis(acyloxy)iodo]benzenes with two different acyloxy groups [6]. Also, heating of iodosylbenzene and an acid (molar ratio 1 2) in chloroform results in the formation of [bis(acyloxy)iodo]benzenes quantitatively. A stable (diformyloxyiodo)arene from... 

A method of determining airborne iodine has also been reported.241 Here, iodine is absorbed into 5% aqueous KI and spectrophotometrically determined at 590 nm in the form of its complex with starch. This method is selective with respect to bromine and chlorine, and the sensitivity of this method is 0.25 mg of I2 per m3 of air. The concentration of the, 31I isotope in water can be determined by a method involving isotope exchange in the starch-iodine complex.242 Flow-injection determination of ascorbic acid (0.1-40 mg/mL) has been proposed.243 Iodine is generated in the flow system as I3- ions, which are in turn exposed to starch to produce a steady signal at 350 and 580 nm. Ascorbic acid provides inversed maxima which are measured. This method is recommended for analysis of ascorbic acid in fruit juice, jam, and vitamin-C preparations. Use of the blue complex has also been reported for determination of sodium dichloro-isocyanurate in air.244 Obviously the blue reaction is applicable in the determination of amylose, amylopectin, and starch,245-252 as well as modified starches.245,253-255... 

The scope of this paper is to provide an overview of methods used to study properties of electrically neutral molecular clusters initiating particle formation in the troposphere, with focus on quantum chemistry. The review of results is intended to be complete with regard to water-sulfuric acid-ammonia clusters. Concerning studies on clusters including other molecular species, we review representative examples and newest publications. Ionic clusters and clusters involving iodine, related to coastal nucleation, are mentioned in passing. 

Decomplexation of ArCr CO)3. The chromium carbonyl complexes of arenes are useful for activation of the aryl group to nucleophilic attack (6, 28, 125-126 7, 71-72). Decomplexation has been effected with iodine or by photochemical oxidation with destruction of the expensive Cr(CO)3 unit. A more recent method involves reflux with pyridine to form Py3-Cr(CO)3 in yields of 70-100%. The pyridine complex in the presence of BF3 can be reused for preparation of ArCr(CO)3. Isomerization of 1,3-dienes. Ergosteryl acetate (1) is isomerized by chromium carbonyl to ergosteryl 83 acetate (2) in 81% yield. Under the same conditions ergosteryl 83 acetate (3) is isomerized to ergosteryl 81 acetate (4). 80th reactions involve isomerization of a cisoid diene to a transpid diene. In contrast iron carbonyl isomerizes steroidal transoid 3,5- and 4,6-dienes to 2,4-dienes. ... 

Following the pioneer work of Kharasch [60], methods involving radical transfer of halides have been developed. The atom transfer method has emerged in the 1980s as one of the best method for conducting intra- and intermolecular radical additions to olefins [61]. This approach is particularly appealing from an atom economy point of view since all atoms remains in the final product. The non-reductive nature of these reactions is also particularly important for the preparation of functionalized molecules. Halides transfers and more particularly iodine atom transfers have found nice applications for cyclizations, annula-tions and cascade reactions [62]. These reactions are based on exothermic radical steps, such as the addition of an alkyl radical to an olefin, followed by an... 

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