Iodate formation

In studies of rates of iodine hydrolysis in the presence of chloride and bromide ions step (1) was also suggested as rate-determining (in iodate formation at pH 6-8). 

A. Skrabal has shown that in the case of hypoiodous acid in alkaline soln., ])robably an alhali hypotri-iodite, MI3O, is formed, which reacts like free iodine and the velocity of iodate formation is determined by the rate of reaction between the hypotri-iodite and the alkali by which iodide and iodate are formed. The kinetic equation, dx/(U k[)ll0Y[Vlf[01i ], based on these assumptions fits the observations of E. L. C. Forster. When the concentration of the iodine is small the reaction... 

The formation of bromate in hypobromite solutions is about one hundred times as fast as that of chlorate in hypochlorite solutions and occurs readily in slightly alkaline solution, because of the greater hydrolysis of the sodium hypobromite. The velocity of iodate formation in hypoiodite solution proceeds at a much greater rate, about 3,000,000 times that for the chlorate. Hence, hypoiodite solutions are stable only in very low concentration or in the presence of a very small excess of alkali. An increase in temperature increases the rate of iodate formation. Ethanol will react readily with a cold solution of iodine in alkali with the... 

Because of the pronounced differences in the reaction kinetics between reactions (1) and (2), in particular at low solution pH, it seems advisable to look separately at the relative concentrations of the different iodine species present in the equilibrium solution for the cases with iodate formation and without iodate formation . The results of both Bell et al. (1982 a) and of Palmer and Lietzke (1982) on the equilibrium state of the main hydrolysis reactions can be summarized as follows ... 

Figure 7.22. I2 as a fraction of total iodine in the equilibirum state of the hydrolysis reactions, with and without iodate formation (Bell, 1981)...

Three typical examples of the results of these calculations which are of interest in reactor accident considerations are shown in Fig. 7.24. The tendency of these results can be summarized as follows At pH 5, 100 °C and an initial I2 concentration of 10 g-atom/1, iodate formation proceeds very slowly, reaching the same concentration as I2 after about 1 day the equilibrium state of the reaction would only be established after about 100 days. This means that over a comparatively long period of time one has to deal with rather high fractions of the molecular species I2 and HOI. Raising pH to about 7 at the same temperatme results in a much faster IO3" formation, with the equilibrium state already being established after about 10 minutes. At lower temperatures, the reaction rates are correspondingly lower. In solutions with very low total iodine concentrations, the I2 fraction decreases very quickly at pH 7 however, HOI disproportionation proceeds rather slowly with the consequence that the equilibrium state of reaction (3) is attained only after several days. 

Figure 7.24. Kinetics of iodate formation (according to Bell et al., 1982 b)...

Hence, the most important parameter for the magnitude of the integral iodine partition coefficient is iodate formation and, consequently, as a result of the rather slow rate of HOI disproportionation in the pH range 5 to 7, a time-dependent value of the partition coefficient can be expected. This effect, quantitatively predicted by the kinetic calculations e. g., of Bell et al. (1982 b), has been confirmed by numerous measurements. As an example, the experimental investigations of Beahm and Shockley (1983) shall be mentioned, which show that at a total iodine concentra-... 

As the experiments showed, iodide and iodate were formed in a ratio of about 5 1 upon contact of I2 with the basic aerosol materials. Apparently, it is easier for I2 to disproportionate on the surface than it is for it to undergo a redox reaction with ions in the crystal to form Csl alone. Thermochemical data show that the formation of iodide and iodate would result in a lower free energy of reaction than formation of iodide alone. Formation of iodate alone would give a lower iodine potential than formation of iodide and iodate however, iodate formation seems to be limited by reaction kinetics. The extent of I2 reaction with anhydrous CS2O and CS2CO3 is probably limited only by the surface concentration of iodide and iodate which prevents or delays further interaction between I2 and the host crystal. In tests in which saturated aqueous solutions of these compounds were present, no such limitation was observed, nor had it been expected. 

In the reaction, the rate of iodate formation should be slower than the oxidation of the aldose. The reaction is slowed down by the presence of buffers such as borax 195). 

Place 2 ml. of the periodic acid reagent in a small test tube, add one drop (no more—otherwise the silver iodate, if formed, will fail to precipitate) of concentrated nitric acid, and shake well. Add one drop or a small crystal of the compound to be tested, shake the mixture for 15-20 seconds, and then add 1-2 drops of 3 per cent, silver nitrate solution. The instantaneous formation of a white precipitate of silver iodate is a positive test. Failure to form a precipitate, or the appearance of a brown precipitate which redissolves on shaking, constitutes a negative test. 

The equihbrium constant of this reaction is 5.4 x 10 at 25°C, ie, iodine hydrolyzes to a much smaller extent than do the other halogens (49). The species concentrations are highly pH dependent at pH = 5, about 99% is present as elemental at pH = 7, the and HIO species are present in almost equal concentrations and at pH = 8, only 12% is present as and 88% as HIO. The dissociation constant for HIO is ca 2.3 x 10 and the pH has tittle effect on the lO ion formation. At higher pH values, the HIO converts to iodate ion. This latter species has been shown to possess no disinfection activity. An aqueous solution containing iodate, iodide, and a free iodine or triodide ion has a pH of about 7. A thorough discussion of the kinetics of iodine hydrolysis is available (49). 

Accordingly, crystallization of iodates from solutions containing an excess of HIO3 sometimes results in the formation of hydrogen biio-dates, M H(I03)2, or even dihydrogen triiodates,... 

For bromates and iodates, disproportionation to halide and perhalate is not thermodynamically feasible and decomposition occurs either with formation of halide and liberation of O2 (as in the catalysed decomposition of CIOs just considered), or by formation of the oxide ... 

Figure 4.101 Formation of zones due to the change of reaction mechanism by applying an electrical field during the oxidation of arsenous acid by iodate ( = 2.0 V cm" ). Numbers show the time intervals after the electric field was switched on. Intermediate product iodine (dark) and iodide (white) [68. ...

Unlike chlorine and fluorine, the free bromine and iodine are produced by chemical methods (reaction of chlorine with bromide or iodide solutions). Electrochemical methods are used to produce the salts of their oxygen-containing acids, the bromates and iodates, from the corresponding bromide and iodide solutions. These reactions are analogous to those in chlorate production [Eqs. (15.31) to (15.34)] and involve the intermediate formation of hypobromites and hypoiodites. 

Wong [8] found that the determination of residual chlorine in seawater by the amperometic titrimetic method, potassium iodide must be added to the sample before the addition of the pH 4 buffer, and the addition of these two reagents should not be more than a minute apart. Serious analytical error may arise if the order of addition of the reagents is reversed. There is no evidence suggesting the formation of iodate by the reaction between hypobromite and iodide. Concentrations of residual chlorine below 1 mg/1 iodate, which occurs naturally in seawater, causes serious analytical uncertainties. 

Wong [8] reported that the losses of chlorine are not related to the formation of iodate by the oxidation of iodide by hypobromite. The presence of iodate in seawater may cause significant uncertainty in the determination of small quantities of residual chlorine in water. Determinations of residual chlorine at the 0.01 mg/1 level are of questionable significance. 

Magnesium acetate bromide chlorate chloride fluorosilicate formate iodate iodide... 

The oxidation-reduction methods with potassium iodate invariably based on the formation of iodine monochloride (ICl) in a medium of strong hydrochloric acid solution. 

The solution becomes acidic due to the formation of sulfuric acid. This solution is treated with an equivalent amount of fresh iodate mother hquor. lodide-iodate reaction in acid medium yields iodine ... 

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