Iodine hydrolysis equilibrium

The equilibrium constant of this reaction is 5.4 x 10-13 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 I2 at pH = 7, the I2 and HIO species are present in almost equal concentrations and at pH = 8, only 12% is present as I2 and 88% as HIO. The dissociation constant for HIO is ca 2.3 x 10 11 and the pH has little effect on the IO 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). 

When calculating the h equilibrium concentration in the containment atmosphere from these data, one has to remember that the partition coefficients usually were determined using pure I2 solutions. The fission product iodine in the sump water, however, mainly consists of non-volatile iodide with smaller fractions of I2 on the order of 10% and less. The airborne fraction of I2, therefore, has to be calculated from the dissolved I2 concentration only. The excess 1 present in the solution will act to shift the I2 hydrolysis equilibrium towards a higher I2 fraction at the low total iodine concentrations present, however, this effect is not expected to result in a significant decrease in the total iodine partition coefficient. 

One of the most important parameters controlling iodine volatility is sump water pH not only will the I2 hydrolysis equilibrium and the iodine partition coefficient be affected by this parameter, but the product yields of radiolytic reactions and the extent of formation of organoiodine compounds as well. Because of the lack of practical experience, the sump water pH to be expected under severe accident conditions has to be calculated on the basis of assumed concentrations of potential sump water ingredients. In Table 7.17. (according to Beahm et al., 1992) an overview of substances to be expected in the sump water, which would effect a shift in solution pH either to lower or to higher values, is given. Besides these chemical substances, radiation may also affect sump water pH irradiation of trisodium phosphate solution (5.3 kGy/h) was reported to decrease the pH from an initial value of 9.0 to about 4.0 after 60 hours of irradiation (Beahm et al., 1992). It is obvious that in such a complicated system definition of the sump water pH to be expected in a real severe reactor accident is a difficult task. Nonetheless, a model for calculation has been developed by Weber et al. (1992). 

I2 The behavior of the activity extracted Into benzene from addle solution arTd back-extracted Into water or aqueous potassium Iodide Is expected for molecular Iodine at low concentrations where the hydrolysis equilibrium l2(aq)... 

It is, however, more likely that the discrepancies observed in the periodate oxidation of malonaldehyde concern mainly the hydroxylation step. In the mechanism proposed (5) for this reaction, it is the enol form of malonaldehyde which is hydroxylated. However, titrations of a solution of malonaldehyde, prepared by hydrolysis of an aqueous solution (33) of carefully distilled 1, 3, 3-tri-ethoxypropene (46, 47), both with strong base and with iodine, indicate that only about 80% of the enol form is present in the equilibrium solution. On the other hand, the thio-barbituric acid test (58, 59) gave consistently higher values for the malonaldehyde content of the solution. The fact that only about 80% of the enol form is present in the equilibrium solution is all the more important as it can be shown (56) by titration with strong base that the enolization is slow, and moreover does not seem to go to completion. 

A soln. of twenty-two grains of iodine and thirty-three grains of iodide of potassium, in one ounce of distilled water forms the liquor iodi of the British Pharmacopoeia. The effects produced by the ammonium salts are attributed to their hydrolysis into ammonium hydroxide, and the consequent formation of ammonium iodide or polyiodide. The effects produced by soln. of the halide salts are doubtless due to the formation of poly iodides as in the analogous case with bromine and potassium bromide. A. A. Jakowkin allowed carbon disulphide to remain in contact with aq. soln. of iodine and potassium iodide until equilibrium was attained and... 

The final partitioning of fission product iodine between the sump water and the atmosphere of the containment is determined by the equilibrium partition coefficient as will be discussed in more detail in Section 7.4.3.1., this figure depends on several parameters, such as iodine concentration and sump water pH and, to a lesser extent, temperature. As can be seen from Fig. 4.13., the iodine partition coefficient increases with decreasing iodine concentration in the solution this behavior is the consequence of the shift in the h hydrolysis reaction towards a lower I2 equilibrium fraction at lower total iodine concentration. Below a concentration of about 10" g/1, however, the partition coefficient remains virtually constant, an effect which has been attributed to the volatility of the hydrolysis product HOI (Lin, 1981). This constant value is about l(f at pH 7 and 25 °C it applies as well for the concentration range to be expected in a loss-of-coolant accident. Concerning the HOI partition coefficient at 100 °C, several measurements yielding quite different values have been reported. The lowest one is 240 given by Lin (1981) Lemire et al. (1981) reported values between 100 and 1(1, while Wren and Sanipelli (1984) measured values between 7 10 and 3 10. Since the HOI compound was... 

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 ... 

The data on the relative concentrations of the different iodine species in the hydrolysis solution which were given above apply only when the equilibrium state has been completely established. However, if important reactions proceed only slowly, compared to the time scale of the accident, the distribution of species will be different from that to be expected for the equilibrium state. Thus, for the evaluation of fission product iodine behavior, the rates of the reactions leading to the equilibrium state also have to be taken into account. 

The degree of blockiness in the structure of partially hydrolysed, poly(vinyl acetate) depends on the method of hydrolysis. Saponification with alkali gives vinyl acetate-alcohol copolymers with a highly blocked structure, transesterification with methanolic methoxides gives intermediate blockiness, and acid-catalysed equilibrium hydrolysis gives near-random copolymers. These structures respond differently in iodination analyses. ... 

---