Hydrolysis of iodine: equilibria at high temperatures Page: 1 of 7
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THE HYDROLYSIS OF IODINE: EQUILIBRIA AT HIGH TEMPERATURES*
NS OF THIS REPORT ARE ILLEGIBLE. it
reproduced from the best-available
permit the broadest possible avail-
Donald A. PALMER, Richard W. RAMETTE+, and Robert E. MESMER
Chemistry Division, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, TN 37831 USA
The hydrolysis (or disproportionation) of molecular iodine to form iodate and iodide
ions has been studied by emf measurements over the temperature range, 3.8 to 209.0*C.
The interpretation of these results required a knowledge of the formation constant for
triiodide ion and the acid dissociation constant of iodic acid, both of which were
measured as a function of temperature. The resulting thermodynamic data have been
incorporated into a general computer model describing the hydrolysis equilibria of
''dine as a function of initial concentration, pH and temperature.
The Three Mile Island nuclear accident has
focussed renewed attention on the behavior of
iodine in aqueous solution. For it was sub-
sequently realized that hydrolysis of iodine in
either of its volatile forms, namely molecular
iodine 12 and hypoiodous acid HOI, would sub-
stantially reduce the release of 1311 to the
atmosphere by converting them to the non-
volatile ionic forms, iodate (+V) and iodide
(-I). Although individual reactions within the
matrix of iodise hydrolysis reactions have been
extensively studied under ambient conditions,
the key disproportionation [equation (3) in the
sequence shown below) has been neglected with
only one investigation involving two experi-
ments being reported1. Turner2 demonstrated
that attempts to estimate the temperature
dependence of this reaction based on various
thermodynamic assumptions lead to widely dif-
fering values for the equilibrium constant at
high temperatures (e.g., at 150*C the predicted
values differ by 16 orders of magnitude).
Although radiolysis, redox and photochemical
processes, as well as interaction with organic
radicals, undoubtedly perturb the hydrolysis in
a nuclear accident setting, an exact knowledge
of the thermodynamic parameters for the prin-
cipal hydrolysis equilibrium is essential in
establishing a model for iodine behavior.
Similarly, the kinetics of iodine hydrolysis
are important, particularly at low temperatures
and low pH values where the rates are slow.
Although this aspect will not be addressed
here, an experimental program to measure these
kinetics has been completed at our laboratory.
In formulating the equilibrium model3, the
following reactions were considered:
12 + I * 13
12 + H20 HOI + I +H+
312 + 3H20 * 103- + 5I- + 6H+
HOI * OIF +H+
HI03 * 103' + H+
H2OI+ * HOI + H+
12 + H20 * I20H~ +H+
*Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences,
U.S. Department of Energy under contract DE-AC05-84OR21lwith the Martin Marietta Energy
+On leave from the Department of Chemistry, Carleton College, Northfield, MN 55057 USA.
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Palmer, D.A.; Ramette, R.W. & Mesmer, R.E. Hydrolysis of iodine: equilibria at high temperatures, article, January 1, 1984; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1207811/m1/1/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.