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Can. J. Chem. Vol. 81, 2003
[5]
[6]
RI(aq) + H2O → ROH(aq) + I− + H+
RI(aq) + e−aq → R·(aq) + I−
compound can still be obtained from the change in
absorbance at 225 nm (see Section 4.1 for the relationship
between the absorbance and the first-order rate constant).
In general, the measurements were repeated at least once.
The reproducibility of the measurements was better (<5%)
when measuring fast hydrolysis rates (i.e., for 2-iodobutane
and (iodomethyl)benzene). During the measurement of slow
hydrolysis rates (i.e., iodomethane and iodoethane), repro-
ducibility was generally in the 20% range. Measurements
lasted between 3–5 half-lives during experiments using the
electrode method. For each spectrometer measurement, the
absorbance at 225 nm was then monitored until a limiting
absorbance was approached (generally > 3 half-lives).
Solutions were prepared by adding the pure compound (as
received) to purified (distilled and deionized) water
(Millipore Milli-Q Plus) in a temperature-controlled cell.
After dissolution of the compound, the electrodes were in-
troduced, and the I– signal was monitored until completion.
For the spectrophotometric method, an aliquot of the solu-
tion was removed and placed in a temperature-controlled
optical cell (10 cm path length). Starting concentrations of
all species for these measurements were approximately 1 ×
10–4 mol dm–3. This concentration is below the saturation
level of the compounds studied, but high enough to ensure
an adequate increase in absorbance as the reaction pro-
gresses. To avoid loss of the compound to the gas phase, the
cell was completely filled to eliminate any headspace. The
water pH was slightly acidic (~pH 6) at the start of a test,
but was not controlled or buffered and generally decreased
to ~4 during the hydrolysis.
Therefore, to determine the overall behaviour of organic
iodides, and their contributions to the gaseous iodine con-
centration under containment accident conditions, the forma-
tion and the decomposition of organic iodides must be
adequately quantified. The rate of the hydrated electron re-
action (rxn. [6]) is diffusion-controlled and is therefore
nearly independent of the type of organic iodide. Therefore,
the current study focuses on the partitioning and the hydro-
lysis of organic iodides. The partition coefficients (see Sec-
tion 3.2 for its definition) and the hydrolysis rate constants
of a range of organic iodides have been measured, and com-
pared with the literature data of organic iodides, if available,
and also with compounds of similar structure that do not
contain iodine. These values were reviewed for the purpose
of providing guidelines to appropriately group organic io-
dides so that modelling of organic iodide behaviour under
radiolytic conditions can be more easily managed.
3. Experimental
A small set of compounds was chosen to cover the major
structural possibilities (methyl, primary, secondary, tertiary,
benzyl, aryl) of organic compounds that may be present in
reactor containment. For example, a primary alcohol
(iodoethanol) and a carboxylic acid (iodoethanoic acid) were
chosen to explore the effect of neighbouring oxygen-
containing functional groups on hydrolysis rate and partition
coefficients. All compounds studied were purchased from
Aldrich Chemical Co., except (iodomethyl)benzene, which
was from Alfa-Aesar (a Johnson Matthey company), and 2-
iodobenzenol, which was from ICN Biomedicals Inc. Com-
pounds were of the highest purity available and were used as
received without further purification.
3.2 Partition coefficient determination
In this report, the partition coefficient (HK) is defined as
the ratio of the aqueous phase concentration to the gas phase
concentration at equilibrium.
[Compound(aq)]eq
[7]
HK =
[Compound(g)]eq
3.1 Measurement of hydrolysis rates
Two methods were used to study the hydrolysis rates of
these compounds. The first method used a solid-state ion se-
lective electrode (ISE) designed for iodide measurements
(Orion 9553) with a double junction reference electrode
(Orion 900200) to measure the concentration of iodide pro-
duced by hydrolysis. These probes were connected to a
Fisher Accumet 950 pH meter. The electrode was calibrated
for each temperature of interest using NaI solutions of
known concentration. The plot of mV vs. log10 [I–] was
found to be linear over the concentration range studied. This
method was successfully used for the hydrolysis of 2-
iodobutane. Due to baseline drift however, this method was
found to be unsuitable for the time periods required to mea-
sure the hydrolysis rates of the other compounds.
For the majority of the studies, the hydrolysis rates were
determined from the growth of the absorbance of I– (ε =
12 980 dm3 mol–1 cm–1 at 225 nm) monitored using a HP
8450A spectrophotometer. In some cases there was minor
absorbance at 225 nm attributed to the starting material and
(or) the resulting alcohol, in addition to the absorbance by I–.
But because the change in concentration of the three compo-
nents (organic iodide, alcohol, and I–) occurs at the same
rate, the first-order hydrolysis rate constant of the organic
Partition coefficients were determined in ~400 mL
temperature-controlled cells at ambient pressure. Prior to be-
ing added to the temperature-controlled cell, 200 mL of pu-
rified water was purged with high purity air for 20–30 min.
The vessel and water was conditioned at a given temperature
for several hours before the pure compound, or an aliquot of
a concentrated stock solution, was added to the aqueous
phase of the vessel. A concentration of approximately 1 ×
10–4 mol dm–3 was typically used for the partition coefficient
measurement. For iodoethanol and iodoethanoic acid, a
higher concentration (~1 × 10–3 mol dm–3), which is still be-
low saturation concentration, was used because of their low
volatility.
Aqueous samples (400–1000 µL) were analyzed using a
Waters high performance liquid chromatography system
(LC-18 DB column, detection by UV at 254 nm). Gas sam-
ples (200–500 µL) were analyzed by gas chromatography
(GC). The volatile species (iodomethane, iodoethane, and 2-
iodobutane) were detected using a Varian GC system fitted
with a photoionization detector, and an Altec polymethyl
siloxane column (30 m, 0.53 mm i.d., 5 µm film thickness).
The less volatile species required thinner phase capillary
© 2003 NRC Canada