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Ned E. E. I. du Pont d e Nemours and Co. Aiken, South Carolina 29801
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The Radiolysiir of Air-Saturated Solutions of Iodine An undergraduate experiment involving an isobestic point
This article describes a simple yet oomprehensive experiment in radiation chemistry suitable for an undergraduate physical chemistry laboratory course. The experiment can be performed easily in 3 hr and does not require special sample purification or preparation procedures that are often necessary in this field. Instruments necessary to perform the experiment are a visual recording spectrophotorneter and an intense (-lo5 mds/hr) radiation source such as an Xray generator or a QCogamma-ray source. The experiment compares the rates of the radiation-induced disappearance of iodine and the subsequent appearance of iodine monochloride in air-saturated CCII-I2 solutions. The rates are compared in two ways, both of which utilize the visible absorption spectra of the irradiated solutions. First, the molar absorptivities of Izand IC1 at the isoshestic point in the spectra are used to calculate the ratio of the two rates directly. Second, this result is compared with the ratio obtained from the slopes of the respective yield-dose curves for Izand IC1. Procedure
Sufficient care must be exercised to prevent the evaporation of the CCl, during the irradiation. The spectra in Figure 1were obtained with a Cary Model 11 recording spectrophotometer. After each scan, the chart paper was rethreaded so that all the traces could he recorded on the same portion of the paper. Results
The isosbestic point in the spectra a t 464 mp indicates the presence of two species that have overlapping absorption spectra and obey Beer's law (8). The spectra of the two solutions irradiated to the largest doses exhibit maximum absorbance at 460 mp, in agreement with that reported for IC1 (9, 4), which indicates that one of the species is ICI; the other is 1%. Since the spectra of the two aliquots'irradiated to the largest doses are coincident, all of the I2 can be reasonably assumed to have been converted to ICl in these samples. This assumption can be tested by calculating with eqn. (1) the molar absorptivity for ICl at the 460 mp absorption peak and comparing it with the value in the literature.
A standard solution M) of TI in CClr is prepared using resublimed I*and reagent grade CCL that has previously been dried by standing overnight over calcium sulfate. Five milliliter aliquots of this solution are then transferred to clean, dry irradiation cells (test tubes closed with tapered joints or any ,other closed vessels) and exposed to successively increasing doses of ionizing radiation a t constant intensity. The usual radiation protection precautions associated with intense radiatio~isources (1) should be followed during these irradiations. After irradiation, the visible absorption spectra between 400 and 550 mp of each of the aliquots and of an unirradiated sample are determined and plotted as in Figure 1. The color of the irradiated solution is stable for a t least 3 hr. The radiation doses are increased until the spectra of the last two aliquots are identical. For a 10W3M 1% solution this will require a dose equivalent to 1.3 X lo5rads in water. A 23,000 Ci W o "Gammacell" 220%source was used to irradiate the samples whose spectra are shown in Figure 1. The information cont,ainedin this article was developed during the course of work under Contract AT(07-2)-1 with the U.S. Atomic Energy- Commission. This work is being sponsored by the Division of Peaceful Nnclear Explosives. lAtomic Energy of Canada, Ltd., Commercial Products Div., Ottawa, Canada.
722 / Journal of Cherniccrl Education
Wavelength, m p Figure 1. Visible obrorption spectra of on irrodioted oir-saturated CCII-brolution.
I n eqn. (I), q a , r s o is the molar absorptivity of ICl a t 460 mp, ( A ~ s is) the ~ ~ absorbance (in a 1-cm cell) at 460 mp of the coincident spectra of the two aliquots, and [I,], is the original concentration of iodine. The factor of 2 in the denominator results from the assump tion that each mole of I, forms two moles of IC1. I n the example in Figure 1, the result is (0.55)/(2)(1.84 X 10-3 M) (1 cm) or 150 *3 1 mole-' cm-'; this value agrees very well with that found by Buckles and Mills (5). This result confirms that all the I, has been converted to IC13and indicates that the spectra of the two aliquots irradiated to the largest doses are due solely to IC1. From these spectra, the molar absorptivity of IC1 can now be calculated a t any desired wavelength. Similarly, the molar ab~orpt~ivities of I2 a t any wavelength can be calculated from the spectrum of the unirradiated solution and the original concentration of 1 2 . Since these values are based on solutions of only one concentration, ,their accuracy may be low, depending upon the care in obtaining the spectra and preparing the original solution. I n the experiment illustrated in Figure 1, an analytical balance accurate to 0.1 mg was used to weigh the 12,which was then dissolved in exactly 50 ml of CCln. The molar absorptivities of I z calculated from the spectra of the solution agreed very well with those reported by de Maine (6). The ratio of the rates of the disappearance of 1% and the appearance of IC1 can now be calculated using the molar absorptivities of the .two compounds .at the isosbestic point.4 The change in the total absorbance at a given wavelength, A, with irradiation time, t, is given by eqn. (2).
+
A(Abs)h/a = t,!,hA[Z%]/M ncrhA[ICl]/At
(2)
At the isosbestic point (X = 464 mp), the absorbance remains constant as the solution is irradiated. Therefore, at this wavelength A(Abs),/At is equal to zero and eqn. (2) can be rearranged to give eqn. (3). A[ICll/A[I~l =
(3)
-er3,demm
The negative sign indicates that the concentration of one species is increasing while that of the other is decreasing. At 464 mp, the calculated molar absorptivities for ICl and I2are 149 1 mole-' cm-' and 304 1mole-' cm-I, respectively, giving a ratio of 2.0. This result indicates as expected that the IC1 appears twice as fast as the Iz disappean. As mentioned earlier, this ratio of rates can also be determined by comparing slopes of the yield-dose curves for I, and ICl. I n order to determine the concentrations of each of the two species in the irradiated aliquots, two simultaneous equations describing the total absorbance a t two different wavelengths can be applied in
T h i s conduaion has also heen supported by demonstrating that no organic iodides are formed during the irradiation. Solntions containing '"1 as a tracer were irradiated. At all doses, greater than 99%of the activity co$d he extracted out of the CCL using aqueous sulfite solotion. 'The cause of this invariant paint ia not included in the usual (ICI). definition of an isosbestic point (8). The eum 2(L) rather than the total concentrrttion of the absorbing species, is eonstant. The dme rate in the dosimeter is corrected to t,hat in CCI
