D. PERNERAND ROBERTH. SCHULER
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Hydrogen Iodide as a Radical Scavenger in the Radiolysis of Hydrocarbons1
by D. Perner and Robert H. Schuler Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania
(Received J w u a r y 4, 1966)
Hydrogen iodide has been used as a scavenger to measure the total radical yield in the radiolysis of saturated alkanes. Initially, the radicals react with the hydrogen iodide to produce iodine. As iodine builds up, competitive scavenging occurs between the two solutes present. It is found possible to treat the competition kinetics analytically and correct the iodine production curves for the reaction with iodine so as to obtain an accurate value for the initial yield. This yield is found to be equal to that of iodine disappearance in the corresponding iodine-alkane system. The ratio for the rate constants for the two scavenging reactions can be obtained from a detailed consideration of the growth and decay of iodine as the irradiation progresses. The value for the ratio ICR+I~/ICR+HI is found to be 2.0 in hexane a t room temperature.
In spite of the fact that hydrogen iodide was suggested some time ago2 as a scavenger to measure the over-all radical yield in hydrocarbon radiolysis, no such studies were carried out until those of Hanrahan and co-~orkers.~-5I n conjunction with studies in which tritium iodide has been used to produce labeled hydrocarbon products from the radicals,6 we have also examined the use of hydrogen iodide in some detail and present the results here. Since a second competing scavenger, iodine, is always present in this system, quantitative treatment of the data involves certain inherent difficulties. Even in the presence of these difficulties, however, this system has a number of advantages which include, as discussed below, certain chemical simplifications of the reactions due to hydrogen atoms, the measurement of product formation rather than reactant disappearance, and the production of stable labeled hydrocarbons through the use of deuterium and tritium iodide. The present results give measurements of radical yields which are quite comparable to those obtained with other scavengers and in addition indicate a ratio between the rate constants for reaction of radicals with iodine and with hydrogen iodide which is quite close to unity. Experimental Section Sample Preparation. Samples of Phillips Research grade hexane, decane, and tridecane were thoroughly dried on a high-vacuum line over freshly distilled sodium and distilled into the irradiation cell. Hydrogen The Journal of Physical Chemistry
iodide was prepared by dehydrating a solution of hydriodic acid (Baker 47% aqueous H I solution) with PzOs.Free iodine and other impurities were removed by distillation a t Dry Ice temperature. A measured gas volume of the hydrogen iodide was added to 3 ml of the hydrocarbon. The vapor volume above the liquid sample was -3 ml. With experience and care it was found possible to prepare HI solutions with no observable iodine ( [Iz]< M). Irradiation and Analysis. The irradiabion cells were 1-cm Suprasil (nonradiation coloring) quartz cells suitable for insertion into the spectrophotometer. These were connected to the vacuum line through graded seals. Irradiations were carried out inside a cylindrical Co6O source a t an absorbed dose rate in hexane of 9.3 X 1Ols ev g-1 hr-1. Standard Fricke dosimetry, with appropriate corrections for the electron density of the absorbing material, was used. ~~~~~
~
(1) Supported in part by the U. S. Atomic Energy Commission. Presented at the 151st National Meeting of the American Chemical Society, Pittsburgh, Pa., March 1966. (2) R. H. Schuler, J . Phys. Chem., 6 2 , 37 (1958). (3) B. N. Hughes and R. J. Hanrahan, ibid., 69, 2707 (1965). (4) I. Mani and R. J. Hanrahan, Abstracts, 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965; J . Phys. Chem., 70, 2233 (1966). ( 5 ) Hydrogen iodide has, however, been used in alkyl iodide systems; cf., R. J. Hanrahan and J. R. Willard, J . Ana. Chem. SOC.,79, 2434 (1957); D. L. Bunbury, R. R. Williams, and W. H. Hamill, ibid., 78, 6228 (1956); H. A. Gillis, R. R. Williams, and W. H. Hamill, ibid., 83, 17 (1961). (6) D. Perner and R. H. Schuler, to be published.
HI AS
A
RADICAL SCAVENGER IN HYDROCARBON RADIOLYSIS
The iodine concentration was determined during the course of the intermittent irradiation by measurement of the absorbance a t 525 mp with a Cary 14 spectrophotometer. Because many of the measurements were made a t low absorbances, the scale expansion available for use with the Cary instrument for absorbances up to 0.2 was used to provide increased accuracy in this region. The extinction coeficients for iodine used in this work were 914 M-I cm-I in hexane, 920 M-’cm-I in decane, and 935 M-’ cm-l in tridecane. For the runs a t -78” with hexane the samples were precooled in a Dry Ice bath and then kept a t this temperature in the source by a thin layer of Dry Ice. The samples were warmed to room temperature for absorbance measurements.
The Idealized Kinetics In this system the radicals produced by the radiation are expected, at conventional intensities and scavenger concentrations, to react with the hydrogen iodide added to the system to form a hydrocarbon and iodine. In the absence of complications, the initial iodine production yields should be equal to the iodine disappearance yield in the iodine-hydrocarbon systems. Because of an increased importance of scavenging of radicals by iodine and decrease in the hydrogen iodide concentration, one qualitatively expects that, as the radiolysis progresses, the iodine will a t first build up in concentration and then later on decrease. The detailed form of the iodine production curve will, of course, depend on the ratio of rate constants for the two scavenging reactions. Since this ratio is known to be of the order of magnitude of unity, an exact solution to the kinetic problem is desirable. Mathematical analysis of the problem is rather complex and one cannot, in fact, obtain an explicit solution for either the iodine or the hydrogen iodide concentration as a function of dose. As is shown here, a general, though somewhat indirect, solution of the problem does exist. Msni and Hanrahan4 have considered this same problem and carried out numerical calculations based on iterative considerations of the rates of the competing scavenging processes as the reaction progresses. The forms of the curves resulting from their calculations are virtually the same as those obtained by the present treatment. We consider here the idealized problem in which radicals produced at a constant rate GD GD
RH --+- R .
+ R’.
react competitively either with HI initially added to the system
2225
R. + H I - % R H + I . (1) or with molecular iodine produced as a result of the combination of the iodine atoms formed in (I).’
Re
+ 1 2 k 2 ’ R I + 1.
(IIP
From the appropriate kinetic expressions, it can be shown that the stoichiometry is represented by the relationship
GDt = 2( [HI10 - [HI] - [I211
(1)
where GDt is the total number of radicals produced, [HI10 is the initial hydrogen iodide concentration, and [HI] and [I21 are the hydrogen iodide and iodine concentrations present a t dose Dt. The number of radicals produced is thus measured by the number of equivalents of H I lost from solution minus the number of equivalents of iodine built up, with 1 mole of each reactant representing 2 equiv in the over-all reaction. Introducing this stoichiometric relationship into the appropriate rate expression, one obtains
Equation 2 is the basic differential equation which must be integrated to give the dependence of the hydrogen iodide and iodine (via eq 1) concentrations upon dose. The Limiting Cases. The two limiting cases, where either the rate of reaction I or I1 dominates the over-all kinetics, are trivial and can be treated by inspection. Where k l / k 2 >> 1 the radicals react exclusively with H I at a rate equal to GD and iodine builds up at a rate equal to GD/2. When the H I becomes exhausted, iodine will be present at a concentration of [HII0/2 and will then disappear a t a rate equal to GD/2. This behavior, which gives the respective lower and upper limits to the HI and I2 concentrations, is illustrated by the dashed curves of Figure l. Where kl/lc2 > kt (dashed curves). When k1
