Scavenger of Radiolytic Products in Liquid n-Hexane
The Journal of
Physical Chemistry, Vol, 82, No. 20, 1978
2149
Iodine as a Scavenger of Radiolytic Products in Liquid n-Hexane Marlja BonlfaEle" and Mllenko Vlatkovle "Rudjer BoSkovi6 Institute, Zagreb, Croatia, Yugoslavia (Received April 14, 1978) Publication costs assisted by " Rudjer BoSkovi6" Institute
The yield of HI formed from the radiolysis of a I,-n-hexane solution has been measured as a function of 1 2 concentration. Kinetic analysis indicates that HI is formed in the reaction in a homogeneous medium. GHI is reduced by the presence of hydrogen scavenger C2H4and electron scavenger SF6in the irradiated solution. The effect of the latter is interpreted by the decrease of the yield of H. atoms which are formed via ionic precursors. Study of the radiolysis of chlorobenzene solution in n-hexane has shown that GHCl does not change when iodine is present in the solution while the addition of alkyl iodide caused a considerable decrease. The experimental evidence indicates that the most significant role of iodine in n-hexane solution is to act as a radical scavenger (e.g., I2 + H. HI + I.) while its ability to scavenge electrons could not be proved by measurements of final radiolytic products.
-
The absorption of high-energy radiation in liquid hydrocarbons leads to excitation and ionization of the molecules,l which can generally be described as RH RH* (1) RH RH"' + e(2) followed by geminate neutralization RH+ e- RH* (3) and the stabilization of excited molecules. The most important chemical processes are the formation of alkyl radicals (R-), H- atoms (eq 4 and 5), and new stable RH* R1* + Rz. (4) R. H. (5) RH(-H2) + H2 (6) molecular products such as alkenes and molecular hydrogen (eq 6). In the absence of any solutes the unstable radicals may undergo any of several chemical reaction such as recombination, disproportionation, hydrogen abstraction, or a similar process which gives rise to stable products. If special solutes are added to the solution before irradiation, radicals can be scavenged by addition and converted to stable products suitable for identification of yield measurements. Radical scavengers should not interfere with the primary neutralization or energy transfer processes. Elementary iodine has been used in numerous studies as a radical scavenger because of the high rate constant for the reaction R. + I2 RI I. (7) However, its ability as an electron scavenger24 and a positive ion scavenger5i6has often been discussed, particularly when iodine was applied at higher concentrations. The total radical yield in irradiated solutions of iodine in hydrocarbons, measured by the disappearance of I2and expressed as G-iId2, was found to be independent of iodine M.' concentrations ranging from 5 X lo4 to 5 X Irradiation dose rates were relatively low as is usual for 6oCoirradiations. On the other hand, measurements of the individual products in irradiated Iz-cyclohexane solutions have shown a constant decrease in molecular hydrogen"1° and the parent cyclohexane radical yields6with increasing iodine concentration. The effect has been assigned to the reaction of elementary iodine with H. atoms H* + 1 2 HI + I. (8)
+
--
-
which otherwise may interact with themselves H. + He Hz +
-
+
+
0022-3654/78/2082-2 149$01.OO/O
-
or may abstract hydrogen from the solvent molecules H. RH H2 + Re (10)
+
HI has been identified as the radiolytic product in irradiated 12-hydrocarbon solution,3>11p12 the yield being dependent on I2 c~ncentration.~Attempts have been made, however, to explain HI as the product of the reaction with electrons3 in accordance with the equations
I2+ e-
-+
+
(9)
I-
+ RH"'
+
I-
-+
+ I-
HI
+ R*
(11)
(12)
The purpose of the present study was to determine more conclusively the role of iodine as a scavenger of radiolytical products in irradiated 12-n-hexane solution. Therefore measurements were done in the presence of various solutes which may have competed with iodine for primary radiolytical species. Some preliminary results of this study have been published earlier.lZ Experimental Section Materials. Spectroscopic grade n-hexane and research grade iodine from Merck, research grade absolute ethyl alcohol and puress. ethylene from Fluka were used as received. Biphenyl (BDH) was purified by recrystallization from benzene. Chlorobenzene (Riedel de Haen, techn.) was distilled through a 1.70-m helix-packed column and only the fraction at 131.6 OC was used. Sulfur hexafluoride (Kali-Chemie A. G. Hanover, techn.) was purified by degassing under vacuum at liquid nitrogen temperature. Sample Preparation and Irradiation. The sample solutions were prepared volumetrically, dried with P2O5, and degassed under vacuum. Gaseous solutes were added to the vessels already filled with outgassed frozen solution to 2f 3 of their volume. Therefore, the actual concentration of SF6and C2H4in the liquid phase (n-hexane) was not known exactly. Irradiations were performed in a 6oCosource at room temperature in the position where the absorbed dose rate waq 2 X lo1' eV/g as determined by Fricke dosimeter. The doses absorbed in n-hexane solutions were calculated by applying corrections for electron density. The radiation dose for each sample was chosen according to the concentration of iodine in order to prevent the dose effect on the observed yields. 0 1978 American Chemical Society
2150
The Journal of Physical Chemistry, Vol. 82, No. 20, 1978
M. BonifaEiC and M. VlatkoviC
A 1
TABLE I : Dependence of G Values for HI and for Iodine Disappearance on Secondary Solute Concentration a S
C,H,-C,H, C,H,-C,H, C,H,-C,H, C,H,-C,H, C,H,-C,H, C,H,-C,H,
SF, SF,
[SI M 9
lo-' 2.0 X lom2 3.5 X lo-' 5.0 X lo-' 7.5 X lo-' 1.0 X lo-' 1 X lo-* 3 x lo-' 1.0 X
G- 1 1 2Ia
GHI
0.89 0.89 1.10
i 0.03
* 0.03
5.2b
*
0.05 0.92 i 0.05 1.32 i 0.03 0.91 i 0.05 1.28 i 0.04
-0.4
4.8 4.3 5.8
99.9%) were dissolved in HN03, dried, and redissolved in molten LiN03-KN03 eutectic (43 mol % LiN03). Concentrations were determined by initial weighing of the oxide and the solid eutectic. In the case of 14'Pm, the assay was based on counting the p activity of aliquots of the aqueous nitrate s ~ l u t i o n .Spectra ~ were recorded on a prism-grating spectrophotometer in 1-cm path length cells at 150-160 "C. Several different concentrations were studied for each lanthanide element. The refractive index of the melt at 160 "C was n = 1.43. The intensity analysis followed the procedures outlined in similar work with the aquo ions;*the reported oscillator strengths represent the average of those obtained in the separate experiments. The absorption spectra were computer analyzed to resolve components of the complex band structure and integrate the curves. To measure the fluorescence lifetime, a xenon flashlamp of 750 ns (fwhm) duration was used as an excitation source. A 0.25-m monochrometer and a narrow pass dielectric filter 0 1978 American Chemical Society
