THEREACTION OF SULFURHEXAFLUORIDE WITH HYDRATED ELECTRONS
4285
The Reaction of Sulfur Hexafluoride with Hydrated Electrons1 by K.-D. Asmus and J. H. Fendler Radiation Research Laboratories, Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pennsylvania (Received J u n e 17,1968)
16225
The reaction of SFP,with eaq-in irradiated aqueous solutions leads to F-, S04*-,and H30+ as stable products via an intermediate with oxidizing properties. The rate constant for this reaction has been determined t o be SF^+^^^-) = 1.65 x 1O1O M-’ sec-l. Since G(F-) = 16.55 was obtained in buffered (pH -6.5) solutioiis and the suggested mechanism requires the formation of 6F- from every molecule of SF6 in its reaction with eaq-, G(e,,-) = 2.76 was calculated. The yield of Sod2ion amounts to l/&(F-) and therefore is equal to the yield of eaq- which is scavenged by the SFe. The Sodz-ion results from the reoxidation of the sulfur from a lower oxidation state. Owing to the specificity of SF6 as an esq- scavenger and the high F- yield, this system can be conveniently used for the determination of relative rate constants in eaq-scavenging competition studies.
Introduction Sulfur hexafluoride is known to be one of the most inert inorganic molecules. Its high electron affinity, however, has rendered i t an extremely valuable specific electron scavenger in the radiolysis of gaseous2 and liquid3 hydrocarbons and of water vapor.4 The use of SF6 in these systems has been limited, however, to those competitive studies where its influence on such measurable products as N2 or alkyl radicals, arising from the reaction of NzO or alkyl halides with electrons, could conveniently be determined. Although of potentially great interest, no use has been made of SF6 as an electron scavenger in liquid water. Presumably such experiments were thought to be unfeasible because of the low solubility of SF6 in water and the paucity of knowledge concerning the radiation chemistry of this system. The aim of the present work was to obtain a mechanistic insight into the title reaction. We essentially achieved this by determining the yields of fluoride ion produced in irradiated, aqueous, air-free solutions of SF6 under various conditions by means of a fluoride ion activity electrode sensitive to 10-6 M F-.
Experimental Section SFs, N20, and C2H4 were purified by repeated distillation. Reagent grade HC104, 12, Na2HP04, KH2PO4, K4[Fe(CN)6],acetone, methanol, and 2-propanol were used without further purification. For the radiolysis of SFB solutions between p H 2 and 5 , the hydrogen ion concentrations were adjusted by adding small amounts of concentrated HC10, and were measured by a calibrated Orion Model 801 pH meter. The Na2HP04and KHzP04 buffer solutions were adjusted by concentrated HCIO4 and NaOH to the required pH. Acetone, I,, and K4[Fe(CN)6]solutions were prepared by either volume or weight. Two separate 60Co y-irradiation facilities were used for the steady-state experiments. The absorbed dose rates were determined to be 1.70 X lo4 and 1.0 X lo6
rads/hr by the Fricke dosimeter taking G(Fe3+) = 15.5.5 The absolute rate constant for the reaction sFs eaq- was determined by pulse radiolysis. Pulses from a 1.5-MeV Van de Graaff electron accelerator of a 0.5psec duration and 8-mA current gave doses of ca. 700 rads/pulse. The decay of the eaQ- absorption in the presence of SF6 at various concentrations in an air-free solution was followed at 720 nm. For the W o y-radiation experiments, a known volume, usually 4.5 ml, of triply distilled water was degassed by repeated freeze pumping to ca. torr. The required amount of SF6 and other gaseous additives were introduced by condensing them from a gas sample vessel of known PVT. After the sealed samples reached room temperature, the gases were equilibrated by vigorous shaking. The total volume of the vessel was measured after the irradiation, and the concentrations of the gaseous additives in the liquid phase were calculated from known solubility data6 using the equation?
+
1000a
po Vo
‘‘ = 22400 X 760 V , + aV1 [M1
(1)
(1) Supported in part by the U.S.Atomic Energy Commission, (2) G. 11. A. Johnson and J. M . Warman, Trans. Faraday Soc., 61, 1709 (1965). (3) J. M. Warman, K.-D. Asmus, and R. H. Schuler, Advances in Chemistry Series No. 82, American Chemical Society, Washington, D. C., 1968, p 25, and references therein, (4) G. R. A. Johnson and M. Simic, J . Phys. Chem., 71, 2775 (1967). (5) J. Weiss, A. 0. Allen, and H. A. Schwara, Proc. Int. Conf. Peaceful Uses Atomic Energy, Geneva, 1965, 14, 179 (1956). (6). CYSF~ = 0.0054 cm3 of SFs/cm3 of HnO at 25O and 1 atm (H. L. O 0.59 em3 Friedman, J . Amer. Chem. SOC.,76, 3294 (1954)); C Y N ~ = of N~O/cm3of HzO at 25O and 1 atm (W. F. Linke, “Solubilities of Inorganic and Metal-Organic Compounds,” I V ed, Vol. 11, American Chemical Society, Washington, D. C., 1965, p 794); and a c 2 ~=, 0.12 cm3 of CnHa/cm3 of Hs0 at 25O and 1 atm (A. Seidel, ”Solubilities of Organic Compounds,” I11 ed, Vol. 11, D. Van Nostrand Co., Inc., New York, N. Y., 1941, p 94). (7) K.-D. Asmus and A. Henglein, Ber. Bunsenges. Phys. Chem., 58, 348 (1964).
Volume 72, ,Vumber 12 Nocember 1968
K.-D. ASMUSAND J. H. FENDLER
4286 where c, is the molar concentration of the dissolved gas in the solution after equilibration, a is the solubility of the gas in cm3/cm3of HzO, poVois the amount of added gas in cm3 torr, V , is the volume of the gas phase in cm3, and VI is the volume of the liquid phase in cm3. The pressure in the gas phase under these conditions did not exceed 7 atm. Fluoride ion concentrations in the irradiated solutions of SF6 were determined by an Orion fluoride ion activity electrode (94-09) in conjunction with an Orion singlejunction reference electrode (90-01) ; the electrode potentials were measured by the Orion Model 801 digital pH meter. The limit of detection by this method is 10-6 M fluoride ion, and by using the Orion microsample dish determinations could be carried out on as little as 1.0ml of sample. In the pH region of 5-7 the electrode is completely selective for fluoride ion. At lower pH values, however, hydrogen ions form H F and HFz- which are not detectable by the electrode.8 When the pH of the SF6 solution was below 5, it was therefore adjusted after the irradiation by a known volume of 15% CH3COONa to ca. 5.5 (usually 1.0 ml of 15% CH3COONa added to 4.0 ml of irradiated solution). The electrodes were calibrated daily with standard sodium fluoride solutions which had the same ionic strength as the irradiated samples. These calibrations gave excellent straight lines. The fluoride ion electrode exhibited a 59.2-mV change in potential for each tenfold change in fluoride ion concentration and was reproducible to *0.2 mV. The electrode potential was, however, slightly different for solutions having different ionic strengths. The response time of the electrode varied between 3 and 10 min; the slowest response time was observed at low fluoride ion concentrations in the presence of buffers. The fluoride ion concentration could be determined within k l % . For each irradiated sample two independent fluoride ion determinations were carried out. K4[Fe(CN)6] solutions were stored and irradiated in dark vessels and used within 4 hr of preparation. Kh [Fe(CN)6]was analyzed spectroscopically at 420 nm (€420 1.0 X lo3 M - l cm-l)9 using the unirradiated Kd [Fe(CN)6]solution as the blank. The estimated accuracy of this determination is f.10%. Sulfate ions were determined by the barium-chloroanilate method using an 80% 2-propanol-water mixture as the solvent medium. The free chloroanilic acid was measured at 310 nm, which is the pH-independent isosbestic point (€310 1.53 X lo4 M-l crn-l).l0 Blank solutions were obtained by treating triply distilled water in a manner exactly analogous to that of the irradiated solutions. The estimated accuracy of this determination is f15%. H2O2 was determined spectrophotometrically by the The Hz yield was determined I 2 method at 350 nm." by collecting the gas from an irradiated sample and analyzing it by mass spectrometry. All irradiations The Journal of Physical Chemistry
and product analysis were carried out at room temperature.
Results Absolute Rate Constant for the Reaction SF6
+ eaq-.
I n neutral air-free solutions of (1.44-2.50) X 10-6 M SF6, irradiated with a ca. 700 rads/0.5-psec pulse, the hydrated electron disappears by a pseudo-first-order decay, since its concentration is negligible compared with that of the SF6. The half-lives of eaq- in such , from a log A(e,,-) vs. time plot, solutions, T ~ / ~derived are listed in Table I. From these data and the SFs concentrations, the absolute second-order rate constant eaq-) = (1.65 f 0.10) X 101OM-lsec-l was k(SF6 calculated. This value indicates that the reaction is diffusion controlled.
+
+
Table I : Rate Constant for the Reaction SFa eaq(Solution, Ar Saturated; Pulse Length, 0.5 psec; Dose per Pulse, 700 rads; 20")
1.44 1.81 2.50 a
2.90 2.35 1.65
1.66 1.64 1.68
Determined by Dr. A. Wigger a t Hahn-Meitner Institut.
Reaction Products. High yields of F- ions are produced together with H30+ and 504'- ions in reaction I. Figure 1 shows the F- yield as a function of dose in
SFe eaq- 4F-, H30+, 504'(1) irradiated buffered solutions of (1-2) X M SF6; the pH was adjusted to -6.5 by using lov3M iSa2HPO4 as a b ~ f f e r . ' ~ From ~ ' ~ the slope of this line G(F-) = 16.55 * 0.30 was calculated. Such a good linear yield-dose relationship (up to 10% conversion of SF6) is only obtained, however, in solutions whose pH was maintained between 6-7 by buffers; otherwise the H 3 0+ ions produced during the irradiation will successfully compete for eaq-. This fact was demonstrated IM SF6 in by irradiating a solution of 1.16 X 10-1 M HClO4 where all eaq- are converted to H . atoms H30+
+ eaq- +He + HzO
(11)
and accordingly no fluoride ions were detected (Table 11). A decrease in fluoride ion yields also was ob(8) K. Srinivasan and G. A. Rechnitz, Anal. Chem., 40, 509 (1968). (9) J. Rabani and G. Stein, Trans. Faraday Soc., 58, 2150 (1962). (10) H. N. S. Schafer, Ana2. Chem., 39, 1719 (1967), and references therein. (11) C. J. Hochanadel, J . Phys. Chem., 56, 587 (1952). (12) The phosphate ion under these conditions does not compete for e,, -.la (13) M. Anbar and P. Neta, Int. J. A p p l . Radiat. Isotopes, 18, 493 (1963), and references therein.
THEREACTION OF SULFUR HEXAFLUORIDE WITH HYDRATED ELECTRONS
'r
Dose
[io4 rods]
Figure 1. F- yield in irradiated buffered (pH -6.5) solutions of (1-2) X 10-3 M SFe as a function of absorbed dose. ~~
~~
~
Table 11: F- Yields in Irradiated Solutions Containing SFa and Competing Electron Scavengers (Dose, 2.87 X Competing scavenger
[Scavenger], M
...
... CHsCOCHa
NzO
H a 0 + (HClO4)
a
terminations were unbuffered in order to avoid foreign ion interferences. Under these conditions G(F-) = 13.1 and G(S0,2-) = 2.1 was obtained in a 2 X 10-3 M SF6 solution. The addition of lov2 M HzOz to the irradiated solution did not increase the 504'- yield. When 0.50 M CH3OH was added to a SFa solution, G(F-) was not affected, but G(S042-) decreased to zero, indicating that the OH radical participates in the production of s04'-. Only G(H2) = 0.48 and no H202 were found in neutral buffered solutions of 1.5 X low3M SF6. Competition between SFSand Other Electron Scavengers for eaq-. The addition of other electron scavengers to a solution of SF6 decreases the fluoride ion yield. G(F-) in solutions containing (1.0-1.8) X M SFe and various concentrations of CH&20CHB,N20, H30+ (HClO,), and 1 2 are listed in Table 11. The competition of SF6 and another electron scavenger, X, for esqis described by
rads)
1OS[SFa1,
M
~ 1 . 5
C(F -)
16.57"
x 10-4 x 10-3 x 10-3 x 10-3 x 10-3 x 10-3 4.61 x 10-4 1.44 x 10-3 2.70 x 1 0 - 3 5.42 x 10-3 6.08 x 10-3 1 . 9 1 x 10-4 3.37 x 10-4 1.10 x 10-3 2.29 x 10-3 3.23 x 10-3 1 . 0 0 x 10-1
0.97 1.10 1.61 1.13 1.60 1.23
14.90 12.47 12.26 9.52 8.79 6.49
1.19 1.24 1.20 1.74 1.29
12.49 9.54 6.82 6.28 4.70
1.53
13.03 10.08 6.92 4.11 3.27
1.74 X 8 . 7 1 x 10-6 2.96 x 10-4 5.23 x 10-4 7.84 x 10-4
1.05 0.99 1.03 0.94 1.02
3.33 1.00 2.00 2.22 4.00 4.44
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1.18 1.22 1.16 1.27 1.16
where G(F-) and G(F-), are the fluoride ion yields in the presence and absence of a competitor, X; kl and kx are the rate constants for eaq- with SFa and X, and [SFe] and [XI are the concentrations of the competing solutes. Rearrangement of eq 2 gives
A plot of l/G(F-) vs. [XI/ [SFe] therefore should give a straight line, with l/G(F-)o as the intercept and (1/ G(F-),) ( k z / k l ) as the slope. Figure 2 shows the experimental data according to this treatment. These straight lines extrapolate to a common intercept representing G(F-)o = 16.6, which is in agreement with the
... 14.05 11.84 7.66 6.17 5.04
Taken from the yield-dose plot (Figure 1).
served (Table 11) in irradiated solutions of SF6 when N20, which converts the eeq- to OH. radicals, was added
NzO $- eaq- +Nz
+ OH. + OH-
(111)
No decrease in G(F-) was observed, however, if 0.50 M CH30Hwas added to a 1.8 X M SFa solution. Since the Sod2-determination is much less sensitive than that for the F-, a high dose of 1.7 X lo4rads had to be used. Furthermore, the solutions for such de-
Figure 2. Dependence of l/G(F-) as a function of [X]/[SF6] according to eq 111. ( [SFe], (1-2) X M; dose, 2.9 X 108 rads). Competing electron scavenger, X: 0 , CHaCOCHs; 0 , NzO; A, HaO'; 0, 13. Volume 72, Number 12 November 1968
K.-D. A s m s AND J. H. FENDLER
4288
fluoride yield derived from the yield-dose plot. The ratios of the rate constants k,/kl, calculated from these slopes are listed in Table III.l4-l8 If k ( e , Q - + ~ ~ e-) 1.65 X 10'0 M-I sec-' is used, the absolute rate constants JC(Xfeaq -) obtained from these competitive experiments are in good agreement with those values determined by other workers. 14-18
not an effective electron scavenger,'3 the decrease cannot be explained by competition for the hydrated electron. However, OH. radicals and H . atoms, which readily react with CzH4, were shown to not participate in the fluoride ion production. Therefore, the slight reduction of G(F-) is tentatively attributed to the scavenging of an intermediate radical in the reaction of SFs with e,,- which might lead to fluorinated ethylene compounds.
Table 111: Relative and Absolute Rate Constants from
Discussion
Competitive Experiments in Irradiated Aqueous Solutions of SFa (k(SF2+eaq-) = 1.65 X 10'O M-' sec-l) Competing scavenger (X)
k(X+ea, -1
-h(X+e,, Present
CHsCOCHa NzO
0.39 0.57
6 . 1 X lo9 9 . 4 X log
H30+(HClOd)
1.57
2.6 X 1Ol0
a Reference 14. Reference 18.
3.08
sec-1-P
Lit. values
work
%%+eas-)
1%
-),
5.9 8.8 5.6 2.4 2.3 5.1
5 . 1 X 1Olo
References 15 and 16.
X X X X X X
Ref
IOB log IOg 1010 1O1O lozo
a
a b a
c d
Reference 17.
+ F- (IV) SF5. + 2H2O +OH* + H30' + SF4 + F- (V) SF, + 9H20 +503'- + 6H30+ + 4F- (VI) 503'- + OH., SF5., H2Oz sod2- (VII) SF6
4-eaq- +SF6*--+
SFj.
-+
Reactions of an Oxidizing Intermediate. The reaction of SF6 with hydrated electrons leads to an intermediate with oxidizing properties. To demonstrate this fact, buffered (pH -6.5) solutions containing SF6 and I
