Sulfur Exchange between CS2 and Ss
41 9
Radiation-Induced Sulfur Exchange between Carbon Disulfide and Elemental Sulfur Francis Johnston Department of Chemistry, University of Georgia, Athens, Georgia 30602 (Received September 3, 1974) Publication costs assisted by The University of Georgia
When CS2 solutions containing elemental sulfur labeled with 35S absorbs ionizing radiation, sulfur exchange between the two species is observed. The tracer also becomes incorporated in a polymer formed in the radiolysis. The G value for the exchange increases linearly with sulfur concentration above 0.01 g-atom 1.-l and is 0.23 sulfur atoms exchanged per 100 eV of absorbed energy a t 0.06 g-atoms 1.-l of elemental sulfur. The data indicate that the exchange is partially a spur reaction with an additional contribution from reactions of the CS molecule with elemental sulfur.
Introduction The decomposition of carbon disulfide resulting from the absorption of ionizing radiation has been described by Janssen, Henglein, and Perner.l The reaction products are elemental sulfur and a polymer having an empirical formula (CS,), in which 1 < m < 2. The value of rn was found to be dependent upon the conditions under which the radiolysis occurred. For liquid CS2 a t 2 7 O , the value of m was approximately 1.3. The net chemical change may be represented by
cs,
-
i/n(csm)n + PS,
(1) The stoichimetric requirement for p is 80 m = 2. The limitations on m require that 0 < p < 0.12. The G value for the overall decomposition was approximately 0.8 at relatively low doses and reached a limiting value of approximately 0.5 with increasing total dose. As possible reaction processes, the above workers suggested the following steps involving nonionic intermediates:
cs, 2cs
s cs + s8 (CS),, + s8
---
+
cs + s
(2)
(CS), (CS),
(3)
‘/*s*
(4)
cs, + s, (csm#)n# + s,
(5) (6)
The occurrence of reaction 5 was indicated by a decrease in CS2 decompasition yields with increasing sulfur concentration, and reaction 6 by the stoichiometric composition of the polymer. Insaddition, the ionic species CS2*+, CS+, and CS2- were suggested as possible intermediates in the reaction, the first two on the basis of mass spectrometric evidence and the latter because of the scavenging ability of CS2 for solvated electrons. This report describes the results of experiments in which CS2 was irradiated in the presence of elemental sulfur labeled with 35S. Total doses were substantially lower than those used in the work described in ref 1 and yields of decomposition and sulfur production were not measured. Experimental Section Materials. The CS2 used in the experiments was Fisher Spectranalyzed material which had been subjected to preirradiation with 6oCoy rays to the extent of 8-15 X lozo
eV ml-l. Before use in the exchange experiments, this product was fractionated and a middle cut used in preparing the reaction mixtures. The sulfur was a product recrystallized from reagent benzene. 35s was obtained commercially as elemental sulfur in benzene. Stock solutions of radiosulfur of known specific activity were prepared by adding weighed amounts of carrier sulfur to the benzene solution, evaporating to dryness, dissolving, and diluting to a fixed volume with purified
cs2.
Procedure. Samples (4.2 ml) of reaction mixtures consisting of solutions of labeled sulfur of known initial concentration and specific activity were deaerated by freezepump-thaw cycles and sealed off in uacuo in Pyrex reaction cells of approximately 6-ml volume. Radiolyses were carried out in a Gammacell-200 6oCo source in which the dose rate in CS2 was, during the period over which the experiments were carried out, 0.965-1.15 X 10l6 eV ml-l sec-l. Irradiations were at ambient source temperature which was 35 f 2’. Following irradiation, the reaction mixtures were frozen, opened, placed on a vacuum line, and the CS2 was separated from sulfur and polymer by distillation into a tube immersed in liquid nitrogen. 35S Counting. The 0’s from 35S (E,,, = 0.167 meV) were measured by liquid scintillation counting. Samples (100 1) of the reactant CS2-labeled sulfur mixtures or exchanged CS2 were dissolved in 20-ml volumes of scintillator solutions and counted in either a Beckman P-MATE I1 or LSlOOC 0 spectrometer system. The scintillator solutions were conventional mixtures consisting of 0.4% by weight of PPO (2,5-diphenyloxazole) and 0.01% by weight of POPOP (1,4-bis[2-(5-phenyloxazoly)]benzene)in Reagent grade toluene. When the activity in the polymer was required, it was measured by difference between that in the total reaction mixture and that in the reaction mixture following irradiation and separation of the polymer by centrifugation. Results and Discussion Equation 1 describes the overall chemical change occurring upon the irradiation of CS2. When the irradiation is carried out in the presence of elemental sulfur labeled with 35S, the radiosulfur becomes incorporated both as CSS* and in the polymer. These processes are represented by The Journal of Physical Chemistry, Vol. 79, No. 5, 1975
420
Francis Johnston
-.
.
0
.04
.02
.06
(SI.g atom 1-1
Figure 2. G values for the exchange of sulfur between CS2 and as a function of sulfur concentration. 12
0
24
Sa
36
ev cc-1 x 10-IS
(a) .12
.08
.04
0
I
1
40
ev c c 80 -1x10-~~
160 1
I
120
(bJ
Figure 1. The fraction of total 35Sradioactivity appearing in the form of CS2 as a function of absorbed dose for several initial sulfur con, 0.0050 g atom i.-‘; (b) 0, centrations: (a) 0, 0.0020 g atom i.-’; . 0.042 g atom I.-’; W , 0.060 g atom I.-’.
-
cs, + s* s, -css* CS, + S* S?
+ s8
( 7)
i/nCSm(i-a~S*am + (P + 1 ) s ~ (8)
a represents the fraction of sulfur in the polymer resulting from reactions such as (6). Since the radiosulfur is both diluted during the radiolysis by sulfur formed from initially untagged CS2 and consumed by incorporation into the polymer, the exchange reaction cannot be described by the usual first-order treatment. The relationship between the rate of reaction 7 and the observed rate of appearance of labeled sulfur in CS2 may be derived in the following ~ a y . Let ~ - R,(t) ~ be the rate, in g atoms 1.-l sec-l, a t which sulfur atoms are being exchanged between SSand CS2. Since the sulfur concentration is increasing during the radiolysis, this rate will be a function of time. Let &(t) represent the rate, in M sec-l, a t which CS2 is decomposed to polymer and elemental sulfur. The amount of CS2 decomposed at time t is then, p ( t ) = J”otRd(t) dt. The fraction of exchanges in which a labeled sulfur atom is transferred from the elemental form to CS2 is x/(S) where x is the concentration, in g atoms l.-l, of 35S in the Sa form and (S)is the total concentration of sulfur as Sa. The fraction of exchanges in which the labeled atom is transferred in the opposite direction is y/2(CS2), where y is The Journal of Physical Chemistry, Vol. 79, No. 5, 1975
ev
cc-1x 1049
.,
Figure 3. The fraction of total 35Sradioactivity incorporated into the polymer as a function of absorbed dose: 0, 0.0020 M sulfur; 0.0050 Msulfur.
the g atomic concentration of labeled sulfur in the CS2 form and (CS2) the total molar concentration of CS2. The net rate of appearance of 35Sin the CS2 form will be the total exchange rate multiplied by the difference between these two fractions minus the rate at which this species is removed from the CS2 form by reaction 8. With a and b representing initial gram atomic concentrations of sulfur in the elemental and CS2 forms, respectively, this net rate is
-
d t = R’(t){ [ , a dY
X
+
8/3p(t)] -
With z as the concentration of the label in the polymer form
Since the concentration of CS2 remains essentially constant
CS2 decomposition rates are not sufficiently well defined at low doses and at varying initial sulfur concentrations to
Sulfur
Exchange between CS2 and Sa
421
allow the direct use of the above expressions. Under initial reactant conditions, however, eq I11 and I1 reduce to
and
Since ylxo and Z / X O are the fractions of total radioactivity occurring as CS2 and polymer, Fcsz and F,, respectively, then
R,(t)o = a(dF,s2/dt)o
(VD
aR,(t)o = a(dF,/dt)o
(VIB
and Figure l a and b shows plots of Fcs2 us. dose for several experiments involving diffeient initial sulfur concentrations. At initial sulfur concentrations above 0.01 g atom l.-l, these plots are linear to at least 15 or 20% exchange. Under these conditions, the exchange behavior approximates that in a system of constant composition, i.e.5
R = - - ab
a + b
d In (1 - F ) dt
(Vim
For small values of F
Since, for this system, b >> a, eq IX reduces to the same form as eq VI. Dose rather than time is used as a coordinate because source decay was significant during the period over which the experiments were performed. G values corresponding to the initial slopes of such plots are summarized in Figure 2. In Figure 3 are shown plots of Fp us. dose for initial sulfur concentrations of 0.002 and 0.005 M. The corresponding values of c&d are 0.12 and 0.18, respectively. The exchange occurs with a relatively small temperature coefficient. The results shown were obtained at 35”. In one series the exchange was followed a t 55O in a system initially 0.002 M in sulfur. The resulting G value for exchange was 0.15 as compared to 0.08 a t the lower temperature. The corresponding apparent activation energy is 6 kcal/mol. The exchange rate data do not afford a sufficiently sensitive probe to define the intermediate species involved in the reaction. They do, however, indicate that at least two
distinct processes are involved in the exchange of sulfur between CSz and Sg. The linear increase in the exchange yield beyond approximately 0.01 M sulfur indicates a spur resction which probably involves ionic and energized intermediates. Since the maximum electron per cent of solute in the systems studied was 0.15, direct adsorption of energy by elemental sulfur can be discounted as a significant factor in the reaction. From 0.01 to a t least 0.06 M sulfur, a nonspur reaction contributes 0.10 to the overall G value for the exchange. Such a process very likely involves the CS molecule in a scavenging reaction such as (5). The formation of “stable” CS molecules by the dissociation of CSz has been observed in an electrical discharge: upon exposure to a heated tungsten ribbon: upon flash photolysis,8 by shock wave heating,g and in a thermal graphite cell.lo It is also an intermediate in the operation of CSz-Oz.laser systems.11J2 The exchange results indicate that at sulfur concentrations of 0.01 M and above, CS molecules escaping dimerization (or higher ordered combinations) in the spurs are effectively scavenged by elemental sulfur. Such a reaction model predicts a higher polymer yield a t lower sulfur concentrations and is consistent with the exDerimenta1 obserwations described in ref 1. If it is assumed that the G value of 0.8 for the decomposition is appropriate for the conditions of these experiments, the results illustrated in Figure 3 indicate that at initial sulfur concentrations of 0.002 and 0.005 M , approximately 15 and 23%, respectively, of the sulfur contained in the polymer results from reactions of growing polymer units with elemental sulfur, e.g., reaction 6. It is probable that all of the “extra” sulfur in the polymer ( m - 1 molecules per carbon atom) results from reactions of this type.
References and Notes (1) 0. Janssen, A. Henglein, and D. Perner,
Z. Naturforsch. B, 19, 1005 (1964). (2) C. P. Luehr, G. E. Challenger, and B. J. Masters, J. Amer. Chem. SOC., 78, 1314 (1956) (3) R. D. Borle, Jr., and F. J. Johnston, Radiat. Res., 15, 724 (1961). (4) F. J. Johnston and J. F. Hinton. J. fhys. Chem., 67, 2812 (1963). (5) G. Friedlander, J. W. Kennedy, and J. M. Miller, “Nuclear and Radiochemistry,” 2nd ed, Wiley, New York, N.Y., 1964, pp 196-198. (6) P. J. Dyne and D. A. Ramsey, J. Chem. fhys., 20, 1055 (1952). (7) L. P. Blanchard and P. LeGoff, Can. J. Chem., 35, 89 (1957). (8) A. B. Callear and R. G. W. Norrish, Nafure(London),188, 53 (1960). (9) S.J. Arnold, W. G. Brownlee, and G. H. Kimball, J. fhys. Chem., 74, 8 (1970). (10) T. C. Peng, J. fhys. Chem., 78, 634 (1974). (11) G. Hancockand I. W. M. Smith, Chem. fhys. Lett., 3,573 (1969). (12) S. J. ArnoldandG. H. Kimbell, Appl. fhys. Lett, 15,351 (1969).
The Journal of Physical Chemistry, Vol. 79, No. 5 , 1975
