3631
NOTES
Radiation-Induced Oxidation of Liquid Sulfur Dioxide Containing Oxygen and Water1 by Siegfried Schonherr, * Helmut Seidel, Bergakademie Freiberg, S e k t w n Chemie, Arbeitsgruppe Reaktionsverhalten und Syntheseprinzipien, Freiberg, East Germany
and Walter G. Rothschild* Department of Chemistry, Scientific Research S t a f f , Ford Motor Company, Dearborn, Michigan 48181 (Received February 1, 1971) Publication costs assisted by the Ford Motor Company
Various recent publications on the radiation chemistry of liquid sulfur dioxide and its solutions make use of a simple radical mechanism to account for the stoichiometry of the products. This mechanism
so2
so + 0
(1)
which, as proposed, forms 0 and SO biradicals in equimolar amounts,2 has afforded a satisfactory explanation of reaction products and reaction steps in various simple liquid sulfur dioxide systems. 2-5 It is quite obvious that other mechanisms, which are equally acceptable in describing the product stoichiometry, can be written down. Furthermore, from the nature of liquid SO2 it is evident that ionic reactions must be considered to fully explain the radiation chemistry of this s o l ~ e n t . ~Yet, * ~ remembering that knowledge about the radiation chemistry of this system is still sparse, it seemed more profitable to push instead the proposed radical mechanism to its limits to ascertain where it may fail and where further work-hopefully using more sophisticated techniques than available to us-is indicated. We have therefore studied, on the basis of eq 1 and some of its logical subsequent step^,^,^,^ the effect of precisely controlled oxygen atmospheres and of exactly known amounts of water on the 6oCo y-ray induced product formation in the respective liquid sulfur dioxide systems. We expect that initially added oxygen will set up competition mechanisms involving primary (in our mechanism) radicals. Addition of water, on the other hand, would permit us to test whether the complications introduced by active species arising from water decomposition6J can be accounted for in the framework of our simple reaction mechanism.
Experimental Section Details of the purification, irradiation, identification and analytical procedures have been previously des~ribed.~Jl8Gas atmospheres and corresponding solution concentrations were precisely controlled by accurate gas mixing, standard analysis, known absorption coefficients,Q and continuous gas saturation of the solutions. Irradiation temperatures were maintained by using conveniently boiling liquids contained in the cooling jacket of the irradiation vessel (-40.8", Freon 22; -33") ammonia; -23.7", CH3C1). The wet liquid SO2solutions mere analyzed for water using conventional techniques; 10 the measured solubilities agreed well with literature data. l1 Dithionic acid (HzSz06) was determined by a titration rnethod.l2 In all cases we convinced ourselves that the applied analytical techniques were reliable within a few per cent error. The dose rate was measured with the help of the Cu-Fe dosimeter;13 it amounted to 0.281 X 10'' eV ~ m sec-I - ~ in the liquid SOz (at - 23.7'). All yields are reported in milligrams per cubic centimeter or in G values (number of molecules oxidized or reduced per 100 eV absorbed in the solution). Significant amounts of dark reactions were not detected. Results and Discussion a. Anhydrous Liquid Sulfur Dioxide-Oxygen System. The points in Figure 1 show the experimental G values of so3 formation as a function of dissolved oxygen concentration, [02], in liquid SO2 which was continuously saturated by 02-Ar mixtures. For relatively low [ O Z ] , the data are seen to extrapolate to G(S03) = G ( 0 ) = 1.35, the radical yield in the deaerated systemU2 For relatively high [ 0 2 ] , G(SO3) attains a value of ~ 2 . 8 , (1) Taken from the Diploma Dissertation of H. Seidel, 1968-1969. (2) W.G. Rothschild, J . A m e r . Chem. Soc., 86, 1307 (1964). (3) W. G. Rothschild, paper 148, Division of Physical Chemistry, 149th National Meeting of the American Chemical Society, Detroit, Mich., April 1965. (4) W. G. Rothschild, J. Chem. Phys., 45, 3594 (1966). (5) S.Schonherr, R. Schrader, and P. Mahlitz, 2. Anorg. Allg. Chem., 365, 262 (1969). (6) R. Schrader and 8 . Schonherr, Naturwissenschaften, 48, 569 (1961). (7) R. Schrader and S. Schonherr, 2. Anorg. Allg. Chem., 331, 289, 298 (1964). (8) P. Mahlitz, Diploma, Dissertation, Freiberg, 1968. (9) M. R. Dean and W. S.Walls, I n d . Eng. Chem., 39, 1049 (1947). (10) E. Eberius, Chem. Rundsch., 7, 337 (1954). (11) D. Murakami and N. Tokura, Bull. Chem. Soc. Jap., 31, 431 (1958). (12) R. Lang and H. Kurtenacker, 2.Anal. Chem., 123, 81 (1942). (13) E. J. Hart, Radiat. Res., 2, 33 (1955).
The Journal of Physical Chemistry, Vol, 76, No. 88, 2071
NOTES
3632
m G(S03) 2.0
t
-40.8OC
9-4
2c
5 -
\
0
E
I .6
l-23.7'C
I
.
I
.
.2
I
.
I
.
.4 .6 0,-Concentration
I
.
I
.8 1.0 (mrnole/l)
.
I
8 1
1.2
which is twice the radical yield of the air-free system, since now SO is converted into an oxidizing species6
+ so 0 + so2 0 + so2 +SO8
(2)
--j.
(3) We therefore expect that for intermediate [OZ]a competition mechanism is taking place. On the premise of our radical reaction scheme, the simplest such mechanism is14
so + +so2 + 0 so + so so2 + s
-
1
0.8
1
1.8
1
1
1
1
3.2
2.4
1
1
4.0
lozoev/cm3
Figure 1. Radiation-induced 100-eV yields of SOSformation in anhydrous liquid sulfur dioxide as a function of oxygen concentration. Dose rate: 0.289 X loi7 (-40.8') and 0.281 X lo" eV cm-a sec-1 (-23.7'). Each point represents the slope of the (linear) plot of mg/cma SO3 us. dose at a constant [O,].
0 2
Ar- satd
W 02-satd
J
Figure 2. Radiation-induced HzSz06 formation in water-saturated liquid sulfur dioxide. The upper curve shows the HzSzOeformation in a medium which was Ar-saturated prior to irradiation, the lower curve shows this for a medium which was 02-saturated prior to exposure.
2.4
2.0
-
-
I
I
I
- 40.Q'C
I
I
I
I
I
-
1.6
-
-
-
(2)
0 2
(4) (We always detected elemental sulfur at low [OZ].) --f
Setting up the corresponding steady-state equations, we obtain15 d[SO,]/dt
aG(S03)R =aG(O)R
+
Developing the expression under the square root into a power series for the cases of low [02]and for high [02], eq 5 approximates to
aG(0)R
d [SOl]/dt
+
IC2
[Oz](aG(O)R/2k*) ''' (low [Ozl)
(6)
and
5
formation in Figure 3. Radiation-induced HzSO~ The solution was water-saturated liquid SO2 a t -40.8'. continually saturated with pure 02. The yield during the linear portion corresponds to the highest G(S0s) in Figure 1.
b. Wuter-Containing System. The principal difference as compared to the reactions in the anhydrous system is (i) appearance of dithionic acid5 (HzSzOe), and (ii) SOa formation by an initial short-chain p r o c e ~ s . ~ Figure 2 shows the rate of H2SzOaformation in wet liquidS02 (0.49 wt % HzOat -23.7', 0.27% at -40.8') which was saturated with wet O2 and Ar, respectively, prior to irradiation. It is evident that the formation of H2Sz06is suppressed in the presence of 02, We rationalize this by the stepsl8
HSO3 (high P 2 l ) (7) if the series developments are broken off after the term to first order and second order, respectively. The rate expressions reproduce satisfactorily the data shown in Figure 1, using a rate constant ratio of h4'"/hZ
-
0.47
f
0.15
1. -'" see'")
(8)
(We obtain this value, according to eq 6, from the initial slope of the curve in Figure 1.) The Journal of Physical Chemistry, Vol. 76, N o . 28, 1971
+ HSOa
4H2SzOa
(9)
and (14) I n our experiments, [SOS]/[SOZ] N 0.001 at the highest dose. In such a case, the back reaction SO8 0 + SO2 4- 0 2 amounts to G ( 0 ) = 1.35 less than 1% of the forward step (3); thus G(S0) (see ref 2). (15) The factor a amounts t o 10-2/N ( N = Avogadro's number) = 0.166 X 10-26,where dose rate R is in eV I.-' sec-1 and concentrations in mol/l. (16) R. Schrader, S. Schonherr, and B. Fritsche, 2. Anorg. Allg. Chem., 339, 67 (1965).
+
3633
NOTES
There are various reaction steps which, together with step 10, could conceivably consume O2 by a chain mechanism, for instance5
H
+
0 2
+HOz
+ SO2 + + OH OH + SO2 +HSOa HS06 + SO2 +2SO3 + OH HO2
so3
Acknowledgments. We are grateful to Professors G. Ackermann, S. Herzog (Freiberg), and L. Kevan (Wayne State University) for their suggestions and helpful comments.
(11) (12)
Thermodynamic Data for the
(13)
Water-Hexamethylenetetramine System
(14)
We have observed, as displayed by the data in Figure 3, that this is indeed the case. However, the chain mechanism involves, as is evident from the low chain yield, rather short chains. After an initial rise, G(SO8) soon drops to a value close to that in the corresponding anhydrous liquid sulfur dioxide-oxygen system (see Figure 1). It is useful to remark here that the yield of SO3in this system is too high to arise by direct interaction with oxidizing species from the radiolysis of water (the water absorbs about 0.3% of the total dose). Nor does it turn out that the product formation in wet liquid SO2is simply a superposition of that of the anhydrous substance and that derived from the aqueous system-even taking the highest (initial) yield of H2S04 formation reported. l7 We find that various observations in the water-containing systems are difficult to describe quantitatively or to explain at all on the basis of our simple mechanisms. For instance, we have measured that the yield of HzSz06formation is 1.6 at -40.8’ (see Figure 2) but 2.5 at -23.7’. We see no way to rationalize this temperature dependence, which is outside the experimental error (and shows a consistent trend), with the help of the above reaction steps. Furthermore, we unexpectedly find some initial shortchain oxidation of SO2to SO3in wet, air-free liquid SO2 a t 16” (vapor pressure 2.9 atm). In fact, the G value of this short-chain oxidation process (-17), as well as its duration, were the greatest observed in our experiments. We are therefore led to the conclusion that the radiation-induced products in anhydrous liquid sulfur dioxide systems can be explained satisfactorily on the basis of a few radical steps involving 0 and SO, the “primary” decomposition products. On the other hand, in the presence of water the mechanism no longer accounts quantitatively for the reaction products nor (even qualitatively) predicts the observed temperature dependence.
+
(J7) For instance, consider formation of HzSOa in wet, 02-saturated liquid SOZ a t a dose of 0.25 X lozoeV cm-8. The observed yield amounts to 0.4 mg/cma (see Figure 3). Regarding the aqueous “part system” (0.42 vol %) as O r and SOz-saturated water, we compute a maximum yield of 0.062 mg/cma HzSOa, based on G(HzSO4) = 510 (see ref 7), for it. For the anhydrous liquid SOz, we compute a yield of 0.10 mg/cm3 SOS, based on G(S0s) = 2.6, for this dose. These individual contributions do not add up to the observed H z S O ~ yield.
by F. Quadrifoglio, V. Crescenzi,” A. Cesho, and F. Delben Istituto d i Chimica, UniversitQd i Trieste, Trieste, Italy (Received February 3, 1971) Publication costs assisted by the Consiglio Nazionale delle Recerche
The physicochemical properties of hexamethylenetetramine (HMT) aqueous solutions have attracted considerable attention. 1-4 In terms of certain features exhibited by these solutions, HMT has been considered as a “structure-maker” solute in water.6 The results reported here should help to gain a more complete physicochemical picture of the water-HMT system and hence also to provide additional, though indirect, information on the influence of HMT on the structural organization of the solvent. Our work has been concerned with measurements of Table I: Heat of Dilution of HMT Solutions a t 25’a Pobsdv 111.2
0.1809 0.3118 0.3277 0.5625 0.8509 0.9671 1.3447 1.6824 2.0680 2,6044 2.8076 4.4004
0.1708 0.2984 0.3120 0,5457 0.8029 0.9078 1,2534 1.5551 1,9486 2,4125 2.6828 4.0973
crtl
x
10-1
2.012 4.602 5.311 8.644 39.84 56.71 117.2 207.0 242.2 457.7 333.1 1252.0
AH7 cal/mol
AH/Am
2.655 3,888 4.426 4,886 13.86 16,998 28.36 40.91 38.63 64.65 42.02 113.3
262.2 290.1 281.9 290.8 288.7 286.6 310.6 321.4 323.6 336.9 336 7 373.8 I
=All measurements have been carried out using a LKB batch-type microcalorimeter. The purification of HMT has been made as already described.28
(1) J. F. Walker, “Formaldehyde,” Reinhold, New York, N. Y., 1944.
(2) (a) V. Crescenai, F. Quadrifoglio, and V. Vitagliano, J. Phys. Chem., 71, 2313 (1967); (b) Ric. Sci., 37, 529 (1967); (c) L. Costantino, V. Crescenzi, and V. Vitagliano, J. Phys. Chem., 72, 2588 (1968).
(3) J. L. Neal and D. A. I. Goring, ibid., 74, 658 (1970). (4) 0 . Nomoto and H. Endo, Bull. Chem. SOC.Jap., 43, 2718 (1970). (5) G. Barone, V. Crescenai, A. M. Liquori, and F. Quadrifoglio, J. Phys. Chem., 71, 984 (1967).
The Journal of Physical Chemistry, Vol. 76,No. 23,1971
