Improved Method for the Preparation of Sulfur Dioxide from Barium Sulfate for Isotope Ratio Studies SIR: In a recent publication, Holt and Engelkemeir ( I ) describe a method for converting barium sulfate t o sulfur dioxide by direct thermal decomposition. We suggest that their combination of conditions for the production of sulfur dioxide is not always easily reproduced. Using this method, we have failed to obtain quantitative yields of the gas, although the recommended procedure has been closely followed. We have, in fact, employed many of the procedures not recommended but, whether the quartz powder is added as a 6-mm thick layer, or a n intimately mixed component, or whether it is omitted entirely, the yield of sulfur dioxide has been consistently less than theoretical. In our experiments, we collected the total condensable gases in a trap cooled in liquid nitrogen following the thermal decomposition of a standard barium sulfate sample. As a routine procedure, water was removed from these products by replacing the liquid nitrogen with solid carbon dioxide/ acetone and distilling the more volatile components (SOz and CO,) into a second trap cooled in liquid nitrogen. Although previous experience has shown that this separation normally presents little difficulty, the behavior of the collected gases was such that a clean fractionation could not be made satisfactorily. The presence of another product was thus indicated and sulfur trioxide became the prime suspect. When the same barium sulfate was converted to sulfur dioxide (97 theoretical yield) via barium and silver sulfides ( 2 ) , the 34Scontent of the gas was some 1.6 per mil greater than that of the sulfur dioxide (85% theoretical yield) produced by direct thermal decomposition. Isotope fractionation studies at equilibrium have shown the heavier sulfur isotope to be concentrated preferentially in the more oxidized species (3). Thus, when sulfur dioxide yields are markedly less than theoretical and the sulfur is therefore likely to be present as both the trioxide and the dioxide, the dioxide would be expected to be depleted in 34S relative to the original barium sulfate. The isotopic measurements indicate that this is the case and strengthen the supposition that sulfur trioxide is formed during, or following, thermal decomposition. While the overall reaction may perhaps be represented by the equation given by Holt and Engelkemeir: BaS04 + BaO
+ SO2 +
0 2
(1)
the following reactions may also be important BaS04 e BaO
so0
* so9 +
+ SOa '/2 0 2
(2)
(3)
If so, the ratio of S02/S03in the final products will decrease with increase in oxygen partial pressure, and increase with increase in temperature. In the method described ( I ) , it is difficult to estimate, and therefore control, the temperature of the barium sulfate, since the only guide is the softening temperature of the silica. Oxygen pressures vary markedly during the heating process, since they are controlled by the (1) B. D. Holt and A. G. Engelkemeir, ANAL.CHEM., 42, 1451 (1970). (2) T. A. Rafter, N.Z.J. Sci. Techno[.,Sect. B , 38, 849 (1957). (3) H. Sakai, Geockini. Cosnzochim.A c / a , 12, 150 (1957). 1542
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
rate of heating of the sulfate, the geometry of the sysiem, and the rate a t which the gas is removed by pumping. In view of the difficulties encountered in duplicating the experimental results of Holt and Engelkemeir, the apparatus was modified to allow better control of experimental conditions t o be established. EXPERIMENTAL
The modified apparatus ( 4 ) as shown in Figure I includes a removable silica tube packed with metallic copper turnings heated to 800 "C. The purpose of this is to control the pressure of the oxygen released during the decomposition of the sulfate and thus to reduce to a minimum the sulfur trioxide content in the gaseous products. Details of the exact experimental procedure employed when using this modified equipment follow. Weigh 20 mg of barium sulfate into the silica envelope and insert into silica combustion tube. Evacuate the system, heat the copper turnings to 800 "C, and pump any volatile products to waste for 10 minutes. Close tap A to isolate the reaction system and heat the barium sulfate with a hydrogen/oxygen flame for 10 minutes as described by Holt and Engelkemeir ( I ) . Close all taps. Place solid carbon dioxidelacetone and liquid nitrogen containers in traps E and F, respectively, and open tap A . After 2 minutes, open tap B, and after a further 2 minutes, slightly open taps C and D to allow all noncondensable gases t o pump to waste. When the pressure in trap E falls to 20 Fm H g (as indicated by the gauge H ) close all taps. Transfer the liquid nitrogen bath to the finger G and with tap D closed, open taps C and J and distil the contents of trap F into the finger. Close tap J. Transfer the liquid nitrogen container back to trap F and immerse the finger G in a bath of iso-hexanelliquid nitrogen (-125 "C). Open tap J and allow any carbon dioxide in the finger to distil over into trap F. Close all taps, remove cooling bath from finger, and measure the sulfur dioxide manometrically. When a direct comparison between this method and that recommended by Holt and Engelkemeir was required, the copper-packed tube was removed and the following variations made t o the previously described procedure. The sample was weighed as before and the whole system was then evacuated, Tap A was not closed and the products of decomposition were drawn directly through the gas collection system, and collected and separated by the procedure outlined. The results, including gas yields and the isotopic composition of the sulfur dioxide produced, from the repeated analyses of a sample of barium sulfate by both methods are listed in Table I. From this table, it is immediately evident that gas yields most nearly approach theoretical when the modified apparatus is used and that the 34Scontent of the sulfur dioxide so produced is similarly at a maximum. These results fully confirm the early observation made during the analysis of the previous barium sulfate; that is, a decrease in gas yield is accompanied by a decrease in 34S content. While this data does not prove that the formation of sulfur trioxide in varying quantities is the main reason for the reported differences in gas yields and isotopic composition, the very limited number of possible decomposition products appears to leave no other alternative. (4) I. R . Kaplan, J. W. Smith, and E. Ruth, Proc. Apollo 11Lunar Sci. Co)7f., 2, 1317 (1970).
Table I. Gas Yields and Isotopic Composition of the Products from Thermal Decomposition of Barium Sulfate Products reacted with copper turnings at 800 "C Products collected directly Gas yield 634.5 Gas yield 634s 2 theoretical meteoritic sulfur Wt BaSOa, mg Wt BaS04, mg theoretical meteoritic sulfur 24.9 33.5 25.7 21 .o 32.1
72 60 64 58 77
+2.3 +1.5 +0.8 +2.2 +1.5
33.2 21 .o 28.8 24.3 26.0 22.9 26.5
97 97 96 100 98 97 99
i-2.9 +2.8 +2.8 +2.7 +2.7 +2.9 +2.9
Figure 1. Apparatus for production and measurement of SOy
It could be argued that poor yields of sulfur dioxide result from the vaporization of barium sulfate into cooler parts of the tube, from inadequate contact between the reacting barium sulfate and silica, or from general deficiencies in the geometry of the apparatus. The excellent gas yields obtained when the copper is included in the apparatus prove that n o problems exist in any of these respects. It should be noted that the addition of powdered silica has not been found to be necessary; at high reaction temperatures there appears t o be sufficient readily available from the walls of the enclosing tube. In conclusion, it is suggested that although Holt and Engelkemeir have, from their own data, demonstrated that excellent yields of sulfur dioxide may be obtained from sulfate by their method, difficulties in reproducing their experimental conditions may be experienced. Whether these difficulties are related to unknown variables, one of which might well be speed of pumping, has not been determined. The main problem in obtaining theoretical yields of sulfur dioxide appears to be the formation of sulfur trioxide. Isotopic measurements and gas yields indicate that when a reaction vessel containing copper metal at 800 " C is included in the gas preparation system. better and easier control over
the conditions for sulfur dioxide generation result. The production of sulfur trioxide is minimized and that of sulfur dioxide is maximized. This modification to the method has proved most successful and in the analysis of over one hundred samples of barium sulfate, yields of sulfur dioxide have rarely been less than 96 of theoretical. ACKNOWLEDGMENT
The authors thank I. R. Kaplan and Chari Petrowski of the Department of Geology, University of California, Los Angeles, for the determination of the early 634Svalues referred to in the script only and for helpful discussion. S. A. BAILEY J. W. SMITH
CSIRO Division of Mineralogy P.O. Box 136 North Ryde, N.S.W. Australia 21 13 RECEIVED for review July 6, 1971. Accepted March 1, 1972.
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