985
Anal. Chem. 1983, 55, 985-987
since it is moire accurate for heterocyclic amines than the colorimetric method. ACKNOWLEDGMENT The authors thank the personnel under the supervison of R. W. Black of the Dow Chemical Co. for performing the Kjeldahl analyses. Registry No. Nitrogen, 7727-37-9; n-butylamine, 109-73-9; 2-aminoheptane, 123-82-0; ethylenediamine, 107-15-3; E-100, 84582-75-2; piperazine, 110-85-0;pyrazine, 290-37-9; selenium, 7782-49-2. LITERATURE CITED (1) Schachter, 12.; Co ony, W. An., Inst. Cercet. Cuff. Cartofulul Sfeclel Zahar, Bratsov, PSer] Cartoful 1071, 2 , 267-273. Chem Abstr.
1873, 78, 40043. (2) "Hach Digesdahl Method vs. Classical Kjeldahl Procedure"; Hach Co: Loveland, CO, 1981 (3) Koch, F. C.; McMeeken, T. L. J . Am. Chem. SOC. 1829, 49, 2066-2069. (4) Lauro, M. F. Ind. Eng. Chem., Anal. Ed. 1831, 3 , 401-402. (5) "Model 21400 Digesdahl Dlgestion Apparatus Methods Manual"; Hach Co.: Loveland, CO, 1981. (6) Bergstedt, L.; Widmark, 0. Acta Chem. Scand. 1870, 2 4 , 2703-2723. (7) Phelps, I. K.; Daudt, H. W. J . Assoc. Off. Agric. Chem. 1920, 3 , 306-307.
for review September
97
1982. Accepted
29, 1982.
Thermal Decomposition of Barium Sulfate-Vanadium Pentaoxide-Silica Glass Mixtures; for Preparation of Sulfur Dioxide in Sulfur Isotope Ratio Measurements Fumitaka Yariaglsawa and Hitoshl Sakal" Institute for Thermal Spring Research, Okayama lJniversi@, Mlsasa, Tottori-ken 682-02, Japan
Thermal decomposition of BaSOl has been used by several workers to prepare SO2for the sulfur isotopic analysis (1-7). Haur e t d.( 4 ) reported that the decomposition temperature of BaS04 was lowered considerably when it was heated with V205and SiO,; the rapid evolution of SO2 started at 700 "C and was completed between IO00 and 1050 "C. More recently, Coleman and Moore (7) used BaS04-Cu20-Si02 mixtures to obtain SO2 at 11120 "C. Compared with the earlier techniques which involved reduction of BaS04to Ag2S, this procedure was potentially simpler and more suitable for routine analysis of a large number of BaS04 samples. However, in these previous techniques, the yield of SOz was often as low as 75% ( 4 ) or the influence of the oxygen isotope ratio of SO2 on the final sulfur isotope ratio was not corrected, although it was discussed (2, 7). We have modified their technique to overcome these difficulties and developed a simple method which routinely yields 9549% of SOzwith a consistent leg/160ratio. This paper describes the experimental procedures in detail and discusses certain aspects of the reaction mechanism. EXPERIMENTAL SECTION (1) Sample Preparation. The preparation line is schemat-
ically illustrated in Figure 1. The extraction tube is of 9 mm 0.d. silica glass. An intimately ground mixture of BaS04 (5 mg), VzO5 (50 mg) and SiOz (50 mg) was placed at the bottom of the tube and covered by silica glass wool. Another wad of glass wool was jammed into the tube about 2 cm above the reaction mixture. The tube was then heated at 450 "C for about 30 min in open air to remove any organic contaminants. Two grams of copper wire was placed on the upper glass wool wad (Figure 2A). The tube was then connected to the preparation line by a Cajon Ultra-Torr union and evacuated through stopcocks A and B (Figure 1). The copper wire was then brought to a red glow by heating the glass tube with a torch until no appreciable outgassing was observed by a Pirani vacuum gauge. This procedure removed oxide coatings as well as any volatile contaminants in the copper. This silica glass tube was then heated to a level just above the copper by a vertical electric furnace which was closed on the bottom end. Thi.is the copper was somewhat cooler than the reaction mixture. When the temperature of the reaction mixture reached 500 "C, stopcock A was closed, and C and D were opened to collect the evolved SO2into trap 1which was immersed in liquid air. The evolution of SOz started at 600 " C and maximized at about 670 "C. In order to complete the thermal decomposition,
the mixture must be heated up to 900-950 OC. However, the temperature should be raised slowly, in order to (1)prevent bumping of the mixture, (2) ensure the consistency of the oxygen isotope ratio of the evolved SOz (see below), and (3) achieve complete reduction of SO3 by the copper. After the reaction was over, stopcocks B, C, and D were closed, the liquid air on trap 1 replaced by an acetone-slush bath (ca. -95 "C), trap 2 cooled with liquid air, and stopcock E opened. This transferred the SO2 from trap 1to trap 2. The volume of SO2was measured prior to storage in a sample tube for the isotope ratio measurement. Since the solid reaction residue was soluble in 6 N HCl, the reaction tube could be used repeatedly after .this cleaning procedure. (2) The Effect of the Oxygen Isotope Composition of SOz on Sulfur Isotope Ratio Measurements. In the mass spectrometric determination of 34S/32Sratios, the results are affected by overlap of the ion currents due to 32S160180+and 34S1602+. Therefore, the 1'30/160 ratio of SO2must be kept constant among samples in order to apply a consistent correction factor for this ratio must effect. A simple calculation shows that the 180/160 be uniform within f l % o in order to maintain a reproducibility of &O.l%o in sulfur isotope determinations (Kamada et al. ( 5 ) ) . In order to achieve this, they decomposed BaS0, in the presence of a large excess of powdered silica glass which buffered the 1s0/160 ratio of SOz by isotope exchange (5),probably between sulfate and oxides in the molten mixtures. In order to demonstrate the effect of the oxygen isotope ratio of S02, two BaS04 samples were prepared which had the same sulfur but different oxygen isotopic abundances. These samples were mixed with varying amounts of a 1:l mixture of Vz05 and SO2,and the apparent 6% values of SOz obtained by the present technique were compared (Figure 3). The 634Svalue is defined by
where X and S?' denote a sample and a standard, respectively. The apparent 6% values of the two BaS04 samples approached each other with increasing (V2O5 + Si02)/BaS04ratios; when this ratio was above 14, the two BaS04 samples gave practically the same P4S values. For this reason, it was decided to use a (Vz05 + Si02)/BaS04ratio of 20. Because the P O values of silica glass and VzO5 may differ among batches, the same batch of each reactant that was used to make SO2from a standard should be used for a series of samples to be measured relative to the standard. The relative amounts of SOz,V205, and BaSO, in the
0003-2700/83/0355-0985$01,50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
086
t o vacuum
Table I. The Sulfur Isotope Ratios of Two BaSO, Standards, MSS-2 and MSS-3
to vacuum
+* Thermocoupd'
to a i r
Manometer
run no. 1
2 3 4 5
@
Reaction tube
6 7 8
Electric furnace
9
Flgure 1. The vacuum line for the preparatlon of SOz.
10 11
12
'
Gw
Copper wire
Reacted copper w i r e
Ca 1OOmmk c o p p e r w i r e
Silica-glass
1
Black colored deDosit
!
no. ~
Reaction products (Black color) (Yellow color)
(A)
(C)
(8)
Figure 2. Schematic representation of the charged silica glass tubes before and after the decomposltbn: (A) before decomposition; (B) after decomposition of a BaS0,-V,05 mixture, (note that the solld reaction products were spread verticaiiy); (C) after decompositlon of a 0,SiOZ-V2O5mixture, (note that the solid reaction products remalned at the bottom and, in contrast to B, only the lower portlon of the copper was oxidized).
-2t 1h -31.
0 IO 20 ( S i 0 2 * V 2 0 5 ) / Bas04 ; ( S i 0 2 : V 2 0 5 , 1 : I )
+
Flgure 3. The effect of the (V205 SiO,)/BaSO, weight ratio on the apparent 634S values (SiOz:V,05, 1:l). The solid and open clrcles represent two different BaSO, samples having the same 634S but different Si8O values. The open circles represent the barium sulfate of 6'*0approximately 20% higher than the other.
mixture should also be kept constant for the given series of measurements. (3) Reaction Products. To investigate the reaction mechanism, BaS04, V205,and Si02 as well as the mixtures of two of these reagents were heated to 1000 "C under vacuum without copper wire. The solid reaction products were examined by X-ray diffraction. RESULTS AND DISCUSSION (1) S u l f u r Isotope Ratios. The results of isotope ratio measurements carried out over a period of 6 months on two standard BaS04 samples, MSS-2 and MSS-3, are summarized in Table I. Each pair of MSS-2 and MSS-3 listed under the same run numbers was prepared and measured at the same time. The tj3*S, values in Table I are relative to the SOz reference gas which varied in its 634Svalue. However, the
MSS-3
18.50 18.61 19.32 19.28 19.43 19.13 19.10 19.10 18.96 19.29 19.29 19.40
0.50 0.45 1.26 1.26 1.28 0.89 1.00
0.97 0.88 1.22 1.03 1.49
difference,
o/oo
18.00 18.16 18.06 18.02 18.15 18.24 18.10 18.13
18.08 18.07 18.26 17.91 av 18.10 ?: 0.09
Table 11. Products of Thermal Decomposition of SiO, , BaSO,, and V,O, and Their Mixtures
copper w i r e
I
6 34sm, o / o o
MSS-2
starting materials
mol
ratios
SiO, (amorphous)
1 SiO,
2 BaSb, 3 vzo, 4 BaSO,, SiO, 5 BaSO,, V,O, 6 V,O,, SiO, 7 BaSO,, V,O,, SiO, 8 BaSO,, V,O,,
reaction products
1:7 1:11 1:3 1:4:11
1:9:8
SiO, a BaSO, + SiO, mixture reacted only above 1400 "C and formed BaSi,O,.
reproducibility of the difference between MSS-2 and MSS-3 was better than k0.1%0and in excellent agreement with the previously assigned value of 18.0%0.The pair for run number 1was measured five times during the 1-month period in order to check the stability of the mass spectrometer. The difference was reproduced within k0.1%0,implying that the reproducibility in Table I might be controlled by the stability of our mass spectrometer. In order to obtain the 634S values of MSS-2, MSS-3, and a pyrite standard relative to the widely used standard, Cafion Diablo troilite (634ScDT),BaS04 was prepared from the troilite and the pyrite. Then SOz values from the three laboratory standards were compared with the troilitic BaSO,. The 6%cm of MSS-2, MSS-3, and the pyrite were found to be +21.6, +3.5, and +4.5%0, respectively. These values are in excellent agreement with the respective values of +21.5, f3.5, and +4.5%0which we have measured on SOz obtained by combustion of Ag,S prepared from these standards and troilite with Cu20. (2) Reaction Products. The results of X-ray diffraction analysis of the solid reaction products, which were obtained by heating various mixtures of BaS04,Si02,and Vz05to 1000 OC,are summarized in Table 11. BaSOl alone and BaS04 + SiOz did not react a t 1000 "C, whereas BaS04 completely decomposed, releasing SO2,in the presence of Vz05+ Si02. Therefore Vz05 is essential for lowering the decomposition temperature of BaS04. Because pure V205melts at 600 "C, it is likely that it acts as a flux which dissolves BaS04 and yields the observed products by reactions such as
BaS04 + 6V205(melt)= BaV12030+ SO3 + 1/20z (2) The mixture of BaS04 and SiOz did not react appreciably below 1400 "C (see footnote a of Table 11). However, silica glass powder also plays an important role in the present
!387
Ana/. Chem. 1983, 55, 987-988
technique as is discussed below. In order to demonstrate the role of silica glass powder, various mixtures of B&04 and Vz05with or without SiOz were heated at 1000 OC and the rate of evolution of SO2 was monitored by recording the output of the Pirani gauge placed between the reaction tube and trap 1 (Figure 1). Without silica glass powder, the rate of SOz evolution increased with increasing VZO5 However, the evolution of SO2did not proceed smoothly, showing several peak rates at different temperatures. The evolution a t low temperature was incomplete, whereas at highLer temperature, SO2 evolved with vigorous bumping or boiling of the molten mixture. On the other hand, the mixtures containing SiOz with a Si02to V205ratio of 1 or greater evolved SO2 completely and smoothly at lower temperatures. This difference in reaction is also reflected in the behavior of the reaction products. They differed in viscosity and affinity for the silica glass wall as schematically illustrated in Figure 2. Without Si02, the molten mixture displayed high fluidity. It crept up the glass wall and also reacted with the silica glass wool (Figure 2B). In contrast, the mixture with: SiOz decreased in volume forming a droplet which adhered tlo the overlying silica glass wool (Figure 2C). The droplet, when cooled, was porous and brittle. However, excessive Si02 lowered the SO2 yield, probably by preventing melting. On the other hand, excessive V205also lowered the yield, probably because it boiled off and reacted with the copper wire as illustrated in Figure 2B. For these reasons, the Vz05/Si02weight ratio of unity is considered to be the most appropriate. Whereas a ratio of (V205+ SiOz)/BaS04 of 6:l gave a good yield of SO2 at a low temperature, a ratio of 20:l was preferred to assure uniformity of the oxygen isotope composition of the SO2 as previously discussed. The presence of BaVI2Omor Ba0.V204.5V205in the reaction product indicates that the partial pressure of oxygen a t the site of BaSOI decomposition was controlled by V205-V@4 equilibrium. The PO,of this system is close to that of Cu20-
CuO and some SO3 should have formed. However it must have been totally reduced to SO2 by the Cu-Cu20 system,,as metal copper remained after the reaction was over. The L!O, of CU-CUZOat 600 to 1000 “C ranges from to lo4 atm and the equilibrium S03/SOzratio under such Po, would be as low as 10” to (8). Therefore, the presence of excess copper is essential to maintain the low Po, and consequently the high yield of SOz. In conclusion, the present technique is very suitable for the preparation of SO2from the small amounts of BaS04 (1-5 mg) obtained typically from various geologic and laboratory specimens. The distinct advantage of the present technique over other methods is that the SO2 product is pure enough to be directly subjected to mass spectrometric analyses. Thus, the technique seems particularly suitable for an automated SO2 preparation and isotope ratio measurement system currently under development in this laboratory.
ACKNOWLEDGMENT We are indebted to H. R. Krouse, University of Calgary, for his valuable discussion and advice in the course of preparation of this paper. We thank N. Kishima and H. Chiba for their help.
LITERATURE CITED (1) (2) (3) (4) (5)
Claypool, J., private communication, 1970. Holt, B. D.; Engelkemelr, A. G. Anal. Chem. 1970, 4 2 , 1451. Bailey, S. A.; Smith, J. W. Anal. Chem. 1972, 4 4 , 1542. Haur, A.; Hladlkovi, J.; Smejkal, V. lsotopenpraxis 1873, 9 , 329. Kamada, E.; Sakal, H.; Klshima, N., Pap. Inst. Therm. Spring Res.,
Okayama Unlv. 1980, 50, 1. (6) Haias, S.; Woiacewlcz, W. P. Anal. Chem. 1981, 5 3 , 686. (7) Coleman, M. L.; Moore, M. P. Anal. Chem. 1978, 50, 1594. (8) Robinson, B. W.; Kusskabe, M. Anal. Chem. 1975, 4 7 , 1179.
RECEIVED for review July 30, 1982. Accepted December 27, 1982. The present research was supported by the Grant in Aid for Scientific Research No. 57430010 by the Ministry of Education.
Thermometric Calibration In Thermogravimetric Analysis Andrew R. McGhie Laboratory for Research on the Structure of Matter, Universjty of Pennsylvania, 323 1 Walnut Street, Philadelphia, Pennsylvanla
A problem exists in accurately determining the temperature of a sample undergoing programmed heating during thermogravimetric analysis. The currently accepted method of determining temperature is to position a thermocouple close to, but not touching, the sample boat. This procedure does not necessarily measure the temperature of the boat as it is very susceptible to temperature variation with the thermocouple position and flow rate of the purge gas. At least two methods of temperature calibration have been tried. First, small metal riders have been placed on the balance beam which fall off when they reach their melting point. This method has several disadvantages: (1)it does not measure the boat bmperature, (2) surface tension of the liquid may prevent the rider from falling off exactly at the melting point, (3) the metal may oxidize, (4) the total sample weight on the beam will change. A more recent attempt has been made to use the Curie point of magnetic materials (I). In this case a small magnetic sample is placed in the boat alone or with an actual sample and a small permanent magnet is
19 104
positioned near the boat (either inside or outside the balance) to cause an apparent weight change of a few percent. When the reference undergoes the Curie point transition this magnetic force disappears and a sharp apparent weight change is observed. Disadvantages in this case include (1) obtaining standard reference materials with well-characterized Curie points over a wide temperature range, (2) the abrupt base line change, (3) the relatively low precision, k3.6 “C, of the method (2).
EXPERIMENTAL SECTION In this paper we wish to describe a variant of the first method in which most of the disadvantages have been eliminated. The technique is best described with reference to Figure 1. A standard platinum boat has a thin, horizontal platinum wire spot-welded to it near the top of the pan. The wire is bent at right angles so that it lies direct1:y above the bottom of the pan. To this wire a hook of some material with a well-characterized melting point, e.g., a metal, is attached and from this hook a platinum weight (approximately30 mg) is hung. The hook should be of low mass
0003-2700!83/0355-0987$01.50/0 0 1963 American Chemical Soclety
