errors introduced in the conventional analysis by variations of the flow-rate, the possibility of extending the counting time, in order t o reduce the statistical uncertainty associated with the radioactivity measurement. Still more important, the much longer residence time of each eluted peak within the furnace ensures a complete combustion of the tritiated compounds. Finally, while an ionization chamber has been used in the present work, it appears that the method described can be
easily modified t o be used in connection with other types of radioactivity detectors. ACKNOWLEDGMENT
The authors thank D. Carrara for his skillful assistance. RECEIVED for review March 26, 1968. Accepted September 4,1968.
Determination of Oxygen in Refractory Oxides Carolyn S. MacDougall, Maynard E. Smith and Glenn R. Waterbury Unic'ersity of California, Los Alamos Scientific Laboratory, Los Alamos. N . M . 87544 THEDEPENDENCE of the properties of refractory oxide fuel materials upon the oxygen to metal atom ratio necessitated the development of a method for accurately determining this ratio. A review of reported methods is given by Marken, Walter, and Bones ( I ) . Some independent methods for measurement of oxygen in these oxide fuels include: reaction of the oxide with bromine trifluoride and measurement of the oxygen formed (2), and reaction of the oxide with carbon in an inert atmosphere o r vacuum and subsequent measurement by conductometry (3,4) or gravimetry (5,6) of the C O formed. Methods based upon the direct measurement of O2 are less empirical than those methods based solely on changes in sample weight, and the gravimetric determination of the off-gases offers the desired precision of measurement. Determination of the contents of metals in these oxides by titration methods (7) permits calculation of the critical oxygen t o metal atom ratio. I n basic concept, the method developed is similar t o previously reported methods (5, 6), but differs significantly in sample preparation, sample size, lack of crucible bath, automatic temperature programming, and the products formed in the inert gas fusion. EXPERIMENTAL
Apparatus. The major features of the analytical train are shown in Figure 1 . The sample is heated in an AUC grade graphite crucible which is contained in a water-cooled fusedsilica furnace tube. Argon carrier gas, purified by passage over hot uranium turnings, sweeps the reaction products from the furnace through a desiccant tube into an absorption tube containing Ascarite to trap the COz. The effluent C O is oxidized over hot CuO and trapped in the second absorption tube. The Schiitze reagent in the second desiccant tube ensures complete oxidation of any CO from the CuO furnace and indicates inefficiency of the CuO furnace by turning brown. The 25-kW induction generator is equipped with a timed, motor-driven power control to raise the power level automatically from a preset minimum to a preset maxi~
(1) T. L.Marken, A. J. Walter, and R. J. Bones, At. Energy Research Establishment, R4608 (1964). (2) H. R. Hoeckstra and J. J. Katz, ANAL.CHEM.,25,1608 (1953). (3) E. J. Beck and E. E. Clark, fbjd.,33,1767-70 (1961). (4) H. L. MacDonnell, R. J. Prossman, and J. P. Williams, ibid., 35, 579 (1963). ( 5 ) H. T. Goodspeed and D. Pettis, U. S. At. Energy Comm. Rept, ANL-7264 (1967). (6) B. D. Holt and J. E. Stoessel, ANAL.CHEM.,36, 1320 (1964). (7) G . B. Nelson, K. S. Bergstresser, G. R. Waterbury, and C . F. Metz, Twelfth Conference on Analytical Chemistry in Nuclear Technology, Gatlinburg, Tenn., Oct. 8-10, 1968. 372
0
ANALYTICAL CHEMISTRY
mum within a desired time interval. Weighing, grinding, and pelletizing of the sample are done in an inert atmosphere enclosure similar to that designed by Smith (8). Procedure. All samples are ground and homogenized in a mixer-mill prior to preparation of the sample pellet. A quantity of oxide, containing approximately 120 mg of 0 2 , and 0.2 gram of SP-I graphite are accurately weighed and transferred to a stainless steel mixer-mill capsule. After a 2-minute mixing period, the sample is pelletized a t 8000-lb gauge pressure in a 1/4-inch pellet die, and then weighed. Proportionate loss of both oxide and graphite during pellet preoaration is assumed. This loss should be no greater than 10%. With the induction generator off and a positive pressure in the fused silica furnace, the pellet is placed in the crucible. When replacing the crucible and furnace caps, care must be taken to purge from the system any air trapped during this operation. The flow of Ar through the apparatus is adjusted to 100 ml/minute, and the automatic programmer raises the crucible temperature from 1000 to 2000 "C in 20 minutes, maintains the maximum temperature for 1 hour, and then turns off the power. The absorption tubes are weighed according to an established gravimetric procedure. Each day, a blank determination is made prior to a series of analyses by repeatidg the procedure using a spent pellet. The weight increase of either tube should not exceed 0.1 mg for the blank. RESULTS AND DISCUSSION
The recently reported inert gas fusion methods for determining O2in oxides used a relatively small sample (10 to 250 mg) and a platinum bath in the reaction crucible. Under these conditions, CO was formed, and the amount of CO?was ne:ligible. In order to obtain representative samples of fuel element materials, quantities of oxides weighing not less than 1 gram are used. Mass spectrometric analysis of the effluent gases produced from reaction of 1 gram of sample with an excess of graphite in a dry crucible showed that appreciable amounts of CO?, as well as C O were produced. This COz must be measured and included in the calculations. The mixing of the graphite with the oxide and pelletizing provides intimate contact of the reacting species and eliminates the necessity of using a Pt bath. As the pellet contains an excess of carbon, the crucible is not appreciably attacked and can be used almost indefinitely. Removal of each pellet
(8) M. E. Smith, J. M. Hansel, and G. R. Waterbury, U. S. At. Energy Comm. Rept, LA-3344 (1965).
FLOW METER
I & (
u
Figure 1. Schematic diagram of analytical train for inert gas fusiongravimetric determination of oxygen in oxides ~
after analysis eliminates the possibility of non-uniform heating caused by several samples in the crucible. The pellet is heated from a temperature of 1000 to 2000 "C during a period of 20 minutes to ensure a smooth release of CO and COZfrom the pellet. Loss of sample is minimized by the graphite crucible cap and by the slow reproducible heating, controlled by the temperature programmer. The length of time necessary for reaction of the pellet was determined by analyzing a series of oxide-carbon pellets. Figure 2 shows the quantities of O2evolved and measured as C02as a function of time of heating for T h o ? , u308, PuOZ, and ( U O PuO.?)Oz. .~ These bar graphs show the total amount of CO?collected by both absorption tubes during the respective time interval and d o not include relative quantities of CO and Con. No more COz was recovered from any sample after 60 minutes, but the time of analysis was extended t o 80 minutes to ensure recovery of all reaction products. The per cent of CO?relative to CO varied from 0 % for PuOz t o approximately 10% for U308. In all cases, the C02 was released during the 20-minute warm-up period. Oxide pellets which had been analyzed at 2000 "C were heated to 2200 "C without further release of 0 2 . Hence, 2000 "C, which is the normal operating temperature of the furnace in this apparatus, was considered adequate, For pellet preparation, a quantity of SP-1 graphite of at least 0.2 gram was necessary to ensure adequate pelletizing properties of the sample mixture and to provide an amount of carbon greater than the stoichiometric amount necessary for the complete reaction of the oxide. Oxide-carbon pellets were prepared with amounts of graphite ranging between 0.25 and 0.4 gram. The larger quantities did not increase the amount of O2 released, and were detrimental to the pelletizing properties. The oxygen content of the graphite was measured by analyzing an all-graphite pellet by this method. The amount of COZproduced was less than could be weighed. Four oxides were analyzed for O2by this method. Table I gives the results of these analyses. Fourteen analyses for 0 2 in T h o z averaged 12.18 as compared with the calculated 12.12%, and the relative standard deviation was 0.1 %. Spec-
z
300
225 I50 75
0
00
10
20
30
12.18 15.19 11.92 11.80
60
Figure 2. Oxygen, evolved and measured as carbon dioxide, as a function of time of heating
trographic analyses of the T h o ? revealed that there were a number of metallic impurities in the oxide which were present in sufficient quantity to account for the bias observed, assuming these impurities were oxides. A small lot of U308 was prepared by slow ignition of uranium metal having less than 100 ppm total detected impurities. The product was weighed to determine the oxygen content accurately. The average of 14 analyses for O2in this U308was 15.19% as compared with the calculated value of 15.20%. The relative standard deviation of these measurements was 0.1 %. A small lot of PuO? was prepared by slow ignition of highpurity plutonium metal having less than 100 ppm total detected impurities. The weight of the product showed that it contained 11.90% oxygen. The average of 14 analyses for 0 2 in the P u 0 2 was 11.92 and the relative standard deviation was 0.2z. Pressing and handling of the highly radioactive pellets in a glove box did not affect the accuracy of the method, but the precision was slightly poorer than for nonradioactive samples. This difference probably was caused by a larger loss of sample during preparation in a gloved enclosure. Mathematical corrections for sample loss are less valid for larger losses, and precision of the method suffers.
z,
Table I. Measurement of Oxygen in Refractory Oxides Oxygen Determinations Found, Calculated, Re1 std dev, 14 14 14 12
50
40
TIME IN MINUTES
12.12 15.20 11.90 ( 11.85)
0.1 0.1 0.2 0.2
Recovery, 100.5 99.9 100.2 (99.6)
VOL. 41,NO. 2, FEBRUARY 1969
0
373
The final tests were the analyses of sintered uraniumplutonium oxide pellets. The total of the concentrations of detected impurities in this lot of pellets was about 400 ppm. The main impurities were iron (125 ppm), tungsten (86 ppm), carbon (52 ppm), and nickel (45 ppm). Nine pellets, each weighing about 1.5 grams were ground together in a mixermill capsule, and 1-gram portions of the homogenized material were taken for analysis. The average oxygen content was 11.80%, and the relative standard deviation was 0.2 per cent. Although the exact oxygen content of this oxide was not known, the total metal concentration of 88.15 =t 0.30 %, measured tritrimetrically, indicated that the maximum oxygen concentration was 11.85% i. 0.30%. These oxygen and metal concentrations were used in calculating
the oxygen to metal atom ratio of this lot of fuel pellets. The ratio was 1.993 and the standard deviation was 0.004 if only the uncertainty of the measurement of oxygen was considered. ACKNOWLEDGMENT The authors thank C. F. Metz for his assistance and suggestions during this investigation, members of the plutonium and uranium chemistry and fabrication groups for the high purity metals, and Battelle Northwest Laboratory for the Oxide
RECEIVED for review September 30, 1968. Accepted November 4, 1968. Work done under auspices of U. S. Atomic Energy Commission.
Magnesium Spinel Interferences in Air-Acetylene V S . Nitrous Oxide-Acetylene Flames in Atomic Absorption Spectrometry W. W. Harrison and W. H. Wadlin Department of Chemistry, Unicersity of Virginia, Cltarlattescille, Vu. 22901
ALTHOUGHinterferences in atomic absorption and flame emission spectrometry are well known (1-4) there is still a lack of understanding of exactly how they arise. Most of the documentations appear as warnings attached t o descriptions of specific analytical methods. Observations by different investigators are often made under widely varying conditions, leading t o certain apparent incongruities in the literature as t o the magnitude and, at times, even the direction of these interferences ( 5 , 6). Published information in the case of many interferences is pertinent only when applied t o systems which can be closely matched t o that of the original investigator. The object of this investigation was a study of the effect of certain experimental conditions upon interferences which are observed in the atomic absorption determination of magnesium. The experiments were selected to demonstrate the range and magnitude of these interferences that can be encountered under various conditions and t o determine possible conditions under which the interference may be minimized. Magnesium was chosen as the analyte for its common incidence and known susceptibility to interferences (7). Aluminum was chosen as the major interferent for its common occurrence and the severity of its interferences with many elements (1, 4). Titanium and zirconium were also studied as elements which have shown interference effects similar t o aluminum.
(1) R. Herrmann and C. T. J. Alkemade, “Chemical Analysis by Flame Photometry,” 2nd revised ed., Interscience, New York, N. Y.?1963, p 310. (2) J. E. Allan, Spectrochirn. Acta, 18, 605 (1962). (3) M. Margoshes and B. L. Vallee, AKALCHEM.,28, 180 (1956). (4) J. A. F. Gidley and J. T. Jones, Analyst (London), 85, 249 (1960). (5) J. H. Gibson, W. E. L. Grossman, and W. D. Cooke, ANAL. 35, 266 (1963). CHEM., (6) L. W. Fleming and W. K. Stewart, Clirr. Chim. Acta, 36, 278 (1966). (7) A. C . Menzies, ANAL.CHEM.,32,898 (1960). 374
ANALYTICAL CHEMISTRY
I
,
1c
,
-_
A_-1 --lp I _ p L - - i i i - - l
22
39
i r sc 6,
G
RC
9,
cs
11c ‘23
i t -
Figure 1. Interference of aluminum with magnesium at 285.2 nm as a function of flame height M g , 0.8 ppm;
acetjlene, 3.7 liters/min; air, 18.5 liters/rnh EXPERIMENTAL
Apparatus. Absorbance data were taken with a PerkinElmer Model 303 atomic absorption spectrometer equipped with a DCR-1 digital readout system. The hollow cathode tube used was a Westinghouse WL22604 combination Ca-MgA1 element. For air-acetylene flames, the Perkin-Elmer burner was equipped with a Boling head. For nitrous oxideacetylene flames, a Perkin-Elmer nitrous oxide burner head was substituted. Variation of oxidant flow rate was controlled by adjustment of the auxiliary supply. To define a base line for height adiustment in terms of the location of the sample beam in the flame, the following procedure was adopted. The top of the burner was adjusted to tangency with the sample beam at its focal point while the burner height scale was set at zero. From the zero line, fine scribe marks were made on the burner shaft up to 30 mm. The upper limit was found to be the practical experimental limit due to instability of higher regions of the flame. The base line described above
