After three seconds the spectrum scan starts. At the end of the scan t h e Beckinan IR7 resets the paper, wave length drive and slits. When the ready light of the IR7 comes on, a second sample in the changer is moved into place in the IR beam automatically. After three seconds the chart drive mechanimi et cetera restarts and the second spectrum is run. This process is continued until the preset number of spectra have been run a t which time the instrument reverts to the standby condition. Thr. cycle stop light comes on indicating complction of the series.
‘TENS’ 5yJl NIXIE
RESULTS
ITpto 25 samples can be run with one loading of the changer. However, it is desirable to include about three standards among the samples to check the operation of the changer. -1 roll of precalihrated chart paper (Beckman g72073) can accommodate 53 spectra of 27-inch sections each. Even after 25 spectra the pen nas reset accurately by the IR7 mechanism at the beginning of the calibrated chart paper. With high resolution spectra being run at the rate of one per 1 3 ’ ( hour it is possible
Figure 4.
Sample selector circuit
to turn out about nine spectra automatically overnight. No serious operating difficulties have been encountered except for normal breakdowns of the mectrometer-e.e.. source burnouts. h i l e this devi& ’is most suitable for a spectrometer using a continuous chart it be possible to adapt it to types using a drum recorder by adding an auxiliary strip chart recorder.
LITERATURE CITED
(1) Cassels, J. W., Brame, E. G., Day, C, E., A ~ ~ 33, ~ 813, ( 1961),
cHEM.
(2) Johnson, D. R., Cassels, J. W., Brame, E. G., Westneat, D. F., Ibid., 34, 1610 (1962). This investigation was supported by Public Health Service Research Grant No. 1-SOL-FR-05528 and National Science Foundation Grant N o . G-20998.
Absorption l u b e for Removal of Interfering Sulfur Dioxide in Analysis of Atmospheric Oxidant Bernard E. Saltzman and Arthur F. Wartburg, Jr.,l Robert A. Taft Sanitary Engineering Center, Education, and Welfare, Cincinnati, Ohio 45226
is a serious negative in the iodometric (3) and phenolphthalin (1) methods of determination of atmospheric oxidant (a large part of which is ozone). I n many areas the quantities of sulfur dioxide present in the atmosphere esceed those of osidant; thus a false zero analysis for the latter may be obtained. S o available methods for oxidant analysis are free from this interference, except for the nitrogen dioxide equivalent method (6, Y), the dimethoxystilbene method ( 2 ) , and the chemiluminescent method ( 5 ) , which are specific for ozone alone. Intwfrrence in the Mast ozone anal yzrv, was es 1 )c, r i i nentally determined for mixtures of 0.2 to 1 p.p.m. of sulfur dioxide and 0.2 to 1.2 p.p.m. of ozone. -1 flow system was used for diluting and blending higher concentrations of each gas contained in separate Xylar bags. The osidant values were based on calibration of the instrument by the neutral potassium iodide method ( 7 ) . The manufacturer’s calibration yielded values only about 60% of these and was believed to be erroneous. Sul-
S interference .
ULFUR D I O X I D E
Present Address, Yational Center for Atmospheric Research, Boulder, Colo.
fur dioxide produced a 100% negative interference on a p.p.m. or molar basis, a finding that agreed with earlier work ( 8 ) . Analysis of data from the Cincinnati station of the Continuous Air -Monitoring Program indicated a sulfur dioxide interference of about 85% with the Beckman instrument. A variety of absorbent materials was examined in a flow system for efficiencies of removing sulfur dioxide from a mixture with ozone, without concurrent loss of the latter (which was the major problem). Prior conditioning of the apparatus, even of clean empty C-tubes and glassware, was necessary for good ozone recovery. Conditioning was accomplished by passage of about 1 p.p.m. of ozone for several hours. Further study was made of a solution of 2l/2% potassium ~ e r m a n g a n a t e - 2 ~ / ~ % sulfuric acid previously suggested (4). This solution removed sulfur dioxide efficiently; however, it suffered the disadvantages of causing a pressure drop and of introducing a high positive blank error. An erroneous .reading as high as 1 p.p.m. of ozone was produced by a solution that previously was unaerated for 24 hours, even though only pure air was being sampled. L-sually a t least 2 hours
U. S.
Department of Health,
of running time was required before the indicated reading returned to zero. This phenomenon could be repeated several times with the same solution. Probably some product of slow decomposition was responsible. The odor of the solution a t these times resembled that of ozone. Less satisfactory were a number of other liquid and solid absorbents. Best results were obtained with glass fiber paper impregnated with chromium trioxide and sulfuric acid. .I certain conditioning time was required before ozone was no longer appreciably absorbed. This process probably destroyed trace impurities; it never needed repeating. The concentration of chromium trioxide did not critically affect the final performance; however, it was difficult to distribute high concentrations uniformly on the paper, and the life of an absorbent containing a low concentration was expected to be short. The concentration of chromium trioxide selected as optimal required the shortest conditioning time. The amount of sulfuric acid used also was not critical; however, high concentrations caused the paper to be hygroscopic, a condition that sometimes resulted in ozone losses as water collected on the VOL. 37, NO. 6, M A Y 1965
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Figure 1 .
Large size absorber
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Fabricated from two 25- X 200-mm. culture tubes (Kimax 45066A) with screw caps containing Teflon liners. Side arms added are 8-mm. 0.d. and 30 mm. long. Connection at bottom is butt-to-butt, with Tygon tubing. Contains 60 sq. inches of absorbent paper.
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surface over a period of several hours and also led to handling difficulties. EXPERIMENTAL
Recommended Method for Preparation of Chromium Trioxide Paper. Fifteen milliliters of aqueous solution, containing 2.5 grams of chromium trioxide and 0.7 ml. of concentrated sulfuric acid, is dropped uniformly over 60 square inches of flash-fired glass fiber paper; the paper is conveniently supported horizontally on glass rods or on a stainless steel screen. The sheets are then dried in a n oven a t 80" to 90" C. for 1 hour, or until they turn pink. The dried sheets are hygroscopic, and exposure to air should be minimized during the cutting and packing process; rubber gloves should be worn to avoid skin irritation and staining. For packing large absorbers] the sheets are folded into l/c-inch accordian pleats and 1/4-inch wide strips are cut a t right angles to the folds in the compressed mass. For small absorbers the sheets are cut into pieces approximately 1/4 X inch; each piece is folded once into a V shape to prevent the pieces from nesting together as they are packed in the C-tube. The packed tube is conditioned by passage of about 0.2 p.p.m. ozone for an hour or two. (If an ozonizer is not available] satisfactory conditioning rnay be achieved by passing room air through the U-tube overnight.) Optimal Size for Absorbers. A careful balance is required between size of the absorber and flow rate of the sample air. Data on the performance of the absorber illustrated in Figure 1 are given in Figure 2. The working range of sample air flow rates is shown to extend from 0.5 to 6 liters per minute, suitable
780 *
ANALYTICAL CHEMISTRY
Figure 2. Paper
Performance of absorber containing
_____
Per cent ozone loss with oir contoining 0.2 p p m . of ozone Per cent interference from sulfur dioxide with air containing 0.2 p.p.m. of ozone and 2.5 p.p.m. of sulfur dioxide
for the Beckman instrument which samples 4 liters per minute I n similar studies with a 100-mm. Schwartz drying tube containing 6 square inches of the paper, appreciable amounts of sulfur dioxide penetrated a t flows exceeding 0.3 liter per minute. This smaller-size absorber was adequate for use with the Mast ozone analyzer, which draws a sample flow of 0.14 liter per minute. For most analytical work the larger absorber would be better; probably an absorber half this size (consisting of only one section of the larger absorber) would be adequate for flows used in manual sampling. Careful studies also were made to determine whether oxidants from natural or synthetic smogs mere lost in these absorbers, because these oxidants consist only partly of ozone, the remainder being various organic compounds and nitrogen dioxide. Monitoring data from downtown Cincinnati and Philadelphia were studied; these data mere obtained from oxidant instruments operated both with and without the absorbers. Consideration also was given to concurrent monitoring data for sulfur dioxide and nitric oxide. In the absence of interfering concentrations of these gases, oxidant readings with and without the absorber usually agreed; the difference was never more than 5%. If any natural oxidant were lost, the fraction was so small that it could not be determined by inspection of the data. Other tests at a facility producing synthetic smog oxidant by artifical irradiation of automobile exhaust yielded similar results.
60 sq.
inches of chromium trioxide
RESULTS AND DISCUSSION
Interference from Nitric Oxide. Iodometric instruments suffer from positive interference caused by nitrogen dioxide. Because the chromium trioxide absorber is capable of oxidizing almost all of the nitric oxide to nitrogen dioxide, it causes the former gas to give a positive interference of about 10%. h different absorbent was developed to eliminate the interference of nitrogen oxides. This consisted of 5 grams of the silica gel saturated with 10 ml. of a solution that was 0.411 sodium dichromate and 0.72M sulfuric acid, dried at 120" C. for several hours, and then packed into a 100-mm. Schwartz drying tube. At a flow rate of 0.14 liters per minute it absorbed 99% of 4 p.p.m. of nitrogen dioxide that was passed through, whereas it transmitted 95% of 0.3 p.p,m. of ozone. As soon as the silica gel became damp, however, only 9% of the nitrogen dioxide was absorbed; the ozone recovery remained a t 95%. The silica gel could be rejuvenated by heating it a t 120' C. for a few hours. The same mixture also absorbed sulfur dioxide with 95 t'o lOOOj0 efficiency. Because moisture is usually a problem in field operations, this mixture would not be satisfactory for continuous use, but it could be useful for intermittent application or for laboratory studies. The chromium trioxide-sulfuric acid-
glass fiber paper absorbent was regarded as more practical because it was insensitive to atmospheric humidity. Common Concentrations of nitric oxide are in the range of 0.1 to 0.2 p.p.m.; higher concentrations are rarely encountered in the atmosphere and then only in conjunction with low oxidant values. Appropriate corrections can be made if available measurenient,s indicate the presence of nitric oxide. Ordinarily this is an all or nothing correction; if there is enough nit,ric oxide present to necessitate a correction, the concentration of ozone is negligible. Use of the chromium trioxide absorber would be disadvantageous in only a few areas such as Los .\ngeles, where the concentrations of nitric oxide are relatively high and those of sulfur dioxide low. Effective Lifetimes and Performance of Absorbers. The chromium trioside paper showed no deterioration of performance in preliminary laboratory tests. A mixture of 15 p.p.m. sulfur dioxide and 0.15 p.p.m. ozone was passed through a 100-mm. Schwartz drying tube containing 6 square inches of paper a t a flow rate of 0.14 liters per minute for 36 hours. At the end of this time 100% of the sulfur dioxide was still being removed with no loss of ozone. Because atmospheric concentrations of sulfur dioxide seldom reach 1 p.p.m., the conditions of this test were extreme. Other tests were made t o determine the effect of sodium chloride aerosol on performance. h midget impinger containing 20 ml. of saturated sodium chloride was inserted in the system just upstream from the absorber C-tube. Passage of purified air through the system for 2 hours caused the paper to become visibly wet. No detectable indication of oxidant was shown by the monitor. After the paper was dried, no adverse effects on its performance with ozonesulfur dioxide mixtures were found. In another test, a bubbler containing dilute silver nitrate was placed in the train downstream from the impinger containing the sodium chloride solution; the turbidity produced in a few minutes demonstrated the presence of chloride aerosol. Polluted air from several sources was passed through similar tubes to determine their effective lifetimes in the field. ,4t intervals, the absorbers were brought into the laboratory and tested for ozone transmittance and efficiency of sulfur dioxide removal. During these tests the papers gradually turned dark, but' their effectiveness apparently was not reduced. When the small absorber was exposed to air at higher than 90% relative humidity, ozone transmittance in some instances dropped to 90%; but as the absorbent
0.60
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OXIDANT ( B E C K M A N 1
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Figure 3.
Continuous analyses of downtown Cincinnati air on June 27, 1963 Tracings produced by four instruments, os indicated
material dried, ozone recovery came back to 100%. Sulfur dioxide interference in all cases was negligible. This work demonstrated that the effective lifetime of the small absorber in continuous use with the Mast instrument was a t least several weeks. Some data from Cincinnati, obtained on a morning when appreciable sulfur dioxide u'as present, are given in Figure 3. The Beckman sulfur dioxide analyzer measures conductivity of a n absorbing reagent containing 0.0005N sulfuric acid and 2.5YG hydrogen peroxide. The Beckman oxidant instrument measures colorimetrically the iodine released from a neutral buffered 10% potassium iodide solution, whereas the Mast oxidant instrument measures coulometrically the iodine released from a neutral buffered 2% potassium iodide solution. When comparing these results, note that the instruments require 10, 5, and 2 minutes, respectively, for full response. Also the Mast data were obtained according to the manufacturer's calibrations believed to be about 60% of the correct value. During the night hours (not shown) all three oxidant instruments indicated zero concentrations. Kote that from 8:30 A . M . to 3:30 P.M. the Mast instrument mith the absorber (bottom tracing)
indicated a continuous oxidant level of reasonable magnitude. During the same period, a t the times when the sulfur dioxide was present, the Mast and Beckman instruments without the absorber showed lower and widely fluctuating oxidant concentrations, dropping to zero for the former and to negative values for the latter. These low values indicated the interference of sulfur dioxide. During the past year the large absorbers have been applied to the Beckman oxidant analyzers a t most stations of the Continuous Air Monitoring Program. Because deterioration in performance (a slight loss of oxidant) was noted after 1 month of continuous use in the most heavily polluted area, an effective lifetime of 2 weeks was established. Losses of oxidant have been determined occasionally by sampling air from an ozone generator and comparing the readings obtained before removal of the used absorber, ivith no absorber, and after installation of the replacement absorber cont,aining unused packing. These losses have been negligible, even when the paper appeared visibly wet from passage of humid air. During the night hours t,he oxidant usually has been negligible after rorVOL. 37, NO. 6, M A Y 1965
781
recting for the 10% interference of nitrogen oxides. In the daytime the stations generally have shown more oxidant than was observed previously without the absorbers. Interesting new information on oxidant pollutants is being obtained. ACKNOWLEDGMENT
The assistance of George A. Jutee, Kirk E. Foster, and Richard J. Lewis
with the field work and in providing reports of absorber operation in Continuous Air Monitoring Stations is acknowledged. LITERATURE CITED
(1) Adarns, D. F., J . Ai7 Pollution Control Assoe. 13, k8 (1963). (2) Bravo, H. A,, Lodge, J. P., Jr., ANAL. CHEM. 36, 671 (1964). (3) Chahk, J., Schafer, L. J., Yeager, D . W., J . Air Pollution Control Assoe. 5 , 227 (1956).
(4) Hersch, P., Deuringer, R., ANAL.
CHEM.35, 897 (1963). H.,J . Geophys. Res. 69,
( 5 ) Regener, ,.,,”,> V. ~~
37bn
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(6) Saltmmn, B. E., Gilbert, N., Am. Ind. H y g . Assoc. J . 20, 379 (1959). (7) Salternan, B. E., Gilbert, N., ANAL. CHEM.31, 1914 (1959). (8) Wartbnrg, A. F., Brewer, A. W., Lodee. J. P.. Jr.. l n t ~ mJ. . Air Water P o l l & n 8 , Zl~(l964).
Division of Water and Waste Chemistry, 145th Meeting, ACS, New York, N. Y . , September 13, 1963.
Determination of Oxygen in Uranium Dicarbide Beads Having Highly Impervious Pyrolytic Carbon Coatings M. E. Smith, J. M. Hansel, and G. R. Waterbury, Lor Alamor Scientific Laboratory, University of California, Lor Alamor, HANDLING DEVICE is defor crushing and loading materials into a crucible within an inert atmosphere of a furnace which is an integral part of an inert carrier gas apparatus for determining oxygen. The crushing operation is prerequisite to the satisfactory release of oxygen at 2600’ C. from uranium dicarhide beads having highly impervious pyrolytic carbon c o a t ings. Increasing the operating temperature of the inert carrier gas method to 2900° C. also releases oxygen from these beads, hut operation at the higher temperature is impractical because of excessively high apparatus blanks and the rapid failure of available crucible materials. Crushing must be done in an inert atmosphere because the exposed cores are extremely reactive and are readily contaminated with oxygen. Some duplex-coated beads and all triplex-coated beads investigated require crushing prior to the determination of oxygen. In the investigation of the effectiveness of the sample handling device, the inert carrier gas apparatus equipped with a current-concentrator furnace, as previously described (21, was used. Other types of furnaces could have heen used for this purpose if they had been modified to support the device.
Ascribed .
SAMPLE
EXPERIMENTAL
Apparatus. T h e sample handling device, shown inserted in the furnace (Figure 1) and in isometric view (Figure 2), is made of stainless steel, with the exception of the crushing surfaces (Figure 2) which are hardened tool-steel disks cemented to the stainless steel with epoxy resin. The bottom of the device is hollow and forms a funnel which may be rotated by a lever at the top t o provide either a crushing surface or a n opening through which the crushed samplr may be poured into the open crucible within the furnace (Figure 2). C r u d i n g is accomplished by screwing down the stainless steel plunger while 782
.
ANALYTICAL CHEMISTRY
Figure 1 . Furnace with sample handling device in position
the sample rests on the crushing surface of the disk. A hole is drilled in the side of the sample handling device to permit argon Ron into the plunger cavity while the sample is added by the longstemmed funnel made of hypodermic tubing. The device is lowered into the furnace against a Row of argon to prevent oxygen contamination. The device is supported in the furnace by an a d j u s e able brass collar fixed in position with a set screw (Figures 1 and 2). Procedure. The operation of the device is shown schematically in Figure 3. T h e shaded area helow the supporting collar represents the sight glans adapter a t the top of the furnace through which the device is inserted. In the flushing position, the crushing plunger is removed and the funnel rotated t o permit the argon from the furnace t o flow through the funnel stem. After a few seconds, the funnel is rotated to provide a crushing surface. T h e weighed sample is added through a long-stemmed funnel while argon continues t o flush the plunger cavity through a hole in the
N. M.
side. The long-stemmed funnel is removed, and the plunger is inserted and screwed down to crush the sample. The device is then lowered, and the funnel stem inserted into the crucible. The crushing pressure is released, and the funnel is again rotated to the position where the sample falls into the crucible, A small electrically operated engraving tool is held q a i n s t the side of the sample handling device to vibrate the funnel and assist in the transfer of material. The device is then removed from the furnace. During this operation, argon flow prevents contamination of the sample with oxygen. The furnace is capped and alloved to flush for 1 minute before the oxygen is determined according t o the previously described method (2). The amount of sample left in the sample handling device is determined by disassembling the device, rinsing any adhering particles with methyl chloroform onto a weighed filter paper contained in a suction funnel, and reweighing the dried paper. This amount, which is subtracted from the original sample weight, generally is less than 3 mg. and often less than 1 mg. RESULTS AND DISCUSSION
The device was first applied to a sample of uncoated uranium dicarbide beads. Oxygen was determined in portions of the uncrushed beads using the original method (2) and the values were compared with those obtained for portions of the beads crushed with the sample handling device prior to analysis. Agreement among the results (Table I) indicates that no observable oxygen contamination was introduced by crushing the sample in the handling device. When this same material was crushed in an inert atmosphere enclosure and then exposed to air, the powder was ohserved to be pyrophoric. Application of the sample handling device to uranium dicarbide beads coated with a single layer of pyrolytic carbon did not increase the values obtained for the oxygen (Table I). This confirmed Drevious results with similar
