V O L U M E 2 3 , NO. 3, M A R C H 1 9 5 1 n-ater, heated almost to boiling. The iodate was reprecipitated by adding 3 grams of potassium iodate, and was cooled, centrifuged, and washed. The iodate was metathesized to the hydroxide by stirring x i t h 35 ml. of 1 Jf ammonia, centrifugation, and removal of the supernatant. The bulk of the hydroxide was transferred t o a measured excess of standard 0.05 M oxalic acid in the titration vessel. The centrifuge tube and stirring rod were washed with 3 to 4 drops of concentrated nitric acid and as much water as was necessary to effect quantitative transfer. The titration vessel was then inserted in the oscillator, a few minutes were allowed to reach equilibrium, and the titration was then completed with 0.025 ;If standard thorium nitrate. Metathesis of the hydroxide to the oxalate in the titration vessel appeared rapid and quantitative. I n sonic cases the iodate was introduced t o the titration vessel without metathesis to the hydroxide first, but the titration of the iodate was not always as satisfactory as t h a t of the hydroxide. ACKNOWLEDGMENT
This work was supported in part by funds from the Wisconsin iilumni Research Foundation, and in part by a grant-in-aid from E. I. du Pont de Nemours & Co.
475 LITERATURE ClTED (1)
Blaedel, W.J., and Malmstadt, H. V.,
..is.4~. CHEM.,22,
1410
(1950). (2) Clifford, R. A , , J . Assoc. Ofic.Agr. Chemists, 23, 303 (1940). (3) Dodgen, H. TI-., and Rollefson, G. K., .J. A m . Chwii. Soc., 71, 2600 (1949). (4) Eichler, A., 2. anal. Cheni., 129, 396 (1949). (5) Gooch, F. A , , and Kobayashi, AT., Ana. J . S c i . , 195, 227 (1918). ( 6 ) Hoskins, W. M.,and Ferris, C. .I.,IND. ESG. CHEM.,AXAL. ED., 8, 6 (1936). (7) Rfoeller, T., Schweitzer, G. K., and Starr, D. D., Chem. Rets., 42, 6 3 (1948). (8) Rider, B. F.,and Mellon, 11. G., dm2. Chini. Acta, 2, 370 (1948).
(9) Rodden, C . J., “Thorium Standard Samples,” L-. S. Atomic
Energy Commission, MDDC-1220 (1947). (10) Rowley, R. J., and Churchill, H. V., IND. ENG.CHmr., A N ~ L . ED.,9, 551 (1937). (11) Scott, IT. W.,“Standaid Methods of Chemical Analysis,” pp. 946-53, Kew York, D. Van Nostrand Co., 1939. (12) Willard, H. H., and Winter, 0. B., ISD. EXG.CHEM.,.IXAL. ED., 5, 7 (1933). RECEIVED August 7, 1950.
Cold-Surface Collection of Volatile Atmospheric Contaminants R . D. CADLE, MYRA HOLSTON, AND P. L. MAGILL Stanford Research I n s t i t u t e , Stanford, Calif.
The analysis of contaminated atmospheres is often complicated by the low concentrations involved. However, condensation on cold surfaces can often be used to concentrate volatile materials. The sampling of volatile atmospheric contaminants by cold-trap techniques was studied in order to determine the efficiency of such methods and to develop improved equipment. Formaldehyde at concentrations in air of a fraction of a part per million was collected with about 8070 efficiency at liquid nitro-
D
URISG the course of an investigation of air pollution in the Los Angeles area it became evident that cold-trap tech-
niques would be extremely valuable for the collection from the air of certain volatile contaminants such as aldehydes, ketones, and organic acids. The condensation of volatile contaminants by draffing them over cold surfaces is a well-known technique, b u t surprisingly little is known about the efficiency of such methods, particularly for the recovery of substances present in air a t fractions of a part per million. Goldman and Dalla Valle ( 1 ) have discussed the efficiency of a dry ice-acetone trap for the collection of water vapor and bromine a t relatively high concentrations. The work described herein consisted of a study of the effect on the efficiency of cold-trap collection devices of changes in design, of the use of liquid nitrogen as well as dry ice as the cooling agent, and of the use of glass and metal packing in the traps. Efficiencies were determined for the collection of sulfur dioxide, formaldehyde, and k n z e n e . The vapor pressure of sulfur dioxide a t the boiling point of liquid nitrogen is about 3 X mm. of mercury and that of benzene is about 1 X 10-18 mm. of mercury. These values were estimated by extrapolation from data presented by Lange ( 2 ) using the equation loglo Pmm, of H~ = - 52.23 ,4 B. Thus, the collection efficiency of these sub-
+
gen temperatures using metal packing in the traps. Without packing, only 50% efficiency was achieved. Similar results were obtained with low concentrations of benzene and sulfur dioxide. A convenient cold-trap system was developed consisting of a dry ice-cooled coil followed by a liquid nitrogen-cooled, metal-packed U-tube. The results of this work have been very helpful in studying the composition of smog in Los Angeles County and should be applicable to many air pollution problems.
stances a t the concentration used in this investigation (greater than 0.1 p.p.m.) was limited by the apparatus rather than by the vapor pressure. APPARATUS 1
A cold-trap train which was used extensively in the Los Angeles area is shown diagrammatically in Figure 1. Figure 2 is a photograph of this apparatus mounted on the chamber in which the contaminated atmospheres were prepared. The apparatus consisted of three 1-liter glass Dewar flasks mounted one above the other in a cascade system. Below the flasks was a more conventional trap consisting of an inlet tube extending nearly to the bottom of an outer tube. This lowest trap xas about 35 em. long and 3 cm. in diameter. T h e three flasks and the bottom trap were connected in series by means of balland-socket glass joints. Air was drawn between the walls of the flasks, through the bottom trap, through a wet-test gas meter, and finally through a vacuum pump. The Dewar flasks were filled with dry ice-isopropyl alcohol mixtures to cool the air passing between the walls. The outer surfaces of the Dewar flasks were insulated by placing them in boxes or cans and filling the remaining space with glass wool. The fourth and lowest trap was cooled by immersing it in a 1-liter Dewar flask containing either a dry iceisopropyl alcohol mixture or liquid nitrogen.
ANALYTICAL CHEMISTRY
476
nitrogen-cooled trap during the 1-hour sampling periods. During thawing, the escaping Formaldehyde, 20-Cubic Foot Samples oxygen carried with it a part P.p.m. by wt. P.p.m. by wt. Benzene, 12-Cubic Foot Samples of the contaminants which P.p.in. by wt. Recovery in air, detd. by in air, detd. Theor., bubbler by cold trap Recovery by p.p.m. in in air, detd. by cold were collected. The amount Operating Conditions sampling recovery cold traps, R air by cold traps traps, of formaldehyde lost in thik No packing 0.139, 0 . l O f i 0.073, 0.083 62. 50 89.0 53.3 60 Glass-bead packing in manner was determined I , \ 0.284 0.185 fi5 ..., .... .. layt trap passing the gases that escaped Stainless steel-washer on thawing through a series of 0 230. 0.2-Lfi 0 196. 0.195 85. 79 89.0 80.8 91 parking in last trap two fritted-glass hubhlers conGlass-head packing in last trap, operated a t taining a 1% sodium bisulfite’ 0.5 ntm. pressure 0.172 0,092 .3i .... .... solution, then analyzing this Dry ice--isopropyl alrohol roolant !:sed for solution hy the chromotropic. all 4 traps 0.10’’ 0.024 24 .... .... .. acid method. With the unu Except where otherwise indicated, coolant i n the 3 Dewar-type flasks was dry ice-isopropyl alcohol and the 4tb packed trap system, 6% of tht. trap was cooled by liquid nitrogen. Sampling rate in all cases was 20 cubic feet per hour. formaldehydein the air sampled __ was lost from the trap3 during this gas blowoff in each of thtb EXPERIMEYT \L two trials When steel washers were used as packing, 0.5% R-RC lost in each of two tests. T h e collection efficiency of this cold-trap train for fornialdeOxygen condensation a as prevented during one run by mainhydr. and benzene vapor was determined under various operating taining the interior of the liquid nitrogen trap a t 0.5 atmosphere conditions. pressure with the aid of stopcocks on each side of the trap. Thv formaldehyde recovery dropped to 53%, making such an arrangeKnown concentrations of formaldehyde (0.1 to 0.3 p.p.m. by ment undesirable. weight) in air were prepared in a chamber of 10 cubic meters in volume, through which air was moving a t a rate of 5 cubic meters per minute. This chamber has been described in detail in other AI CONDENSING TRAPS MADE FROM reports from this laboratory ( 4 , 6). Formaldehyde vapor, preIN I LITER DEWAR BLANKS pared by dripping a dilute solution from a motor-driven hypodermic syringe into a hot flask, was mixed with the air streain before it entered the chamber. As a check on the amount of formaldeINSULATING BOXES hyde in the chamber atmosphere, the air was drawn through two fritted-glass bubblers in series which contained a 1% sodium bisulfite solution. Previous work in this laboratory has shown this sampling method to be almost 100% efficient for the collection of formaldehyde. The formaldehyde recovered by both bubbler9 and freeze-out train was measured by the chromotropic acid method described by MacFadyen (5). I n all experiments the sampling rate through the freeze-out train was approximately 20 WET TEST METER, cubic feet per hour, and sampling was continued for about 1 hour. I AIR VACUUM P U M P 7 I Benzene (about 90 p.p.m. by weight) was vaporized in the air ERSED TRAP = n ! stream entering the test chamber; the concentration of benzene in the chamber was estimated from the rate of vaporization and thc rate of air flow through the chamber. The amount of benzene retained by the traps was determined by measuring the optical density at 254 mp of an aliquot of the combined eolution collected and the washings. The benzene was completely soluble in the water which was simultaneouely collected. This relatively high concentration of benzene was ueed to provide a measurable ahFigure 1. Schematic Drawing of Cascade norption a t 254 mp. Cold-Trap Device Table I.
Collection Efficiency of Various iModifications of Cascade-Type Cold-Trap SystemG
5~~~ ~
I-d
The results are shown in Table I . Glass beads in the last trap increased the efficiency from 50 to about 65% and the use of metal packing (9-mm. diameter, stainlpss-steel washers) increased the recoveries to about 80%. It is probable that the low collection efficiency of the unpacked train was due partly to the formation of a fine mist that was not retained by the walls of the traps. The visible mist disappeared when either glass or metal packing was used. In an attempt to increase the efficiency of the apparatus without the use of packing, an electrostatic precipitator was built into a trap, immersed in liquid nitrogen, and placed a t the end of the train. This precipitator trap collected a considerable quantity of organic material, but this material had been chemically changed by the electrical discharge. (This was s h o m by the yellowbrown color which developed in place of the usual purple in the chromotropic acid-formaldehyde analysis; also, the ultraviolet absorption curve of the condensate from a benzene-contaminated atmosphere was quite different from the absorption r u n e of benzene, showing stronger absorption in the 230 to 250 mp range.) Several milliliters of liquid oxygen accumulated in the liquid
One test was made in which the dry ice-isopropyl alcohol mixture was used throughout (replacing liquid nitrogen in the last trap). Only 24% of the formaldehyde was retained by the traps, 20% in the first three traps, and 4% in the last. .4ccordingly, in the experiments which follox, liquid nitrogen was used as coolant for the fourth trap; the use of dry ice-isopropyl alcohol was continued in the three Dewar-type traps. APPARATUS I1
The preceding apparatus was bulky and therefore difficdt to use for field sampling. Consequently, a freeze-out system of quite different design was built (Figure 3). This system consisted essentially of two parts: the first was a dry ice-cooled trap, the main function of which was to remove water from the air, and the second was a liquid nitrogen-cooled trap. The dry ice trap consisted of an &foot coil of 18-mm. outside diameter borosilicate glass tubing which was immersed in a dry ice-isopropyl alcohol mixture in a 1-gallon, wide-inoutheti Dewar flask. This coil afforded approximately the same area oi cooled surface for condensation as did the three Dewar traps of
V O L U M E 23, NO. 3, M A R C H 1 9 5 1
Figure 2.
477
Cascade Cold-Trap Device
Mounted on side ef chamber in which oonfsminenta dispersed
~
~
~~
hydroxide solution. In both the cold trap condensate and washings, and the bubbler solutions, measurement of sulfur dioxide content was made by oxidizing the sulfite to sulfate with hydrogen peroxide and analyzing the sulfate turbidimetrically after the addition of barium chloride, as in the method described by Sheen et al. (5). I n all cases the sampling rate w&s 20 cubic feet per hour. After eachsample was taken and t h e condensate was allowed to thaw and was removed from the traps, both the spiral trap and the packing in the liquid nitrogen trap were washed with three aliquot8 of distilled water, and the rr-ashings were added to the condensate for analysis. A total of about 150 ml. of condensate and washings was obtained by this procedure. The results of these tests are given in Table 11. Because of the probability t h a t additional contaminant could be collected by using additional traps in the train, a second washer-filled, liquid nitrogen-cooled U-tube was plseed after the first, and the resulting series of three traps was used t o collect formaldehyde. The condensate and washings from each trap were analyzed separately, and the recoveries were compared with recoveries obtained by hubhler sampling. The results are shown in Table 111.
~~
side was 35.mm. outsid&dia&tmoter glass tubbg, and the outlet arm was 8-nun. outsidediameter tubing; the over-all length was 30 em. The top of the large arm was connected to the spiral of the preceding trap by a ground-glass joint. The wide arm of the liquid nitroaen trao W&E filled with 9-mm. diameter stainless-steel washers. The ushof a U-tube greatly simplified both the introduction of the packing material and the washing of it a t the end of a sampling period. Air samples drawn through this system passed next throueh a eas meter and then through a vacuum pump.
Table 11. Efficiency of Second Type of Cold-Trap System for Formaldehyde, Benzene, and Sulfur Dioxide Collection Contaminant Formaldehyde Sulfur dioxide Benzene
KPERIMEWAL
i n e emciency 01 tnis cold-trap trsin w a determined by using it to sample atmospheres, prepared as previously described, containing known amounts of formaldehyde and benzene vapors. The collection efficiency for sulfur dioxide was alia found by metering this gas into the air stream entering the test chamber, and then sampling the resulting atmosphere. As with formaldehyde, the sulfur dioxide concentration in the chamber was checked by parallcl sampling through two fritted-glass bubblers in series. I n the case of sulfur diosidc, the hulhlers contained z 0.1 N sodium
Gemple Sire,
Cubic Feet
concentration Of Contaminant, P . P . ~by . Wt. 0.362, O..ZR2 2.8
13 12 6 13
Recovery by Cold Traps, % 79. 77 64
88 93
186
169
T a b l e 111. Efficiency of Individual Traps in Second Type of S y s t e m Peroentase of Total P..~".~,,bL..A~
Pemenh..~ of Total
n",i""*"A
"-"."l,,",...,L
Dry ice
Firstli uidnit Second?iwid I
The results of this investigation indicate t h a t fairly efficient collection of many volatile contaminants from contaminated atmospheres can he achieved by the U E of a simple cold-surface condensation system. The presence of proper packing, such 88 the steel washers, considerably increased t h e efficiency. Probably the most important function of this packing was t o increase the surface on which the volatile material could condense, thus decreasing the m o u n t of condensation on nuclei in the air. Thia n.ould prevent the formation of fogs which could eseme from the traps. LITERATURE CITED Goidman, H. F., and Dslla Valle. J. M., U.8.Pub. Health Senice, Pub. Health H y l t h Repts., 54, 172833 (1939). Lange. N. A,. Lanee. A,, "Handbook Handbook 01 Chemistrv." Chemistry." Sandusky, Ohio, Hand.. Sanduskv. - . Ohio.
bookPublishers, biokPublishers,1941.
MacFadyen, D. A,, J . Bid. C h m . . 158, 10733 (1945). Magill. P P..L., L., I d . Eng. Chem.. 41, 247685 (1949).
Sheen. R. T.,Xahler, H. L., and Ross, E. M.. IND.ENB.CHEM., ANAL.ED.. ED.,7. 7, 26S5 1 (1935). 1935). 1natitute;"The Stanlord Research Institute, "The Smog Problem in Los Angela County," Second Interim Report, 1949.
Figure 3. Simplified Cold-Trap Device
RECEIVED October 2. 1950
