V O L U M E 2 4 , NO. 12, D E C E M B E R 1 9 5 2 a t the appropriate setting of the residual current compensator. Here reduction waves could be measured on some linear portions of the curve, but this blank still leaves much to be desired. Curve C shows the result obtained when a trace derived from curve B is used on the curve follower. Here the blank is fairly linear from zero to a potential of -0.5 volt. The instrument has been applied to the determination of lead in a potassium nitrate medium. As shown in Figure 7 by using the appropriate compensating curve on the curve follower, a blank curve approaching a straight line can be obtained. Using this same compensating curve, polarograms were drawn on a series of lead concentrations in 1 M potassium nitrate ranging from 0.2 X M to 2.0 X 10-6 M . One of these curves is shown in Figure 7 (curve D) and the measured diffusion currents (measured by extrapolation to the half-wave potential) are tabulated in Table I. The same data are shown in graphical form in Figure 8. The figures in Table I show that the diffusion current at these low roncentrations is very nearly a linear function of concentration. The precision of measurement is relatively poor, but it is
1899 hoped that further study of the factors influencing polarography in these very dilute solutions will permit an improvement in precision to at least & 10%. The figures at higher concentrations are given in Table I to show that the diffusion current constant, id/^, appears to change with large concentration changes but to be linear over a small range. The diffusion constant predicted by the IlkoviE equation (3) for this electrode was 8.14 pa. per millimole per liter, agreeing very well with 8.10 observed a t 0.72 X 10-3 M . LITERATURE CITED
(1) Coor, Thomas, Jr., and Smith, D. C., Rev. Sci. Instrurne?its, 18, 173-81 (1947). (2) Ilkovi?, D., and Semerano, G., Collection Czechoslov. Chem. Communs., 4, 176 (1932). (3) Kolthoff, I. M,, and Lingane, J. J., “Polarography,” New York, Interscience Publishers, 1941. (4) Rosenbaum, F. J., and Stanton, L., ANAL.CHEM., 19, 794-8 (1947). RECEIVED for review July 25, 1952. Accepted August 29, 1952. Presented before t h e Division of Analytical Chemistry a t t h e 118th Meeting of t h e AMERICASCHEMICALSOCIETY,Chicago, Ill.
Determination of Small Amounts of Hydrocarbons in the Atmosphere PAUL P. MADER, MERLYN W. HEDDON, ROBERT T. LOFBERG, AND RUTH H. KOEHLER Los Angeles C o u n t y .4ir Pollution Control District, Los Angeles, Calif.
h spectrophotometric method for determining small concentrations of hydrocarbons in the atmosphere has been developed. It employs the use of a specially built 100-cm. gas cell supplied with rock salt windows and inserted between the light source compartment and receiver compartment of a Beckman infrared spectrophotometer. Because of the large volume of the new cell, the entire gas sample can be swept into the light path of the instrument. The hydrocarbon sample is collected by a freeze-out tech-
T
HE;studies made by the Los Angeles Air Pollution Control District show hydrocarbons to be the largest single group of gaseous and vapor phase contaminants that are being discharged into the atmosphere. Olefins, branched-chain, and perhaps other hydrocarbons represent the original raw materials u hich, in the presence of air oxygen, ultraviolet radiation, ozone, and oxides of nitrogen, may be converted into oxygenated products ( f - 3 , 6). These hydrocarbons in their oxidized form contribute substantially to the smog conditions prevalent in the I,os Angeles area in days of low wind speeds and temperature inversion. I t has been demonstrated that oxidized forms of hydrocarbons reduce the visibility ( 3 , 5 ) in the affected area and produce eye, nose, and throat irritation, as ell as damage to leafy vegetables ( 2 , 4 ) . In view of the significant effects caused by the hydrocarbon o\idation products, it became desirable to find a method for the determination of small amounts of hydrocarbons in the atmosphere at various times of the day. Specific measurements were needed in air pollution investigations to establish the correlation M hich might exist between the absolute amounts of hydrocarbons found in the atmosphere and the harmful effects experienced by humans and observed on vegetation.
nique, using a Shepherd sampler immersed in liquid oxygen. Based on the collection of the condensates of a 60-liter air sample, the instrument records the presence of 23 micrograms of hexane, or less than 0.1 p.p.m. of this hydrocarbon. The results obtained for known hydrocarbon concentrations are accurate and consistent. This method was successfully applied to the determination of hydrocarbons from air streams over polluted areas or inside industrial establishments.
Previous methods used for determination of minute amounts of hydrocarbons are based on the combustion of the collected sample to carbon dioxide, which is absorbed in an excess of barium hydroxide. The excess of barium hydroxide is back-titrated with a standard hydrochloric acid solution. The amount of carbon dioxide developed corresponds to the hydrocarbon content of the sample. The disadvantage of this method lies in the fact that the determination is executed against a background of 300 p.p.m. of carbon dioxide in the surrounding air, while the amounts of hydrocarbons evpected to be present in the atomosphere may be of the order of a few parts per million only. Any loose connection in the combustion train assembly or any air contact of the titration vessel receiving the generated carbon dioxide will lead to erroneous results. Frequent “blank” determinations must be performed. JThenever the combustion train is not in use for several days, it is necessary to heat the entire assembly for several hours to drive all accumulated carbon dioxide from the combustion tubes and the rest of the system. In spite of these inherent shortcomings, this method, laborious and timeconsuming as it is, can still be used by a skillful and patient operator. -4fairly simple and straightforward method for determination
ANALYTICAL CHEMISTRY
1900
by this method are consistent and reproducible even for minute concentrations. CALIBRATION OF INSTRUMENT AND SAMPLING
The method employed for the calibration of the infrared instrument with the special gas cell consisted in generating kuoa-n hydrocarhon concentrations in an 8 X 8 X 7 foot Plexiglas chamber (Figure 2). Representative samples were withdrawn from the chamber by the same method applied in the atmosphere.
Figure 2. Plexiglas C h a m b e r Used t o Generate Known Hydrocarbon Concentrations u n d e r Dynamic Conditions
of minute quantities of hydrocarbons in the atmosphere ums developed by this laboratory. It involves the use of a Beckman infrared spectrophotometer, Model IR2. I n its standard form, the instrument is equipped with two IC-om. gas cells. The COLItained volume of gas is usually sufficient in all cases where concentrated samples w e available for analysis. I n the case of analyses of air samples, however, where the total amount of the air pollutants present is very small, it was found that only a small fraction of the dilute sample entered into the gas cells, while a oonsiderahk portion stayed in the glass tubing of the gashandling system and the remainder was left in We collector v e 5 sel used to trap the pollutants from the atmosphere. This handicap was overcome by the construction of a specially built gas cell. This cell is cylindrically shaped, 100 cm. long and 3.5 om. in diameter; both ends are supplied with rock salt windows. Two glass tubes near the ends of the cell connect We gas cell with the gas-handling system and vacuum pump. The gas cell is inserted between the light source compartment and receiver Compartment (Figure 1). Because of the large volume of the gas cell, the entire sample can now be swept completely into the gas cell with dry nitrogen. By introducing the entire gas sample instead of a fraction into the light path of the instrument, it heoame possible to observe absorption bands for hydrocarbons present in the atmosphere a t concentrations of less than 0.1 p.p.m. by volume. Based on the collection of condensate from a 60-liter air sample, the instrument, with its specid gas cell built in, will register 23 micrograms of hexane. The results obtained
side the house was in operation. %Hexane (Phillips, redistilled, with boiling point 6&69" ,C.) was selected as the hydrocarbon to be used for the calibration. The initial calibration of the infrared spectrophotometer was performed with a blend of gasoliues comprising the products of five major oil producers in this area. When individual hydrocarbons from C, to CIOwere used for the standardization, i t was found
known hydrocarbon concentrations n-hexane seemed particularly suitahle for this purpose beosuse t i i s hydrocarbon is available in a high degree of purity while the composition of gasolines and their blends changes on standing, However, these reference oalibrations can be carried out with either compound, as long as material used exhibits essentially the same absorption characteristics For
TO PUMP
n
sigueu ana aevempeu uy nernmn n u u a liw u ~ i i v u ~ m UL~ y Southern California for the Los Angeles County Air Pollutiau Control District, 1 ) designed to dispense a liquid a t a known rate. The drops from the syringe were allowed to fall on a hot plate; the liquid hexane was vaporized when i t came in contact with the hot surface of the plate. The initial and find volumes of the hy-
1901
V O L U M E 24, N O . 12, D E C E M B E R 1 9 5 2 The fallowing method is suggested for filling this trap: ..-...~.. .~ ~~
~~
~~
measured by an orifi;: meter. "concentrations within the chamber oould be varied by changing the amounts of n-hexane dispensed or by adjusting the flow of filtered air. Mixing inside the chamber was accomplished by means of a fan. Temperature inside tho house was noted before and after each run.
Borosilicate glass wool is packed loosely through the male joint until i t reaches the constrictions a t point A . Ascarite is added through the other end t o point B. Another glass wool plug is now
ihould be taken to leave a free mace between 3 and E.. Any "_. .... -. ...-. . ., .
~
~
~~~~
.
this way the trap also acts a8 a flow indicator ana the necessity of using the manometer of the gas handling system is elimineted. PROCEDURE OF ANALYSIS
carite tmp. By this time the pressure of the sample had increased t o about 2 atmospheres, although the sampler was still cold. This increase in pressure was relieved by allorvlng the vapors to flow from the sampler into the gas cell. The closed trap was then immersed in a water bath of about 50' C. to melt any ice crystals whioh
showed that the flow intothe gas cell had stopped (by the returh
~~~~
Figure 4.
Shepherd Sampler for Collection of Minute Amounts of Gases and Vapors
Samples of tho house air were then taken with the he!p of a freeze-out assembly (Figure 3). The same arrangement 18 used for the collection of hydrocarbons from the atmosphere. It consisted of an orifice flowmeter, calibrated to deliver 0.5, 1.0, 1.5, and 2.0 liters per minute, and a drying tube filled with Ascarite for the removal of water vauor. carbon dioxide. aldehydes, ketones, and organic acids from the ineqm'ng air-hexane-mixture. The hydrocarbon components of the air stream are not affected hv t.he Aann,rit,n trn,n. The samnle itself N&S frozen out in the _i .... . ...-~ ~ . . Shepherd callect&t~&(5);whicg was immersed in a Dewar flask filled with liauid oxygen. The Shepherd sampler contained a
~
~
~
were transferred td the cell." It was calculated from the dimensions of the system that a total of eight transfers swept 99.3% of the concentrate into the cell, provided the temperature of the sampler was brought up to 50' C. Both valves were then opened, and rhore drv nitroeen was sweut throueh the t r m and sashandling s y s k n unci1 atmasphe;ic press& was reaohed. -AIthough nitrogen does not exhibit absorption in infrared, the total pressure inside the gas cell Nits increased t o 1 atmosphere in all determinations. TO
GAS HANDLING SYSTEM
GLASS WOOL ASCARITE GLASS WOOL ASCARlTk
,:m- . .,
B
....- .~.~.~~~. r~~~~ ~~
UL
* A
-
~
idly cooled t o low tenipkrature, from following a 'heamlined flow and escauins through the warm sampler exit. -~~~~~~~~ Asearite. "Blanks" far hydroc&bans were taken a t a paint 'm the duct connecting the blower unit and the Plexiglas chamber, after the incoming air had passed through the filtering system. The concentrated samples were maintained a t liquid oxygen temperature until they were ready for analysis.
Figure 5. Apparatus for Removing Residual Moisture
At first the evacuated cell N&S scanned lo the 3.0- to 4.0-micron
~~~~
MODIFICATIONS IN USE OF IR2
A 1-meter gas cell.with rock ~ a l windows t was installed (see above). The original IO-cm. xas cells. although still available to the system, were %eptin hlankposition. An automatic connection between the slit width, ,wavelength drive m d cain ront,rol N ~ Sinstalled. therehv maintainine an -... . -~. amplifier output of blank mns close to 100% trksmittance All unnecessaw sections of the standard gas-handling system, ~~
-~~-~ ~~~~~
~
the sample N&S found for the carbon-hydrogen absorption hand a t 3.45 microns, by comparimn of the results obtained for the evacuated cell and for the sample. At the beginning it ~ 8 deemed 8 advisable t o run several samples in duplicate. A number of samples of the n-hexane-air mixtures were also collected a t various sampling rates. The results obtained in all cases were consistent and therefore the collection m d analysis of more than one sample st one concentration were no longer considered necessary. The results found by these measurements are shown in Figure 6 and are tabulated in Section A of Table I.
I n addition to generating laown hydrocarbon concentrrttions under dynamic conditions, number of n-hexanne-air mixtures were generated under static conditions. The method used consisted of introducing into a 226-liter Plexiglas box ora %liter borosilicate glass flask concentrations of hydrocarbons approximating those existing in the amosphere. After thorough mixing, representative samples were withdrawn into
~
ANALYTICAL CHEMISTRY
1902 nine sampling tubes of known volumes. The samples were then transferred into the gas cell and the absorption a t 3.45 microns wae again determined as before,
Table I.
Comparison of Calculated and Experimentally Determined Values of Hexane
The results obtained are also shown in Figure 6 and are tabulated in Sections B and C of Table I.
Weight of Hexane, AIg.
DISCUSSION OF FIGURE 6 AND TABLE I
0.276 0.283 0.371 0.396 0.536 0.553 0.557 0.565 0.742 0.792 0.823 0.829 0.848 1.072 1.106 1.188 1.584 1.608 1.645 2.468 3.290
Of the 21 different concentrations of n-hexane collected under dynamic conditions and used for the calibration of the infrared spectrophotometer, 13 results, when plotted, fall on a straight line which approaches a transmittance of 100% when the concentration of sample decreases to zero. Except for one determination, where the measured transmittance is about 3% higher than the calculated value, the deviations of the other points from the straight line are minor. The group of values obtained from samples collected under static conditions and involving lower concentrations also fall close to the same straight line. The straight-line relationship between the logarithm of per cent transmittance and parts per million of hexane used in the tests can be expressed by the following equation: P.p.m./31.03
+ log T % = 2
9c
1.38 1.41 1.84 1.96 2.71 2.76 2.76 2.83 3.68 3.92 4.15 4.14 4.24 5.42 5.52 5.88 7.84 8.13 8.30 12.45 16.60
90.0 90.3 85.1 85.4 81.7 80.7 80.5 83.8 76.4 72.6 70.9 72.7 74.9 67.1 65 7 65 1 55.0 55,6
51.4 40.3 32.8
-0.2 +0.2 -2.2 -1.1
-0.1 -0 8 -1 0 +2.7
:
0.021 0.021 0.041 0.041 0.083
-1 6 -0.9 +1 9 +o 2 -0.7 +0.5 -0.9 +0.9 -2.6 +O 6 +3.6 1.2
Borosilicate Glass Flask, 22 Liters 0.30 0.50 0.60 0.89 1.04 1.34 1.98 1.98 1.98
Section C .
60
70
90.2 90.1 87.3 86.5 81.8 81.5 81.5 81.1 76.1 74.8 73.5 73.6 73.0 66.9 66.4 64.6 55.9 54.7 54.0 39.7 29.2
99.0 96.0 96.0 94.5 93.0 92.0 85.0 86.0 87.0
97.7 96.4 95.7 93.6 92.6 90.5 86.5 86.5 86.5
8C 7c
Deviation,
AV.
Section B. 0.064 0.106 0.128 0.192 0.213 0.236 0.426 0.426 0.426
IOC
P.P.31. in Sample Diluted t o 60 % Transmittance Liters Found Calculated Section A . Plexiglas House, 11,000 Liters
0.10 0.10 0.19 0.19 0.39
+1.3 -0.4 +0.3 f0.9 f0.4
+1.5 -0.5
+0.5 Av.
f0.4 0.7
Av.
-0.9 -0.8 -1.2 -1.7 -2.1 1.3
Plexiglas Box, 226 Liters 98.4 98.5 97.5 97.0 95.0
99.3 99.3 98.7 98.7 97.1
50 SUMMARY
40
30
20 0
2:5
5:O
?.I5
1010
1215
1510
PARTS PER MILLION FOR 60 LITER SAMPLE
Figure 6. Measurement of Hexane in Air
On the basis of this equation the per cent transmittance can be calculated for every concentration of n-hexane used. The comparison between the experimentally found values and the calculated values is shown in Table I. In 13 determinations collected under dynamic conditions (Section A), the deviations of the calculated values from the experimental values were 1% transmittance or below. The average deviation between all the calculated and the found values taken under dynamic conditions is 1.2% transmittance. The results of the hydrocarbons collected under static conditions, using a 22-liter flask and a 220liter Plexiglas box, show an average deviation of 0.7 and 1.3% transmittance, respectively, between the found and calculated values (Sections B and C of Table I).
Known hydrocarbon concentrations were generated under dynamic and static conditions. Representative samples xere withdrawn by two different methods and analyzed by infrared analysis. The fact that of the many samples analyzed most results, when plotted, fall on a straight line, while the rest show only minute deviations indicates that the method of collection is effective and the results of the infrared analyses are consistent and reproducible, even xhen very low concentrations of hydrocarbons are involved. After the calibration of the infrared instrument, the method was successfully employed for the collection and determination of smog samples. In view of the accurate results obtained, this method is recommended for the determination of trace amounts of hydrocarbons in gases and in air, as a relatively simple and reproducible spectrophotometric procedure. LlTERATURE CITED
(1) Air Pollution Control District of Los Angeles County, Technical and Administrative Report on Air Pollution Control in Los rlngeles County, Annuaf Report, 1949-50 Ibid., 1950-51. Haagen-Smit, A. J., Ind. Eng. Chem., 44, 1342 (1952). Haagen-Smit, A. J., Darley, E. F., Zaitlin, Milton, Hull, Herbert, and Noble, Wilfred, Plant Physiol., 27, 18 (1952). Mader, P. P., MacPhee, R. D., Lofberg. R. T., and Larson, G. P., Ind. Eng. Chem., 44, 1352-5 (1952). . . Shepherd. M..Rock, S. hl.. Howard, R., and Stormes, J., ANAL. CHmr., 23,1431 (1951). RECEIVED January 21, 1952. Accepted September 24, 1952.
