Determination of Traces of Acetylene in Air T. A. GEISSMAN', SARIUEL KAUFMAN*, AND DAVID Y. DOLL31.4X Thermodynamics Research Laboratory, Universitv of Pennsylvania, Philadelphia, Pa.
A method has been deFeloped for the determination of acetylene in air w h e n its concentration is as low as about 1 p.p.m. W-hen calibrated against pure acetylene as a primary standard, agreement with air-acetylene mixtures of known composition is within about 0.1 to 0.5 p.p.m. over the concentration range 1 to 15 p.p.m.
A
METHOD for analyzing air-acetylene mixtures containing less than 10 parts per million of acetylene was required in connection with studies being carried out in this laboratory on the quality of air fed to units producing oxygen by the liquefaction and rectification of air. The procedure described in this paper differs in several respects from that recently described by McKoon and Eddy ( 2 ) , both in the method of measuring the amount of acetylene collected in the condensing coils and in the fact that the authors' studies were made using compressed mixtures of acetylene and air while McKoon and Eddy were concerned with the acetylene content of liquid oxygen. The problems arising from the accumulation of acetylene in tlie rchoiler of the fractionating column have been reviewed by hlcKoon and Eddy ( 2 ) . Experience in the oxygen industry has shown that present methods of dealing with explosion hazards are not completely effective, although a careful selection of the air supply and routine checks of the acetylene content of the liquid in the fractionating column minimize the dangers. h careful consideration of the problem in this laboratory has led t o the conviction that complete safety in the operation of an oxygen plant, particularly when gaseous oxygen is produced, is likely to be achieved only if the air feed can be kept substantially free of acetylene. This condition has been partially realized in some commercial installations by estending the air intake lines for considerable distances into regions where the air can be assumed to be free of contamination. Even this costly expedient does not obviate the possibility of contamination, since hydrocarbons may be introduced into the air during compression. One of the possible methods of ensuring that the air entering the liquefaction unit is free of acetylene is to remove the acetylene from the comprrssed feed. The use of catalytic oxidation to achieve this result has been studied in this laboratory with the aid of the analytical method described in the present paper. The air-acetylene mixtures used viere prepared in high-pressure storage cylinders. The system n-hich included the catalyst being examined n-as supplied viith suitable outlets for collection of inlet and exit samples.
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Figure 1. Liquid -4ir-Cooled Condensing Coil for Collecting Samples
The modifications n-hich !yere introduced in the present work \yere (a)the use of a photoelectric colorimeter for the measurement of color intensity, using as standards a series of carefully made air-acetylene mixtures which were treated in exactly the same way as the samples of the unknown mixtures; and ( b ) the agitation of a measured sample of the air-acetylene mixture in a closed container Tvith a standard amount of the cuprous reagent. This obviated errors arising from changes in absorption rate with changing partial pressure of acetylene in the samples.
A consideration of existing methods for the collection and analysis of samples of air-acetylene mixtures led to the conclusion that the most promising method was that of Coulson-Smith and Sepfang ( I ) , the method adopted with some modification by McKoon and Eddy ( 2 ) . I n the authors' experience, however, this procedure proved unsatisfactory for two reasons: (1) while the colors of colloidal cuprous acetylide and ferric thiocyanate solutions bore a striking superficial resemblance, there was a distinct difference in tone which made reproducible color matching difficult at lox concentrations: and (2) the rate of passagc of the Concentrated acetylene-air sample-i.e., the time of contact1
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COPPER TUBING
LITERATURE REVIEW AND PRELIRIINARY EXPERIMENTS
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through the reagent had to be carefully controlled and standardized to give reproducible results. Stated in another way, with a standard method of passing the gas through the reagent there was not a linear relationship between acetylene content and color intensity when the acetylene-air samples became very dilute. The i-esult of these factors would be that while reproducibility of results could be achieved by strict standardization of procedure, the method would give relative and not absolute values.
APPARATUS AND REAGENTS
Samples of the mixture t o be analyzed were passed over sodalime and Drierite and then through a coil cooled in liquid air. The condensed sample was allowed to expand into a standard 300ml. gas-sample bottle into which the reagent was introduced as described under Procedure. Condensing Coils. The coils in which the acetylene samples were collected vere constructed of 18 turns of 3/~-inchstandard copper tubing, wound on a 2.5-cm. (1-inch) mandrel. The bottom turn led t o a 1 x 2-inch copper chamber packed with glass wool, from which the exit tube issued (Figure 1). Later experi-
Present address, University of California, Los Angeles, Calif. Present address, 1115 Paokard S t . , Ann Arbor, hlich.
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V O L U M E 19, NO. 1 1
nients showed t h a t this trap was not necessary, probably because the acetylene condenses as a film on the tube walls and not as a fog in the PRESSURE gas stream. However, the trap was always used as a precaution against the possible escape of condensed acetylene. The condensing coils were placed in 1-liter, wide-mouthed Dewar flasks and liquid air or oxygen was used as the coolant. Copper rather than glass coils were used largely because of their greater convenience of fabrication and their sturdiness. T h e greater heat transfer rate in copper coils would undoubtedly make it possible t o use much higher sample .flow rates. Direct comparlsons of this for copper and glass coils were not made, but temperature measurements of the efRuent stream in the copper coils under standard operating conditions showed that the temperature of the issuing Figure 2. Gas-Sample Botgas was not measurably tle and Device for Introduchigher than that of the tion of Reagent coolant. Other Apparatus. The ,condensed acetylene sampies were expanded and flushed with Toom air into standard 300-ml. gas-sample bottles. One outlet of these bottles was fitted with a stopper Of the proper size t o accommodate a 6-inch test tube (Figfire 2). To the bottom of a 6-inch test tube was sealed a 4-cm. length of glass capillary tubing. To this was connected, through a length of rubber tubing, a rubber bulb by means of which pressure could be applied to force the reagent from the test tube into the sample bottle which contained the acetylene sample. Reagent. The following solutions were prepared: Copper sulfate pentahydrate, 100 g r a m made up to 500 ml. with distilled water. Hydroxylamine hydrochloride, 150 grams made up to 500 ml. with distilled water. Ammonium hydroxide, concentrated ammonia diluted 1 to 1 with distilled water. Gelatin, 5 grams dissolved in 500 ml. of distilled water. The reagent was freshly prepared each day by mixing in the order listed: 10 ml. of copper sulfate solution, 60 ml. of hydroxylamine hydrochloride solution, 30 ml. of ammonia solution, 50 m1. of gelatin solution, and 50 ml. of distilled water. PROCEDURE
from the sample bottle, the inlet to the coil was opened and air was pulled through the coil until all the water had drained from the sample bottle. The stopcocks on the latter were then closed. In the 6-inch test tube were placed 10 ml. of the reagent and 10 ml. of alcohol (to reduce foaming). The test tube was fitted t o the stopper on the gas-sample bottle, pressure was applied to the test tube by means of the hand bulb, and the stopcock was opened long enough to allow the reagent t o be blown into the sample bottle, and then closed. The sample bottle was shaken vigorously for one minute, during which time the reagent acquired a pink to burgundy-red color. The solution was then transferred quantitatively to a 100-ml. volumetric flask and made up to the mark with distilled water. A 5- or 10-cc. aliquot (depending upon the amount of acetylene present) of the diluted solution was transferred to a colorimeter cell of 100-mm. light path and the transmission read in a model 402E Photovolt Lumetron colorimeter equipped with a 515 mp filter. The amount of acetylene was read from a standard curve prepared as described below, and the concentration (in p.p.m.) of the original gas was calculated from the measured volume of air-acetylene mixture taken.
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Remarks. Separate experiments showed that a t flow rates of 4 to 6 cubic feet per hour no detectable amount of acetylene escaped condensation in one coil. This was demonstrated by using two such coils in series: in no experiment was any trace of color developed in the reagent with which the content of the second coil was treated. Calculations indicated that a t flow rates of this order the gas stream reached substantially the coolant temperature a t a point comfortably distant from the exit end of the coil. Although liquid or solid acetylene is known to be unstable, and the possibility exists that the use of copper coils might allow the formation of explosive copper acetylide, it is felt that a t the temperature of liquid air and with the smallamounts of acetylene that are involved in an analysis these hazards are negligible. The amount of sample taken should be such that the final concentrated mixture in the gas-sample bottle contains in the neighborhood of 0.3 $0 0.6 ml. of acetylene. When amounts of acetYlene exceeding 1 ml. are present in the gas-sample bottle the cuprous acetylide occasionally shoR7s a tendency to flocculate. The water drained from the gas-sample bottle during the transfer of the sample from the condensing coil never contained any detectable amount of acetylene when tested with the cuprous reagent. A standard cpntact time between the reagent and the acetylene sa%$G-irable but is not very critical. Upon prolonged shaking the transmission changes somewhat, and it is cornequently desirable to treat unknown samples in the same way as
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The gas stream t o be analyzed was passed through a drying tower containing sodalime and (indicating) Drierite, into the condensing coil, and finally through a n-et meter. When a suitable volume (determined by the acetylene content of the gas) had been passed, the exit of the coil was connected to the upper nipple of a gas sample bottle ivhich was filled with water and supported in a vertical position on a ring stand. The stopcocks of the gas sample bottle were opened, the entrance to the condensing coil was closed with a pinch clamp and the coil was removed from the liquid air and placed in a beaker of water. After the coil had warmed up and water had ceased to issue
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I IA Figure 3.
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Acetylene Purification Train and Means for Delivering Known Amounts of Acetylene for Standardization of Method A. B. C. D,
Acetylene cylinder Milligan gan-washing bottle containing NaHSOs Milligan gas-washing bottle containiqg 20% NeOH E. CaCh towers F. Glam wool acked tower C. CalibratedTP-ml. Mohr pipet H . Thermometer I . Mercury reservoir (Tygon tubing) J. Gam-sample bottle
N O V E M B E R 1947
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colorimetric method with the result obtained by measuring the amount of condensed acetylene by standard methods (fuming sulfuric acid) of gas analysis. Satisfactory agreement was found. Experience with preparing compressed mixtures of air and acetylene has shown, however, that the concentrations of such mixtures found by analysis occasionally differ in an erratic manner from the expected (calculated) values, although any given mixture gives reproducible results by the method of analysis described in this paper. It is probable that the presence of grease, oil, rubber, etc., in the equipment and lines used may account for these differences.
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RESULTS
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The accuracy of the method can only be inferred by examining the results obtained on standard high-pressure mixtures, since the authors feel that the analytical method is more reliable than the accuracy with which the concentrations of such mixtures can be known. The pfecision of the method is not high when expressed percentagewise; however, it is felt that a precision of 0.1 to 0.5 p.p.m. is easy to achieve over the range of 1.0 to 15.0 p.p.m. An occasional result falls outside these limits. In Table I are given some results obtained on standard mixtures, made up in highpressure storage tanks. The concentrations of the standard mixtures are nominal values, and are seen to correspond rather well with the analytical values.
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Calibration Curve
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the standard samples from which the calibration curve is constructed. Ideally, the 5- or 10-cc. aliquot of the diluted reagent should again be diluted to an accurately-measured volume before the transmission is measured. If, as was done in this work, a single colorimeter cell is used for both the standardization and the analyses, and the reagent aliquot is diluted in this cell to some selected level in the filling neck, no appreciable error is introduced. STANDARDIZATION
Samples of purified acetylene were transferred from a caliibrated 1-nil. Rlohr pipet into a gas-sample bottle and treated with the cuprous reagent exactly as described above. The system used for this purpose is shown in Figure 3. The plot of “log % ’ transmission us. standard ml. of acetylene” was a straight line over the range used (Figure 4). It was found that if the sample of pure acetylene is transferfed to a sample bottle which is completely filled with water at the start, some of the acetylene dissolves in the water a t the point of entrance of the gas. This difficulty was overcome by introducing the acetylene sample into a,sample bottle not more than half filled with watw a t the start. The entering acetylene is then prevented from reaching the surface of the water in a concentration high enough to permit detectable loss (as determined by testing the effluent water with the cuprous reagent). In Figure 4 the points on the 5-cc. curves which are marked F were found by introducing acetylene into a sample bottle filled with water. The point marked H was found when the sample bottle was half filled with air a t the start. Secondary standardizations of the method were carried out by (a) preparing standard mixtures of air and acetylene by introducing a measured amount of acetylene into a bomb and adding compressed air to a measured pressure, and analyzing the resulting mixtures by condensing, etc., in the standard way; and ( b ) comparing the result of the analysis of such a mixture made by the
Results of Analyses of Standard Air-Acetylene Mixtures
Standard Mixture P . p . m . (noma’nu1)a 1.0 4.0 5.0 15.0
Found P.p.m. 0 . 9 , 0.9. 1.0, 1 . 1 , 1.1, 1.0, 1.0,1.1,1.0,1.0 4.1 4.1 4.2 4.1 4.8:4.8,’4.9,’4.9,5.0,5.2,(6.1), (5.8),5.4,5.1,(4.5),4.9 16.2, 15.8. 15.7, 16.1, 1 5 . 5 , 15.1, ( 1 6 . 7 ) , 1 5 . 9 , 1 5 . 6 , (14.9), 15.3, 16.0, 1 6 . 3 , (14.9)
Xfaximum Deviation from Average P.p.m.b - 0 . 1 : +O.l
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-0.7: +0.5
0 Prepared by compressing air into a bomb containing known amount of CaHa. b Figures in parenthesis omitted in computing average.
The method has been in continual use in this laboratory since 1944 and has adequately served the purposes to which it has been put. I t is probable that where greater precision is required the procedure could be refined to fit more exacting requirements. One possible way, which has been the object of a preliminary study, is to burn the acetylene in a combustion tube and determine the amount of carbon dioxide produced. For the’performance of a large number of routine analyses such a procedure would be less adaptable, however, than the colorimetric method described. ACKNOWLEDGMENTS
The authors wish to acknowledge the assistance of numerous members of the supervisory and operating staffs of the Central Engineering Laboratory (now the Thermodynamics Research Laboratory) in developing the method and carrying out the many routine analyses required. LITERATURE CITED
(1) Coulson-Smith and Seyfang, Analyst, 67,3941 (1942). (2) McKoon a n d E d d y , IND.EXQ.CHEM.,ANAL.ED.,18, 133, 136 (1946). RECEIVED January 20, 1947. This work was part of a program of research carried out in 1944-5 under Division 1. Section 11, of the National Defense Research Committee, under contract OEMsr 934 with the University of Pennsylvania.
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