Microanalysis of Gaseous Hydrocarbons LEO MARION AND ARCHIE E. LEDINGHAM Division of Chemistry, National Research Laboratories, Ottawa, Canada
I
N THE study of gaseous reactions the gaseous products
Apparatus
are often obtained in such small quantities t h a t ordinary methods of gas analysis may not readily apply. Furthermore, these methods do not render it possible to differentiate between a homogeneous gaseous hydrocarbon and a mixture of hydrocarbons. A procedure has therefore been developed applying Pregl's microanalytical method for the determination of carbon and hydrogen t o the analysis of gaseous hydrocarbons obtained in such small quantities. B y introducing about 2 cc. of the gas into the combustion tube b y means of a buret inserted into the Pregl train and slightly modifying the wellknown procedure, extremely good results are obtained. As in the ordinary microdetermination (1, 2) of carbon and hydrogen in organic substances, the water and carbon dioxide are weighed. From these the weights of carbon and of hydrogen are calculated and the sum of the two is obviously equal to the weight of the sample taken. This weight makes i t possible not only t o determine the percentage composition b u t t o calculate the molecular weight of the hydrocarbon from the relation of the weight of the sample to the volume of gas taken.
The gas buret designed for this urpose is illustrated in Figure 1. It consists of a glass tube, A b-mm. bore), approximately 1 meter in length, branched at its lower end into a parallel compensating tube, B , and a short tube, C, which is closed with a s t o p cock and connected to a leveling bulb filled with mercury. The upper end of A is sealed to a capillary, D (1-mm. bore), carrying a stopcock and terminating in a ground tip, E, inside of which there extends a fine capillary tube, F (0.5-mm. bore), which is sealed to D. A piece of glass tubing, G, of the same diameter as the Pregl combustion tube, I , is drawn at one end to a tip of the size of the thermometer capillary, J, prescribed by Pregl (2, p. 26) to join the bubble counter, H , to I . G is connected to E by means of a ground-glass joint and can be secured in place with springs held by small glass projections on the two pieces of the joint. F should be sufficiently long to reach the center of G as shown. The buret is mounted on a supporting board on which a strip of graph paper has been pasted and is inserted in the combustion train between J and I . The buret is calibrated and the relation of centimeter length to volume is plotted on a graph which can be used to determine the volume of gas between any two levels of mercury in the buret. T o fill the buret with a sample of gas requires special precautions in order t o exclude the possibility of contaminating the sample with air. The sample is best introduced into the buret b y the apparatus illustrated in Figure 2.
n
It includes a manifold, A , one end of which, B, is connected to a vacuum pump while the opposite end, which is equipped with a stopcock, C, is joined to a mercury blow-off, D, and to a gas cylinder, when such is the source of the sample. A branches a t K into a parallel tube, E,which is connected by means of groundglass joints, F and G, to the as buret and to a receiver, H . The end of E carries a large hoflow-barreled stopcock, I , which is sealed to a cap with a ground-glass lip fitting over a ground-glass container, J .
Procedure When a as sample is to be taken from a steel cylinder the latter is joinei to the manifold, A , and the buret, after removal of its T-shaped head (G, Figure l), is connected at F. After the system, including the buret, has been evacuated, P is closed and stopcock C set to allow but a small opening through which the gas is then allowed to enter the system while the surplus escapes throu h the mercury blow-off. More than the required quantity of hyjrocarbon is condensed into the receiver, H , by cooling the latter in liquid air. C is then closed, the liquid air removed, and the pump allowed to run while the solidified hydrocarbon liquefies. I n order to remove noncondensable impurities completely the process of alternate solidifying and liquefying of the gas under the action of the pump is repeated once or twice. After the final removal of the liquid air from H , P is opened, K is closed, and the hydrocarbon is allowed to distill into the buret until atmospheric pressure has been reached. If the sample to be analyzed is in a sealed glass am ode, L, the latter is placed in J and enough mercury added to ti-ing the
I
I
FIGURE 1 269
270
Vol. 13, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
B
A
C
taken, corrected for normal temperature and pressure, the molecular weight is calculated. This method of analysis has been applied to a series of saturated and unsaturated hydrocarbons and the results obtained are shown in Table I. It will be seen on examining Table I that samples of hydrocarbons as small as 2 to 3 cc. may be analyzed and the carbon and hydrogen content determined with an accuracy of *0.2 per cent.
TABLEI. ANALYSESO F HYDROCARBONS
Ethane
U
Propane Butane
FIGURE 2 Cyclopropane
tip, M , of the am ule into the bore of I . D is cut off at C and the system, i n c l u g g the buret, is evacuated. P is then closed, J is immersed in liquid air to condense the gas, and K is closed. After the gas has solidified, M is broken by a sharp turn of I. The liquid air flask is removed from J and raised around H and the gas is allowed to distill over and condense in H . Stopcock R is closed, K is opened, and the system is pumped while the hydrocarbon li uefies. As in the case mentioned above, the alternate process o? freezing and pumping is repeated once or twice. Finally, the liquid air is removed, the system is cut off from the r m p a t K , P is opened, and the gas is allowed to distill into the uret until atmospheric ressure has been reached. After removal of the Buret from the filling apparatus the Tshaped head (G, Figure 1) is attached to it and the buret is inserted in the combustion train between J and I , as shown in Figure 1. The usual precautions prior to combustion are followed closely, except as outlined below. For the combustion of solids Pregl found that the optimum speed of the stream of oxygen oing through the train as measured in the bubble counter should e 5 cc. per minute. I n the case of gases it is essential to slow this down to 3 cc. per minute. It is also preferable to insert an empty platinum boat in the combustion tube and to keep it hot with the movable burner, exactly as for the combustion of solids. To carry out the combustion the weighed absorption tubes are connected into the train, the flow of gas is adjusted, and the barometric pressure, temperature, and buret reading are recorded. The lower stopcock is then opened and the leveling bulb adjusted so that the mercury in B shows a head of 2 to 3 mm. The upper stopcock is now opened and the gas forced into the combustion tube by the gradual and careful raising of the leveling bulb at such a rate that the time required to introduce the sample is from 15 to 20 minutes, depending on the volume of gas. After the sample has been introduced into the combustion tube, the upper stopcock is closed, the level of the mercury adjusted, and the reading recorded. The stream of oxygen is kept up at the same speed for 5 minutes more and the sweeping operation is effected by a stream of air forced through the train at the usual rate of 5 cc. per minute. I n the course of this sweeping operation 200 cc. of water are collected from the Mariotte flask in 40 minutes. After the products of the combustion have been so swept out from the combustion tube the absorption tubes are disconnected, wiped, and weighed as usual. The volume of gas used in a combustion will obviously vary with the hydrocarbon and should be such as to correspond to a weight of 3.5 to 4.5 mg., as in the ordinary microanalytical technique.
%
From the weights of carbon dioxide and water the weights of carbon and of hydrogen in the sample of hydrocarbon are obtained and their sum is equal to the weight of the sample. From these the percentage composition is determined. Furthermore, from the weight of the sample thus obtained (sum of carbon and hydrogen) and the volume of hydrocarbon
Acetylene Ethylene
W t . oi Carbon
Volume Wt. of (Cor- Hydrorected) gen Cc. Mg.
Gas Sample
Mg.
1:862 2.263
0:iQl 0.599
1:9iO 2.427
11974 1.981
o:fO7 0,705
3:i77 3.192
1:k24 1.392
0:669 0.625
31245 3.000
21176 2.112
0:568
0,564
3:i66 3.398
3:2;2 3.138
0:280
0.278
3:447 3 374
2:241 2.186
0:400
2:401 2.338
0,394
Hydrogen
Carbon
%
%
20.00 19.95 19.79 18.18 18.22 18.11 17.24 17.09 17.25 14.29 14.08 14.24 7.69 7.52 7 63 14.29 14.28 14.42
80.00 80.05 80.21 81.81 81.78 81.89
82.76 82.91 82.76 85.71 85.92 85.76 92.31 92.48 92.37 85.71 85.72 85.58
Ratio of CtoH
Mol. Wt.
30.00 29.63 29.95 44.00 44.08 44.08 58.00 57.53 58.37 42.00 41.52 42.03 26.00 25.84 26.08 28.00 28.00 28.00
4.00 4.01 4.03 4.50 4.49 4.52 4.80 4.85 4.80 6.00 6.10 6.02 12.00
12.28 12.10 6.00 6.00 5.94
TABLE 11. ANALYSESOF ETHYLENE OXIDE W t . of Sample Volume Calcd. from (Corrected) Mol. W t .
Wt. of Hs0
W t . oi
coz
Hz
C
cc.
M g.
Mo,
M g.
70
1:986 1.939
31901 3.808
3:ii2 3.122
7:774 7.589
9.09 9.04 9.17
% 54.54 54.35 54.34
With an unknown gas, the weight of the sample is obtained from the sum of the weights of carbon and hydrogen and thus the percentage composition will always add u p to 100 per cent. This method, however, does not seem to involve any appreciable error, as evidenced by the results given in Table I, since in all cases, although the gas was known, the weight of the sample was obtained in this manner. The percentage composition of the gas does not always enable one to judge of its homogeneity since, for instance, an olefin contaminated with another olefin will have the same percentage composition as if i t were homogeneous. The purity of the gas, however, can be judged from the molecular weight which can be calculated with a n accuracy of *0.5 gram, from the corrected volume and the sum of the weights of carbon and hydrogen. Since the calculation of the molecular weight involves both the weight of the sample and the corrected volume, the value obtained is affected by the presence of any impurity having a different molecular weight. For instance, butylene contaminated with ethylene will have the same percentage composition as pure butylene but a much lower molecular weight. The identity of the sample is based on the correlation of the molecular weight and the percentage composition. This method can be applied to the analysis of gases containing oxygen as well as carbon and hydrogen, such as ethylene oxide (see Table 11). I n such cases the weight of the sample cannot be obtained from the analytical results. When dealing with a gas of unknown identity this difficulty could be overcome by determining the density of the gas by a micromethod prior to carrying out the analysis.
April 15, 1941
271
ANALYTICAL EDITION
Acknowledgment
Literature Cited
The authors wish to acknowledge their indebtedness to E. W. R. Steacie, of these laboratories, who built the buret, and to D. J. LeRoy, also of these laboratories, who filled the buret with the various gases analyzed.
(1) Niederl, J. B., and Niederl, V., “Micromethods of Quantitative Organic Elementary Analysis”, New York, John Wiley & Sons, 1938. (2) Pregl, F., “Quantitative Organic Microanalysis", Philadelphia, P. Blakiston’s Son & Co.. 1930. PUBLIBBED as N. R.C. No. 953.
Interferences Occurring with Selected Drop Reactions LOTHROP SMITH AND PHILIP W. WEST’ State University of Iowa, Iowa City, Iowa
A
S A G E N E R A L rule drop tests and other color reactions
are developed especially for use for specific tests. There are, however, few truly specific reactions and there is need for information concerning such interferences as may occur and steps that may be taken to remove them. Systems of semi8, 10) i n microqualitative analysis have been developed (1, I, which spot tests are used for the identification of the different ions after the usual group separations. Gutzeit (6), Krumholz (4),and Heller (6) have proposed schemes of qualitative analysis based on the more simple separations. The interests of the authors have been more closely allied to those of the latter group of workers, and a system of analysis has been developed (9). Special consideration has been given t o the adaptation of this system t o use in portable kits. In the system employed by the authors. a two-group separation of the elements under consideration is used. The first step is either fusing the sample with sodium carbonate and sodium peroxide, or adding sodium carbonate and then sodium peroxide to a solution of the unknown. In either case, the water-soluble carbonates and hydroxides are separated from the insoluble oxides, carbonates, hydroxides, etc., by filtration. This accomplishes a n almost equal division of the more common elements into two major groups. One of the main advantages of this procedure is that oxidation by sodium peroxide yields the elements in a constant state of oxidation, usually the highest. This adjustment of valence has two main advantages: (1) since all forms of a n element are converted to one common valence, many tests are eliminated; (2) the elimination of extra valence forms reduces the number of possible interferences. When this procedure was first investigated, no study of interferences was made. Instead, the interferences listed by Feigl (3, 4)were noted, and the effect of the proposed separation procedure in eliminating these interferences was taken into account. Throughout 4 years of observation and use the two-group separation has proved to be of great value in conducting microchemical analyses by means of drop tests. In the course of this work i t has been observed t h a t there are a considerable number of interferences not listed in the general literature on drop reactions. For this reason the investigation on which the present report is based was undertaken.
Solutions and Reagents For the study of interferences, solutions were made up to contain approximately 5.0 mg. per ml. of each element to be studied. 1
Present addresa, Louisiana State University, Baton Rouge, La.
I n the preparation of each such solution, an appropriate amount of the c. P. chemical was fused in a platinum crucible with sodium carbonate and sodium peroxide. The melt was then dissolved in water, the pH was ad’usted to a value as near 7.0 as possible, and the solution was boded to decompose the remaining hydro en peroxide. After being cooled, the solutions were made up to b e re uired volume and placed in convenient Pyrex dropping tubes. this procedure, solutions were prepared for lithium, sodium, potassium, copper, silver, gold, beryllium, magnesium, calcium, zinc, strontium, cadmium, barium, mercury, boron, aluminum, carbon, silicon, titanium, zirconium, tin, lead, thorium, nitrogen, phosphorus, vanadium, arsenic, antimony, bismuth, sulfur, chromium, selenium, molybdenum, tungsten, uranium, fluorine, chlorine, manganese, bromine, iodine, iron, cobalt, and nickel. Only one solution was needed for each element, irrespective of the number of variations in valence in which it occurs, since the action of the sodium peroxide in the preparation of the solution leaves each element (except manganese) in a single state of oxidation, usually the highest. I n the case of manganese, a fresh known solution was made up each day, since the solution gradually decomposed to give variable mixtures of manganese dioxide, manganates, and manganous salts. The reagents and apparatus used in this work have been described previously (9).
8y
Method of Studying Interferences The actual investigation of interferences was carried out according t o a definite scheme. Because of the varied forms of phosphate interference met with, special attention was paid to the behavior of this ion under each set of conditions studied. The following series of solutions was prepared and used in studying the interferences with each test. 1. Blank 2. Element under consideration 3. Element plus phosphate
Individual members of periodic group I, in the absence of the element to be studied Individual members of periodic group 11, in the absence of the element to be studied, etc. 5 . Individual members of periodic group I lus element Individual members of periodic group If plus element, etc. 6. Individual members of periodic group I plus phosphate, in the absence of the element Individual members of periodic group I1 plus phosphate, in the absence of the element, etc. 7. Individual members of periodic group I plus phosphate plus element Individual members of periodic group I1 plus phosphate plus element, etc. 4.
The interferences found were reported as positive if a false test was obtained in the absence of the element sought, and negative if the test failed to indicate presence of the element
