INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 15. No. 6
T h e Analysis of Gas Mixtures Containing Methyl Chloridea By Ralph H. McKee and Stephen P. Burke DEPARTMENT OF CHEMICAL ENGINEERING, COLUMBIA UNIVERSITY, NEWYORK,N. Y .
P TO T H E present time no satisfactory method has been published for the determination of methyl chloride in gas mixtures containing other components soluble in organic solvents. In the absence of such components it has been shown2 that the absorption of methyl chloride in glacial acetic acid furnishes a simple and ready method for its determination. However, when the concentration of methyl chloride is below 40 per cent, this method gives results appreciably in error, especially if considerable amounts of water vapor or methane are present. In an investigation on the conversion of methyl chloride to methanol, which will be published later, gaseous mixtures of methyl chloride, methyl ether, and methane, together with various other components, were encountered. Obviously, such mixtures are not amenable to analysis by the acetic acid method. A new method was devised, which was based on the combustion of the methyl chloride. It was found that in an excess of oxygen the methyl chloride will burn, under proper conditions, according to the equation:
U
CHaCl
+ 3/202 = COz f HzO + HCl
The method of analysis, therefore, was to burn the gas mixture over strong potassium hydroxide in a combustion pipet, using an excess of oxygen, and then to analyze the alkaline solution for chlorides. Since the products of reaction are all absorbed in the hydroxide solution, the final gas residue consists of the excess of oxygen used and any noncombustible gases present.
PROCEDURE The apparatus employed is shown in Fig. 1. A layer about 1.5 cm. deep of approximately 3 M potassium hydroxide is first drawn into the combustion pipet B, using mercury as the containing fluid. A measured amount of oxygen, in excess of the amount necessary for the subsequent combustion, is then introduced into the pipet from the waterjacketed buret A, mercury being used as the containing fluid in the buret also. This buret is equipped with a 3-way stopcock. Sufficient current is sent through the platinum spiral to cause it to glow bright red, and the gas sample to be analyzed is then sent very slowly from the buret into t h e pipet. Combustion takes place smoothly and readily if the proper rate and pressure are maintained so that the gases are burned as rapidly as they are mixed. When all the gas has been sent into the pipet, about 3 or 4 cc. of 1 M potassium hydroxide solution are drawn into the buret through the capillary C to assist in absorbing carbon dioxide and hydrochloric acid, care being taken to reject any air. The gases in the pipet are then slowly returned to the buret and this passage back and forth over the platinum spiral is kept up until there is no further diminution in volume-that is, until combustion is complete. The potassium hydroxide solution from the buret is then sent over into the pipet, the gases are drawn back into the buret, and the volume is read. This final reading of volume is slightly in error, because the partial pressure of water in the gas is that corresponding to the 1 Received December 4, 1922. This is the first of a series of articles on making methanol from methyl chloride. Allison and Meighan, THIS JOURNAL, 11 (1919), 943.
potassium hydroxide solution and not that of pure water. This error, however, is negligible (less than 1 mm. a t 20" to 2 5 O C.). The potassium hydroxide solution is then removed through C, and the entire interior of the buret, the pipet, and the connections are very carefully washed out with distilled water, passing the water in and out several times by raising and lowering the mercury, until the washings no longer give a test for chloride. The interior is then washed out with dilute sulfuric acid. The entire washings, together with the original potassium hydroxide solution, should be made strongly alkaline (if they are not so) and filtered. The total volume of this filtrate should not exceed 250 cc. This filtration removes, as a black precipitate of mercurous oxide, a small amount of mercury which goes into solution during t,he combustion. In this way the mercury which may be present in solution is quantitatively removed. This filtration must not be overlooked, for otherwise when the solution is neutralized mercurous chloride appears as a faint, cloudy white precipitate, and the subsequent determination of chloride is erroneous. As a precaut,ion the filtrate may be tested for free chlorine or oxidation products of chlorine. These, however, were never found in this work. The filtrate is then properly neutralized and the chloride determined by Mohr's or any other satisfactory m e t h ~ d . ~ From the quantity of chloride formed it is possible to calculate the amount of methyl chloride present, assuming the absence of any other chlorine compound. It is necessary, of course, to record the pressure and temperature a t which the gas sample is measured in order to know the actual mass of gas used. Moreover, in calculating the percentage of methyl chloride, since the latter deviates considerably from the gas laws under normal conditions, a correction for its density must be applied. The methyl chloride content having been calculated from the determination of chloride, the result can be checked by the volume of oxygen consumed, provided the amount and nature of any other combustible components of the gas sample are known. If the methyl chloride is passed into the pipet too rapidly, or if the oxygen is insufficient, combustion may be incomplete. In this case a small amount of carbon may form, but nevertheless all the chlorine will be converted to hydrogen ~ h l o r i d ehence, ;~ the chloride determination will not be affected although the amount of oxygen used may not agree with the chloride obtained. If the analysis is properly performed, however, a discrepancy between the oxygen cocsumption and the amount of chloride obtained indicates the presence either of a higher chlorinated compound or of some combustible gas not taken into consideration. It is evident that if methane and methylene chloride were present as impurities in equal volume they could not be detected by this combustion method, for the yield of chlorine and the oxygen consumption would be exactly the same as those given by an equivalent total volume of methyl chloride. 8 A method of analysis for methyl chloride quite similar to this in theory was investigated by Allison and Meighan.9 They were unable to obtain accurate results. By using the procedure given here, no difficulties were encountered, and the method gives a satisfactory degree of precision. 4 Above 400° C. methyl chloride is decomposed into carbon, methane, hydrogen, hydrochloric acid, and other products. [Perrot, Ann., 101 (1857). 375 ] ,
J N D USTRIAL A N D ENGINEERIYIG CHEXISTRY
June, 1923
However, they could be detected by absorption in acetic acid, for the methyl chloride and methylene chloride would dissolve, while methane would remain for the most part undissolved.2 The new method was checked by analyzing gas mixtures containing known amounts of methyl chloride. The error, in general, was less than 0.7 per cent. If proper care is taken, the oxygen consumption will agree very well with the result obtained by the chloride determination. Since the results are independent, one serves as a check upon the other. This combustion method, in conjunction with the acetic acid absorption method, furnishes a means of analyzing methyl chloride mixtures containing other components soluble in organic solvents. In the investigation already referred to, mixtures of methyl chloride, methyl ether, carbon monoxide, carbon dioxide, hydrogen, methane, oxygen, and nitrogen were encountered. These were analyzed using the buret A shown in Fig. 1 with standard Hemple pipets. The methyl chloride and methyl ether were first removed by absorption in glacial acetic acid, and the remaining gas mixture was analyzed in the usual way. A second sample of the gas was then taken and the methyl chloride determined by the method just described. The difference between the total absorption in acetic acid and the volume of methyl chloride found gave the volume of methyl ether present. The result was readily checked by the oxygen consumption. This method can best be made clear by an example.
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in both of these reagents. Fifty per cent saturated bromine water will brominate methyl chloride very slowly in the dark, while fuming sulfuric acid slowly absorbs methyl chloride, forming methylchlorosulfonic acid.
A sample of gas gave the following analysis: Methyl chloride 4- methyl ethers Carbon monoxide Oxygen Nitrogen Methane Hydrogen
Per cent 89.2 0.0 1.0 3.5 2.6 4.0
100.3 a
Absorption in glacial acetic acid.
On analyzing for methyl chloride by combustion, 175 cc. of commercial oxygen (98 0 per cent oxygen, 2.0 per cent hydrogen) were used to burn 80.0 cc. of the gas mixture. The gases were measured a t 21.5” C. and 755 mm. pressure. The gas residue after combustion was 33.5 cc. The chloride obtained by titration amounted to 0.00226 mol. From these data we have: 0.00226 X 21900 (mol. vol. CHaCl) = 49.5 cc. a t O o C. and 760 mm. Kow 80.0 cc. saturated with Hz0 a t 21.5’ C. and 755 mm. = 71.5 CC. a t 0’ C. a n d 760 mm. Therefore, percentage CHIC1 = 49.5/71.8 = 65.8 per cent CHaC1 And the (CHa)zO = 89.2 - 68.8 = 20.4 per cent (CHa)zO
To check these results by the oxygen consumption: The gas residue was 35.5 cc. Since the only noncombustible gases present in the original 80.0-cc. sample were nitrogen and oxygen (sum = 4.5 per cent), the excess oxygen was
-
35.5 (80.0 X 0.045) = 35.5 - 3.6 = 31.9 cc. oxygen Hence, total volume oxygen used = 175 - 31.9 = 143.1 cc. From t h e foregoing analysis, the oxygen requived is: ((0.688 X 80.0 X.;.5 X 224/219) t (0.204 X80.0 X 3) 4- (7.2 X,j(8))1.03
+
L‘%
CfiJ
iL)
or, (54 5 T 49 0 5 8 ) l 03 = 139 3 X 1 0 3 = 143 5 cc , as compared with 143 I cc oxygen actually used where (A) = t h e volume of oxygen required for t h e combustion of t h e methyl chloride (B) = the volume of oxygen required for the combustion of t h e methyl ether (C) = t h e volume of oxygen required for the combustion of t h e hydrogen and methane and 1 03 = the factor t o correct for the 2 per cent of hydrogen present in t h e oxygen as an impurity
Hence, the oxygen consumed agrees very well with the result of the chloride determination. The presence of ethylene in gas mixtures containing methyl ether and methyl chloride introduces additional difficulties. I n the method just outlined, any ethylene present is removed in the preliminary absorption in glacial acetic acid. On the other hand, the ethylene cannot be determined directly by the use of bromine or fuming sulfuric acid, as is the usual procedure, on account of the solubility of the methyl chloride
FIG.
1
It was found possible, however, to estimate the volume of ethylene approximately by the following method: Three hundred cc. of a mixture of 10 per cent ethylene and 90 per cent air were shaken with bromine water of known concentration, and the maximum volume of the bromine solution which it would decolorize was determined. An equal volume of the gas sample was then treated under the same conditions of light, temperature, etc., and the maximum amount of solution decolorized was likewise determined The per cent by volume of ethylene present was calculated by proportion. Undoubtedly, some of the bromine reacted with the methyl chloride in the second case, but a blank on pure methyl chloride showed that this amount was inappreciable.
By this new method it is now practicable to analyze mixtures of methyl chloride with methyl ether, ethylene, carbon monoxide, carbon dioxide, hydrogen, methane, oxygen, and nitrogen with the accuracy of ordinary gas analysis.
Priestleyana Admirers of Joseph Priestley, Jr., will be interested in a recent booklet, “Priestleyana,” by Edgar Fahs Smith, which describes some interesting relics that were recently found in his old home in Northumberland, Pa. These include a part of his library, interesting documents such as the original manuscript of Priestley’s “Memoirs,” his will, family pictures, pieces of apparatus, and other possessions, which give an insight into the life and character of this eminent scientist, as well as of the development of the history of chemistry in America. This collection is now preserved in the library of the University of Pennsylvania.
