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If oxygen is passed through the tube while hot, some combines with the reduced copper to form CuO, thus giving too large a value for the contraction. If the available space in the copper oxide U tube is known, the air or oxygen used for combustion may be passed
Bulz42 FIGURE 18.-Wooden support for Orsat apparatus directly into the slow-combustion pipette without passing through the copper oxide tube and a correction applied for the amount of methane and ethane remaining in the tube. The time of analysis may be reduced to about 40 minutes, as it is unnecessary to cool the copper oxide tube after each combustion of the carbon monoxide and hydrogen. If this method is used, the furnace should be left in
place around the copper oxide before and throughout the analysis and maintained at a constant temperature.
In the preparation of the copper oxide tube, particles of metallic copper should be added to the copper oxide to serve as a catalytic agent when the tube is first used. Unless these particles are added the action of the first few samples is very slow until some of the copper oxide is reduced to the metal; then action is rapid.
The Orsat apparatus should be supported on a wooden fixture, as Figure 18 shows.
USE OF WATER IN BURETTE AND COMBUSTION PIPETTE
When a large number of analyses are being performed, as at plants where blast-furnace, producer, or illuminating gas is manufactured, water is usually substituted for mercury in a gas burette or a combustion pipette. Water may be used without the sacrifice of much accuracy if readings are taken promptly, and if several analyses are made earlier to saturate the water with carbon dioxide. As a further precaution against absorption of carbon dioxide the water should be acidified slightly with sulphuric acid.
An auxiliary compensating tube inclosed in the water jacket surrounding the burette compensates for changes of temperature and pressure in the Orsat apparatus described, so that any such changes during the analysis affect the gas in the burette and the air in the tube to the same extent. During the analysis the readings are all made by making connection to the compensating tube and bringing a mercury manometer to a certain mark. This method eliminates the effect of change of temperature and pressure during the analysis, and is more fully discussed under operation of the apparatus.
SOLUBILITY OF GASES IN ABSORBENTS
The errors caused by change of water-vapor content and solubility of gases in the confining liquid have been discussed under the operation of the Haldane apparatus. The solubility of gases in absorbents, however, requires further discussion. The precision to which an analysis can be carried is limited by the absorbents that are available for removing the different constituents. Not only do some absorbents remove chemically the constituents desired to be removed, but also there is a physical solution of some of the other constituents in the absorbents. The physical solubility of the simple gases, such as carbon monoxide, hydrogen, oxygen, nitrogen, and methane, is small, and when two or more analyses are made after fresh solutions have been put into the pipettes the error due to the solubility of the gases mentioned is very slight. Fuming sulphuric acid and bromine solution, used for the removal of unsaturated
hydrocarbons, and cuprous chloride solution, used in some apparatus for the removal of carbon monoxide, are the greatest offenders in this respect.
SOLUBILITY OF METHANE AND ETHANE IN FUMING SULPHURIC ACID AND
CUPROUS CHLORIDE SOLUTION
Pure methane and ethane were prepared by the method of Gladstone and Tribe 22 and passed into pipettes containing fuming sulphuric acid and cuprous chloride solution, with the following results:
No contraction in volume occurred when 100 c. c. of methane remained in contact with the acid 5 minutes. The gas was passed back and forth several times between the pipette and burette. No measurable contraction in volume occurred under the same conditions when the gas was brought in contact with the cuprous chloride solution.
When 100 c. c. of ethane was passed back and forth into fuming sulphuric acid for 2 minutes, it lost 1 per cent of its volume. At the end of 5 minutes a contraction of 2 per cent had taken place. When the same amount of ethane was passed back and forth into the cuprous chloride solution for 2 minutes, it lost 0.6 per cent of its volume, and at the end of 5 minutes the contraction was 1.4 per cent.
R. A. Worstall 23 determined that the paraffin hydrocarbons are soluble to a certain extent in fuming sulphuric acid, the higher members more so than the lower. He says that in examining a gas mixture containing members of the paraffin series of hydrocarbons, the use of fuming sulphuric acid to determine unsaturated hydrocarbons leads to no error if paraffins higher than ethane are absent, and if the gas mixture is in contact with the acid not longer than 15 minutes. Worstall's determinations were carried out by leaving the gas quietly in contact with the acid and having no increased absorption surface, such as is provided by the use of glass rods or beads.
Orndorf and Young 24 found that propane is slowly soluble in fuming sulphuric acid. Phillips 25 states that the higher members of the paraffin series are somewhat soluble in bromine water and in cuprous chloride solution. It follows, then, that if a natural-gas
22 Gladstone, T. H., and Tribe, Alfred, Notes on the preparation of marsh gas: Jour. Chem. Soc., vol. 45, 1884, p. 154. It was found that unsaturated hydrocarbons are produced in the preparation of methane by this method. They were removed by means of fuming sulphuric acid. F. G. Phillips, of the University of Pittsburgh, in conversation with the authors, reported a similar experience.
23 Worstall, R. A., The absorption of methane and ethane by fuming sulphuric acid : Jour. Am. Chem. Soc., vol. 21, 1899, p. 145.
24 Orndorf, W. R., and Young, S. W., The products of the condensation of acetone with concentrated sulphuric acid : Am. Chem. Jour., vol. 15, 1893, p. 249.
25 Phillips, F. C., Oil and gas levels : West Virginia Geol. Survey, vol., 1A, 1904, p. 522,
mixture or other mixture containing higher paraffins is being examined and undergoes a reduction in volume when treated with the above reagents other tests should be performed to verify the presence of olefin hydrocarbons or of carbon monoxide. Qualitative tests of the use of palladium chloride 26 and blood solutions and quantitative tests by the iodine pentoxide method can be performed for such verification.
ACTION OF FUMING SULPHURIC ACID ON PITSBURGH NATURAL GAS
According to combustion analyses performed at the bureau's laboratory, the natural gas used in Pittsburgh contains about 83 per cent methane and 16 per cent ethane. This 99 per cent of paraffins includes some propane and possibly some butane. When 100 c. C. of the gas was treated with fuming sulphuric acid and a cuprous chloride solution the following results were obtained:
Action of fuming sulphuric acid on the natural gas used in Pittsburgh
Action of cuprous chloride on the natural gas used in Pittsburgh
The action of fuming sulphuric acid on a natural gas rich in the higher paraffins is shown below. The gas contained about 23 per cent ethane, 76 per cent propane, and 1 per cent nitrogen, and is used for the production of gasoline.
Action of fuming sulphuric acid on a natural gas rich in the higher paraffin
hydrocarbons Number of passages of
Reduction in volume, gas into pipette
12.5 12 -------
---------- 14.5 The authors have also determined that bromine water has an appreciable solvent action on the higher paraffin hydrocarbons.
ELIMINATION OF ERRORS DUE TO PHYSICAL ABSORPTION
Most errors due to physical absorption are eliminated after several analyses of the same general composition have been made, because
26 Phillips, F. C., Researches upon the phenomena of oxidation and chemical properties of gases : Am. Chem. Jour., vol. 16, 1894, p. 267.
the solutions become saturated with the different gases. When fresh solutions are put into the apparatus at least two analyses should be made before results are accepted as correct.
GRADUATION OF THE BURETTE
Accurate results can not be obtained unless the burette is properly calibrated. Makers of burettes usually calibrate them with fair accuracy, but it is difficult to obtain a uniform bore throughout the entire length, consequently a check calibration should be made. To make it, proceed as follows: Place the burette in a water jacket in the same positions as used and keep the temperature of the water as constant as possible. Seal a three-way stopcock to the lower end of the burette and attach a leveling bottle to one projection of the stopcock, so that if by accident the mercury is allowed to fall below a certain graduation mark on the burette during the calibrating operation the leveling bottle can be raised and the mercury easily brought back to the mark for calibration. Weigh successive 5-c. c. portions of mercury; calculate the volume of mercury at the observed temperature from the weight.
CAPILLARY ERROR OF THE APPARATUS The apparatus is so constructed that the gas can be passed to the different pipettes without connecting or disconnecting any parts. The glass manifold and the connections leading to the cocks above the absorption pipettes through which the gas is distributed during an analysis contain about 3 c. c. of space. If this space contains an inert gas (nitrogen) before and after the analysis the error is largely overcome. Most of the capillary error is eliminated by sweeping out the manifold with nitrogen after each analysis. However, there is a small error because some of the gas is left below each stopcock above the absorbents. To minimize this error, the distance between the cocks and surface of the absorbents is made as small as possible, and small capillary tubing is used for the connections. If the gases being analyzed do not differ appreciably in composition, the capillary error largely compensates itself; that is, the gases introduced into the capillaries and removed have about the same composition, and the gases gained and lost in the capillaries as the analysis progresses are relatively the same.
FORMATION OF OXIDES OF NITROGEN Are oxides of nitrogen 27 formed during the combustion analysis? Gas analysts have raised this question as affecting the accuracy of their
27 White, A. H., The oxidation of nitrogen as a source of error in the estimation of hydrogen and methane: Jour. Am. Chem. Soc., vol. 23, 1901, p. 476,