« ForrigeFortsett »
switch k, allowing the electric current to pass through the platinum spiral, and adjust the temperature by throwing resistance in or out by sliding contact u along the wire on the front of the apparatus. Adjust the temperature of the wire until the wire is bright yellow. If the gas contains appreciable amounts of combustibles, add more resistance by sliding u to the left, otherwise the platinum spiral may be burned out. Pass the gas back and forth over the platinum spiral by raising and lowering r at least twelve times. Keep the pipette cold by directing a stream of air over it while the spiral is heated, using a curved glass tube with many small holes in the bottom (see Pl. IV). With a rubber tube the blower may be connected to a compressed-air line, rotary blower, foot bellows, or other suitable source of air.
After the gas has been passed back and forth over the glowing platinum spiral twelve time or more, open switch k and cool the pipette. Withdraw the gas into the burette and when the mercury level reaches the mark ó' close cock d. Read the contraction due to burning by opening cock e and bringing the level of the caustic solution to o" and 0'' before reading. To find the amount of carbon dioxide produced pass the gas six times or more into the caustic pipette and read the volume. If the contraction produced on burning is greater than 0.200 C. C., pass the gas into the combustion pipette, burn it again, and note the contraction and carbon dioxide produced by the second burning. Add the two contractions and carbon dioxide values.
REMOVAL OF OXYGEN
Pass the gas into pipette m, which contains alkaline pyrogallate solution; there the oxygen is absorbed. When no further contraction appears to be produced, as indicated by the height of the mercury in the burette when the pyrogallate solution is brought to the mark, remove the oxygen from the capillary tubes connecting the pipettes 1 and n before taking a reading. Close cock c and pass the gas once or twice into pipette 1, then a like number of times into pipette n to dilute the oxygen in these capillaries. Pass the gas into the pyrogallate pipette again several times, and measure the reduction in volume of gas. Check this value by passing the gas into the pyrogallate pipette again to make sure that the oxygen is completely removed. It must be remembered that the time necessary for removing the oxygen is much longer than for the other constituents, and the contraction due to oxygen should be checked each time before the sample is discarded. Add the oxygen consumed by the combustion of the methane to the oxygen determined by absorption with alkaline pyrogallate solution, if methane is the only combustible gas present.
Make the different readings in the same manner as described for the original gas volume taken for analysis.
DETERMINATION OF COMBUSTIBLE GAS
The relation between the carbon dioxide and the contraction produced by the combustion indicates the character of the combustible gas present. When the volume of carbon dioxide is just one-half of the contraction, the sample probably contains methane only. The result is confirmed by a second analysis, in which the carbon dioxide is removed and the gas mixture passed directly into the alkaline pyrogallate solution pipette, where the oxygen is absorbed. The difference between the oxygen found by direct determination and that found by the first analysis represents the oxygen consumed in the oxidation of the combustible gases. If methane is the only combustible gas present, the amount of the consumed oxygen should be twice the amount of carbon dioxide formed by combustion. Because methane is the combustible gas usually contained in normal mine atmospheres, a second determination of oxygen is hardly necessary in the analysis of samples of such atmospheres, but the second determination is necessary when from inspection of the combustion data or from the origin of the sample the presence of more than one combustible gas is suspected.
A typical analysis of a sample of gas containing only methane as the combustible constituent follows. The sample was taken from the main return of a coal mine.
Analyses of return air in a coal mine
Volume of sample taken-----
Contraction due to carbon dioxide absorption--
0.11 20. 65
.14 79. 10
CALCULATIONS FOR DETERMINING COMBUSTIBLES
Hydrogen, methane, and carbon monoxide react with oxygen as follows:
(3) 2 C0+02=2 CO2. When only two of these gases are present in a gas sample, the carbon dioxide and the contraction resulting from burning the gases in the combustion pipette provide enough data for calculating the amount of each combustible gas present.
METHANE AND CARBON MONOXIDE
When only methane and carbon monoxide are present the following relations are obtained from equations (2) and (3):
(4) Contraction=2xCH4+1/2 CO.
(5) Carbon dioxide=CH:+CO. Solving equations (4) and (5)
When only methane and hydrogen are present the following relations are obtained from equations (1) and (2):
(9) Carbon dioxide=CH.. Solving equations (6) and (7):
(10) CH=carbon dioxide.
HYDROGEN AND CARBON MONOXIDE
When only hydrogen and carbon monoxide are present the following relations are obtained from equations (1) and (2):
(13) Carbon dioxide=co. Solving equations (8) and (9):
(14) Co=carbon dioxide.
HYDROGEN, METHANE, AND CARBON MONOXIDE
When all three gases are present, however, a determination of the amount of oxygen consumed by the combustion of the gases is neces