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SOLUBILITY OF GAS IN CONFINING LIQUIDS All gases, especially carbon dioxide, some of the unsaturated hydrocarbons, and gases of high molecular weight, are soluble to some extent in water. To eliminate this error of solubility, mercury is used as the confining liquid in the burette and combustion pipette, and for trapping the gas samples during the analysis.
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SOLUBILITY OF GASES BY ABSORBENTS In gas analysis the constituents are either removed by solid or liquid absorbents, which take out certain constituents and leave the remainder; or, if they are combustibles, they are determined by certain combustion relations resulting from the burning of the combustibles with oxygen to carbon dioxide and water. The range of precision of an analysis is limited by the absorbents available for removing the different constituents. Not only do the absorbents remove some constituents chemically, but certain other constituents dissolve physically in the absorbent. The physical solubility of the gases usually analyzed with the Haldane apparatus is not greatoxygen, nitrogen, methane, carbon monoxide, and hydrogen are but slightly soluble in a solution of potassium hydroxide, and after one or two analyses have been made the solubility error becomes inappreciable. In a solution of alkaline pyrogallate, however, the solubility of the four last gases is somewhat larger. Whenever a fresh solution of caustic pyrogallate is put in the apparatus, at least two analyses should be made before the oxygen percentage is taken as the true result, because a fresh solution of alkaline pyrogallate absorbs some nitrogen gas as well as oxygen, and abnormally high results are obtained for the first one or two analyses.
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 accurate calibration of the entire length of the bore is difficult if the burette is narrow and graduated into hundredths of a cubic centimeter; consequently a check calibration should be made as follows: Leave the burette in the water jacket in the same position 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 calibration the bottle can be raised and the mercury conveniently brought back to the mark for the calibration. Weigh successive 1-c. c. portions of mercury, and calculate the volume of the mercury at the observed temperature from the weight. Plate III shows a diagrammatic section of the apparatus.
DESCRIPTION OF APPARATUS The burette has a total capacity of 21 c. c. The bulb at the upper part has a capacity of 15 c. c., and is not graduated. The stem has a capacity of 6 c. c., and is graduated in hundredths of a cubic centimeter. The burette is graduated from the bottom of stopcock g, at the point where the sample is drawn into it.
BURETTE AND PIPETTES
The burette is connected to pipettes m, l, and n by a capillary glass manifold, as shown. Pipette n contains caustic potash for removing carbon dioxide; pipette l has a platinum spiral, which, when heated bright yellow, burns the combustibles to carbon dioxide anl water; and pipette m contains alkaline pyrogallate solution for removing the oxygen. Pipettes m and I are connected with storage bulbs w and on the back of the apparatus by heavy-walled rubber tubing.
A compensator tube is provided in the water jacket 2 surrounding the burette; it connects by way of the three-way cock f and capillary tubing to pipette n, as shown.
TRANSFER OF SAMPLES
One branch of the three-way parallel cock g at the top of the burette connects to three-way cock h, and thence by capillary tubing to the gooseneck j, and the sample of gas to be analyzed. The figure shows the method by which the gas is removed from a vacuum bottle, the receptacle recommended by the Bureau of Mines for sampling gases. The neck of the gas sample-bottle is broken off under mercury in the bell jar i, and the gooseneck j inserted. In this manner gas can leave the sample only through the capillary connected to cock h; likewise air can not enter and contaminate the sample. As the gas is withdrawn, mercury rises from the bell jar and fills the space occupied by the gas. If this method of sampling is not used, the gooseneck is removed and the sample bottle is connected direct to the right branch of stopcock h.
ELECTRIC CONNECTIONS The sketch shows a small transformer which is connected to the source of energy. At the gas laboratory of the Pittsburgh experiment station of the Bureau of Mines the transformer receives alternating current at 110 volts, which is reduced to 6 volts and connected to the apparatus. The transformer has a variable range of 112 to 30 volts, so that different sizes and lengths of wires may be used. One of the leads b from the transformer connects with switch k, then to a sliding connection, u, which can be moved along a 22-gauge nichrome resistance wire as shown. From one of the binding posts the lead connects directly to the combustion pipette, as shown in the detail drawing, Plate IV (p. 24). A loop of heavy platinum wire (No. 22, Brown & Sharpe gauge) is sealed in the bottom of each glass electrode to which the leads connect. The glass electrodes are filled with mercury and serve as the circuit to the platinum spiral at the top. The electrodes are open at the top, so that they can be filled with mercury, and the ends of the 21/2-inch platinum spiral of No. 30 Brown & Sharpe gauge wire are fastened in the tops of the electrodes by small glass buttons. Thus the spiral is kept in the proper position at all times and makes good contact with the mercury.
The other lead (61) from the transformer, connects directly to one of the glass electrodes on the pipette.
The nichrome wire allows more careful regulation of the temperature of the platinum spiral than can be obtained by the transformer. A gas containing an unduly large amount of combustibles causes the wire to become very hot, and unless some additional resistance is added the wire may be burned out. By sliding the contact along the nichrome wire the operator keeps the temperature of the · wire completely under control at all times.
If a transformer is not available a storage battery that will produce an electric current of 4 amperes at 5 volts may be used to heat the platinum wire to incandescence. Dry cells may be used in emergency, but they are not as satisfactory as a storage battery, since long-continued use runs them down.
An electric light bank may be used if a suitable number of carbon electric lights is connected in parallel, and the platinum spiral connected in series with the electric lighting circuit.
OTHER PARTS OF APPARATUS A rubber bag is attached to the rear branch of the pyrogallate pipette to keep the solution from coming in contact with the air, otherwise the solution would soon be exhausted by the absorption of atmospheric oxygen. If a rubber bag is not used the pyrogallate solution may be covered with a layer of mineral oil to exclude the outside oxygen.
Plate III shows a device, y, for raising and lowering the mercury leveling bulb r; it is convenient for bringing the solutions to the marks. An ordinary ring support with a screw clamp may be used as easily.