SLOW-COMBUSTION PIPETTE

Methane and ethane are determined by slow combustion in pipette t, which contains 8, a 24-inch coil of No. 30 (B. & S. gauge) platinum wire. Insert the ends of s in the tops of two glass tubes extending through the rubber stopper in the bottom of the pipette.

(See fig. 13.) Seal stout loops of platinum wire into the bottoms of these tubes. For 110volt alternating current connect these loops to the secondary of a 150-watt toy transformer, which has a variable voltage ranging from 1.5 to 30 volts. Stretch a piece of No. 22 (B. & S. gauge) nichrome wire about 16 inches long between two binding posts mounted on the bottom board of the wood frame above the switch u. Connect the binding post at one end of this wire and a sliding contact on it to the circuit, to permit close adjustment of the temperature of the platinum coil. A rough adjustment is made in steps on the transformer. The wire mentioned above requires about 6 volts. Use mercury for the confining fluid in the slow-combustion pipette.

Before the pipette is assembled fill the glass tubes in the combustion pipette t with mercury to complete the electrical circuit. After the pipette is mounted in place, pass mercury into the bulb m until the pipette is filled to the mark o'”.

FIGURE 14.-Details of copper oxide tube

COPPER OXIDE TUBE AND MANIFOLD

Figure 14 gives details of the copper oxide tube. Bend thinwalled glass tubing, preferably pyrex, 4 mm. inside diameter, into the form of a U. Cut it off to comply with the dimensions given in the figure, and fill it with about 3.5 grams of fine wire copper oxide containing a few particles of metallic copper. The oxide is held in place by small plugs of glass wool at both ends of the tube. Seal

capillary tubing of 1.5 mm. internal diameter to the ends of the thinwalled U tube for connection to the manifold at the stopcocks j and k.

FIGURE 16.-Electric heater for copper oxide tube

(See fig. 15.) The entire free space of the U tube from one stopcock to the other should not exceed 2.5 c. c.

ELECTRIC HEATER

Figure 16 shows the construction of the electric heater for raising the temperature of the copper oxide to 300° C. The heating element is made of 50 feet of No. 22 (B. & S. gauge) nichrome wire, wound in a helical coil. This element is easily made by using a lathe to wind the wire on a spindle three-sixteenths inch in diameter, and then coiling the helix around a grooved alundum tube of 2 inches internal diameter. Adjacent coils should be far enough apart for ample electrical insulation. Heat insulation is obtained by covering the outside of the heating element with a 1-inch layer of a mixture of magnesium oxide, asbestos fiber, and sodium silicate. The outer covering may be layers of asbestos paper or sheet metal. Close the ends with circular pieces of transite board held together by staybolts, as shown. Bring out the two ends of the wire coil to two binding posts mounted on the base of the heater. Before use, dry the furnace thoroughly for several hours at 100° C. The winding described is for use with 110-volt current, and will raise the temperature to 300° C. in 15 minutes, when a current of 1.1 amperes is used. When this temperature is attained reduce the current to about 0.7 ampere by use of a 50-ohm slide-wire rheostat. Insert a thermometer through a small hole in the top of the furnace.

OPERATING PROCEDURE

TESTING APPARATUS FOR TIGHTNESS

To test the apparatus for leaks, draw into the burette about 50 c. c. of air and turn cock c (fig. 10) to connect with the manifold (glass assembly through which gas is distributed to the different pipettes of the apparatus, fig. 15). Raise mercury-leveling bulb a so as to place the gas under 8 inches or more of mercury pressure. Any leaks in the stopcocks or rubber connections are shown by the mercury rising slowly in the burette. This, however, does not indicate the tightness of the rubber connections below stopcocks n, n', n', and n'. Leaks at these connections are shown by the solutions falling from marks o, o', o", and o'"' after they have been standing a short time.

MEASURING THE SAMPLE

Before a series of analyses is begun adjust the gas in the compensator tube to atmospheric pressure by momentarily removing three-way cock d and compensator cock e. Adjust all the reagents in the absorption pipettes and the mercury in the combustion pipette to the marks on the capillaries. Sweep the manifold and U tube ¿ free of oxygen or residual gas from a previous determination by drawing a sample of air into the burette and passing it into the alka

line pyrogallate pipetter to remove the oxygen. Then pass this residual nitrogen through the copper oxide tube i and pipettes p, q, and t to sweep out the remaining oxygen, which may have been contained therein. Much time can be saved by having a supply of nitrogen at hand for sweeping out the manifold and capillaries.

With the heater 7 raised above the copper oxide tube, switch on the current, and regulate the resistance so that the furnace will be at the desired temperature (about 300° C.) by the time the fractional combustion of the hydrogen and carbon monoxide is to be made. Turn stopcocks d, c, and b to communicate with the outside air at w. Raise leveling bulb a until the mercury reaches b. By means of a suitable capillary tube connect the sample of gas to be analyzed to the left branch of the three-way cock b and turn b to communicate with the sample. Lower leveling bulb a, drawing about 20 c. c. of gas into the burette. This sample is contaminated with air from the connections. Discard it by turning cock b to connect with w, and raise the leveling bulb a, expelling the gas at w. When the mercury reaches b, turn b to communicate again with the sample, and draw 50 to 60 c. c. of the gas into the burette as previously directed. Turn cock c through 180°, making connection with the manifold leading to the pipettes and turn b to connect with the air at w. Read the volume of gas by turning cock d through 180°, communicating with the compensator f. Raise or lower the leveling bulb a until the mercury in the compensator comes to mark f. As the burette contains a measured quantity of sample and the fluids in the pipettes have been adjusted to the marks on the capillaries, and the cocks n, n', n'', n''', j, and k closed with respect to the capillary train, the sample is ready for analyses.

CARBON DIOXIDE

Raise leveling bulb a slightly to put the gas under slight pressure and avoid any possibility of pulling the caustic solution into cock n and the manifold, then turn cocks d and n to admit the sample from the burette to caustic pipette p; continue raising a until the mercury reaches d and lower the leveling bottle, drawing the gas back into the burette until the level of the caustic solution covers the glass tubes in the pipette; again pass the gas four times into the caustic pipette, and return it to the burette in the same manner. On the last pass, return the gas to the burette, bringing the level of the reagent to mark o very carefully to avoid getting any solution into the stopcock and the manifold. Close stopcock n, turn d to communicate with the compensator, and adjust the mercury level in compensator manometer to mark f. Read and record the contraction. The analyst, unless he is familiar with the gas and knows that the solution is fresh, has no assurance that all the carbon