Figure. 10.
Mercury Pump and
Azotometer.
A glass bulb I is attached, by means of a heavy rubber tube carrying a screw clamp, to the glass tube A, having heavy walls and a small internal diameter, and being one meter or more in length. The tube A is continued in the form of a U, the two arms being joined by very heavy rubber tubing securely wired. The ends of the glass tubes in the rubber should be bent so that they come near together and form the bend of the U, the rubber simply holding them in place. This is better then to have the tube continuous, avoiding danger of breaking. A tee tube, T, made of the same kind of glass as A, is connected by one arm, a, with the manometer B, by a heavy rubber union well wired. The union is made perfectly air-tight by the tube filled with mercury held by a rubber stopper. The middle arm of the tee, a′, is expanded into a bulb, E, branching into two arms, one of which is connected with A and the other with the delivery-tube F, by the mercury-rubber unions, MM′, just described. The interior of the bulb E should be of such a shape as to allow each drop of mercury to fall at once into F without accumulating in large quantity and being discharged in mass. The third arm of the tee a″ is bent upwards at the end and passes into a mercury sealing tube, D, where it is connected by means of a rubber tube with the delivery-tube from the furnace. The flow of the mercury is regulated by the clamp C, and care should be taken that the supply does not get so low in I as to permit air bubbles to enter A. The manometer B dips into the tube of mercury H. A pump thus constructed is simple, flexible, and perfectly tight. The only part which needs to be specially made is the tee and the one in use here was blown in our own laboratory. The bent end of the delivery-tube F may also be united to the main tube by a rubber joint thus aiding in inserting it into the V-shaped nozzle of the azotometer.
The azotometer used is the one devised by Schiff and modified by Johnson and Jenkins.[134]
We prefer to get the V nozzles separately and join them to any good burette by a rubber tube. The water-jacket is not necessary, but the apparatus can be left exposed until it reaches room temperature.
Any form of mercury pump capable of securing a vacuum may be used, but the one just described is commended by simplicity, economy, effectiveness, and long use.
158. The Pump and Combustion Furnace.—The pump and combustion furnace, as used in the laboratory, are shown in [Fig. 10]. The pump is constructed as just described, and rests in a wooden tray which catches and holds any mercury which may be spilled. The furnace is placed under a hood which carries off the products of the burning lamps and the hot air. A well-ventilated hood is an important accessory to this process, especially when it is carried on in summer. A small mercury pneumatic trough catches the overflow from the pump and also serves to immerse the end of the delivery-tube during the exhaustion of the combustion tube.
The other details of the arrangement and connections have been sufficiently shown in the previous paragraph.
159. Volumetric Method in this Laboratory.—It has been found convenient here to vary slightly the method of the official chemists in the following respects: The tube used for the combustion is made of hard refractory glass, which will keep its shape at a high red heat. It is drawn out and sealed at one end after being well cleaned and dried. It should be about eighty centimeters in length and from twelve to fourteen millimeters in internal diameter. The relative lengths of the spaces occupied by the several contents of the tube are approximately as follows: Sodium bicarbonate, two; asbestos, three; coarse copper oxid, eight; fine copper oxid, containing sample, sixteen; coarse copper oxid, twenty-five; spiral copper gauze, ten to fifteen; copper oxid, eight; and asbestos plug, five centimeters, respectively.