Hofmann screw

CC, while C is rubber. A Hofmann screw is attached at the point E. A large bottle,... 

Process Loosen the Hofmann screw, pour into A water that has been standing in the laboratory for at least twenty-four hours, until the reservoir, E, is two-thirds full then tighten the screw and add more water until the level tube, is three-fourths full clamp A at any convenient height to the iron stand which supports the apparatus. Pinch B to remove any imprisoned air. Fill the eudiometer, F, with some of the same water, invert in the reservoir, and clamp it so that the open end is just below the surface and near the wall of E. 

Introduce into about lo cc. of oxygen free from air. The oxygen may be taken from a gas holder, or it may be prepared from the usual mixture in a test tube, since only a small volume is needed. Unclamp F and press it down upon the stopper, C clamp F again, loosen the Hofmann screw, stand a thermometer in E and allow the whole apparatus to remain undisturbed for at least ten minutes. Meanwhile, read the barometer, and write all necessary notes. Hold the top of the level tube near the eudiometer and raise or lower the level tube, until the water is at the same height in both A and F, then read the volume of oxygen in the eudiometer and the temperature of the water in E. Clamp A again in any convenient position. Record all readings as illustrated below. 

Slip the end of the rubber tube over the exhaust nozzle of an air pump, and exhaust the air by rapid, short strokes of the handle. Tighten the Hofmann screw so that it will be air tight. The screw should not be more than five centimeters from the end. Detach from the pump, and weigh again — on the same scales. If less than i gm. of air has been removed, exhaust again. Call this weight d, and record as shown below. 

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