A MODIFICATION OF THE PYROGALLOL METHOD FOR DETERMINING THE AMOUNT OF OXYGEN IN THE AIR The pyrogallol method for determining the percentage of oxygen (by 4, 638-9 (May, volume) in the air, described by Foster in THISJOURNAL, 1927), has been used in the first-year laboratory of this department for several years. A modification of the above method bas been found to possess certain advantages. The past session has shown it to be more rapid, to give better results, and to be cleaner in manipulation than the old method.
The Modified Method The apparatus consists of a test tube 8" x 1" fitted with a two-hole stopper, glass plug, and glass tube, connected to a leveling bulb by a sec-
tion of rubber tubing. A fluted cork fits loosely into the top of the bulb. The apparatus is first set up as shown in the diagram, the inverted test tube with the plug removed being supported by a clamp and the bulb by a retort ring. At the start, the level of the stopper is above the level of the centre of the bulb ( A ) . The alkaline pyrogallol (30 cc.) is next prepared and immediately poured into the bulb and the fluted cork inserted. The test tube is now lowered by means of the clamp until the pyrogallol appears a t the end of the tube a t (B). The glass plug is then
inserted and the tube lowered further. This forces some pyrogallol into the tube and hastens the reaction. The level of the stopper is marked with gummed paper. When an interval of ten minutes shows no further rise in level of the liquid in the tube, the bulb and tube are brought to the same level (C). The level of the liquid is marked with gummed paper and the percentage of oxygen calculated in the usual way.
Advantages In the old method, the tube which projects through the stopper was filled by opening the pinchcock, allowing the liquid to flow through the tube and then closing the pinchcock. The stopper was then inserted in the test tube. Filling the tube is accomplished more conveniently and without soiling the hands, by the modified method. This method is also more rapid. The use of a bulb with a loose-fitting cork, in place of the open funnel, is an advantage. As the oxygen in the bulb is absorbed, the bulb soon becomes filled with nitrogen (except for the small amount of oxygen admitted by diffusion). The pyrogallol is therefore kept active and the determination is consequently more rapid. It has not been the experience of the writer that a determination can be made "in a few minutes," by either method. More accurate results have in general been obtained by the students using this method. This is probably due to the fact that with the inverted test tube, the occasional leaks occurring around the glass plug, rubber stopper, etc., are eliminated as a source of error, for the pyrogallol on entering the inverted tube "seals" any leaks without changing the volume or the pressure of the enclosed air. With the old method, a slight leak, unnoticed by the student, resulted in a high result due to the escape of air and its replacement by the pyrogallol. Sometimes also, an error was introduced by the escape of a bubble of air through the vertical tube owing to temperature changes. This is eliminated in the modified method. The leveling bulbs were made by our technician from ordinary soft glass 150 cc. flasks. The use of dropping-funnels is not recommended.
