DETERMINATION of the PERCENTAGE by VOLUME of OXYGEN in AIR A. T. LINCOLN AND HAROLD P. KLUG Carleton College, Northfield, Minnesota
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NE O F the common exercises in general chemistj is the determination of the percentage of oxygen in air. Various methods are employed and the most common is the absorption of oxygen by alkaline pyrogallol in a Hempel pipet or directly in the buret. Everyone who has employed this method can testify to its inconvenience and to the difficulties inherent in the manipulation. During the past ten years we have been employing, as the absorbing medium, copper in an ammoniacal ammonium chloride solution in a Hempel pipet connected with a Hempel buret.* Duting this time we have gradually improved the apparatus w that i t is now very compact, and the manipulation .is so simple that general chemistry students have no difficulty in obtaining satisfactory results. THE APPARATUS
The complete set-up comprises the Hempel buret attached to an absorption pipet for solids by means of a three-way stopcock. The units are all mounted on a heavy ring-stand, thus making the whole apparatus compact and easily transportable. The accompanying figure shows a detailed view of the three-way capillary stopcock, its connections with the buret and the pipet for solids which is shown filled with thin copper foil and the ammoniacal solution of ammonium chloride. This is a 14% NHa solution saturated with NHPCI. The metallic copper oxidizes readily in the presence of the ammoniacal solution which in turn removes the oxide * This method is described by DENNIS,"Gas analysis," The Macmillan Co.,New York City, 1913,p. 167. ~
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DETAILEDREPRESENTATION OF
THE
THREE-WAY
STOPCOCKSITOW~NG ITS CONNECTIONS WlrH PIPET. THE BURET, AND THE ATMOSPHERE
THE
of copper formed. The absorption of oxygen is rapid and complete. We use the same solution for about two weeks, making about twenty analyses with each filling.
results obtained by the students in different classes and Werent years. Pn~~nlog~
Readings Second Third
PROCEDURE
a. Filling the Buret with Distilled Water.-See that the buret is open, through the stopcock, to the air. Half fill the huret and leveling tube with distilled water. Now connect the huret with the pipet through the stopcock. - Lower the leveling tube slowly until the air is withdrawn from the pipet and the column of liquid stands in the capillary tube on a level with the horizontal portion of the pipet. Mark this paint and always bring the liquid in the pipet back t o this point. Next shut off the pipet by means of the stopcock and open the huret to the air. Raise the leveling tube and force out the air in the buret until the surface of the water in the buret is a t the zero mark and the surfaces of the liquids in the two tubes are level. b. Taking the Sample of Air.-With the surfaces of the water in the leveling tube and buret level, open the stopcock to the air and gradually lower the leveling tuhe. This lets the air enter the buret. Continue lowering the leveling tube until the surfaces of the water in the tubes are level a t the 100-ml. mark. To make the reading more accurate, place the two tubes in contact with the water levels the same, comparing the undersurfaces of the menisci. Now close the stopcock. Repeat reading and assure yourself that you have exactly 100 ml. of air under atmospheric pressure. c. T o n s f m c e of Sample of Air to Pipet.-See that the pipet is full of liquid, the surface in the vertical capillary tube being even with the horizontal portion of the capillary tube of the pipet. By means of the stopcock, connect the pipet with the buret. Raise the leveling tube slowly and force th! air sample into the pipet. It is necessary t o keep the surface of the water in the leveling tuhe a few inches above the surface in the buret. When all the air has been transferred t o the pipet, allow i t t o stand in contact with the copper for about five minutes. Then return the air to the buret by lowering the leveling tube, level the liquid surfaces, and read the volume of the air in the huret. Record the reading. Now return the air to the pipet. Allow it to remain in contact with the copper and solution about five minutes, then return it to the buret. Level the water surfaces and read the volume of air remaining. Repeat this process three or more times until the last two readings of the residual gas volume are the same. Record this as your final reading. Connect the buret with the atmosphere, raise the leveling tube, and remove all the residual gas from the huret and the pipet. Introduce a new sample of air into the buret and measure it in the manner described above. Transfh it t o the pipet three successive times or until the volume of the residual gas is constant and take the final constant reading as described. Record this value. Take a third sample of air and treat it in the manner just described. The average af the values for the three different samples of air gives the volume of the residual gas and this deducted from 100 ml. gives the volume of oxygen in 100 ml. of air. Calculate the percentage of oxygen in air. RESULTS OBTAINED BY STUDENTS
In the following table are presented a few of the
Df OXJYCI
Fowtk
zn Aw
Aa.
I
20.2 11
19.73 111
20.3 IV
20.3 V
19.8 VI
20.26 VII
19.53 VIII
19.6 IX
19.8 X
20.07
Each Roman figure represents the experiments of one pair of students and there are-listed in each case three analyses, indicating the exercise was done in triplicate. These are typical student results, as ohtained by class after class. The time required for the exercise is one laboratory period of two hours, the students working in pairs, because of the necessity imposed by limited equipment. No effort i s made to correct the gas volumes for the vapor pressure of water a t room temperature, since with us this exercise is performed before the gas laws have been considered. The correction would be smaller than the manipulative ability of the students a t this stage justifies.
