A METHOD FOR THE QUANTITATIVE DETERMINATION OF OXYGEN IN THE ATMOSPHERE
A very satisfactory and accurate determination of the percentage of oxygen in the atmosphere can be made by employing a eudiometer. Two platinum wires are fused in the top of the eudiometer tube and allowed to project within the tube, with a small gap between their points. The wires are connected to a battery and induction coil, by which a spark is made to jump between the two platinum points. A definite volume of air is col-
lected in the eudiometer. The temperature of the water and the harometric pressure are recorded. I t is convenient to have the eudiometer extend downward into a tall cylinder so that i t can be adjusted to water level before readings are taken. The water should he drawn off into the cylinder and allowed to stand for several hours before the demonstration. The water and the air in the room will then be a t the same temperature. Now a few cubic centimeters of hydrogen, more than is required to combine with the oxygen of the enclosed air, are slowly bubbled into the eudiometer. Adjustment to water level is made and the volume of the air plus the hydrogen is recorded. The gases are now exploded, the nitrogen acting gs
a cushion. The eudiometer is lowered into the water for a few minutes so that the temperature of the residual gas may return to the temperature of the water. After adjusting the eudiometer to water level, the volume of the residual gas is recorded. The loss in volume is due to formation of water by hydrogen and oxygen. One-third of the loss in volume is due to oxygen alone. By dividing the volume of oxygen by the volume of air, the percentage of oxygen is obtained. The following problem will show how the calculations are made. Temperature of water (air) Barometer reading Aqueous tension Volume air Corrected volume hydrogen Volume air hydrogen Corrected volume air Volume residual gas Corrected volume residual gas Loss in volume (30.92-19.55) = Loss in volume due t o oxygen 11.38/3 = Percentage of oxygen in air = 3.79/18.2 =
+
+
18% 752.4 mm. 15.4 mm. 20 cc. 18.2 cc. 34 cc. 30.93 21.50 19.56 11.38 3.79 20.82
This method of determining the percentage of oxygen in the atmosphere is suitable to classes in general chemistry, whether high-school or college freshmen. The method possesses the following advantages: 1. 2. 3. 4. 5.
Easy to carry out. Easy for the student to understand Requires only a short time. Gives accurate results. Reviews the student on gas laws.
