Concentration of Oxygen in the Air
This is an attempt to use a microscale method to measure the concentrationof oxygen in air. The method is still not perfected. I would welcome feedbackfrom anyone who would like to try out the method: how successful do you findit? Do you find it consistent? What difficulties do you find? Any suggestionsfor improvement? Comments and suggestions to Mike Clark,
For monitoring of the temperature of the experiment, I have been using aCBL temperature probe rather than a cheaper digital thermometer or aliquid-in-glass thermometer.
PROPORTION OF OXYGEN IN AIR
AIM To measure the proportion of oxygen, by volume, in a sample ofair.
MATERIALS REQUIRED One graduated 1 mL pipette, pipette filler bulb,digital thermometer to read to 0.1 degrees C, watch or clock, retort standwith clamp, about 0.6 mL of 1.0M solution (1.9 g dissolved in 10 mL of solution)of sodium pyrosulfite, Na2S2O5, (also knownas sodium metabisulfite).
PREDICTIONS AND PLANNING
Some chemical substances in aqueous solution react with oxygen gas quitereadily. Examples include sulfites, iron(II) ions and tin(II) ions. If sucha substance were used to remove all the oxygen from a measured volume ofair, then the loss of volume of the sample would indicate the proportionof oxygen in the sample.
A volume of air of about 0.7 mL can be held as a bubble in a horizontal 1mL graduated pipette between two volumes of solution that can absorb oxygen.The volume of the bubble can be read to 0.001 mL (one tenth of a graduation).If the pipette is left for a length of time sufficient that no further changeoccurs in the volume of the bubble, then loss in the bubbleís volumeshould represent the volume of oxygen that was absorbed.
What solution should be chosen to absorb the oxygen? Sodium pyrosulfite producesfairly consistent results within a reasonable length of time. Solutions oftin(II) ions are difficult to prepare; they absorb oxygen well when the oxygenconcentration of the air is high, but tend to react slowly and unreliablyas the concentration of oxygen becomes low. Iron(II) sulfate solutions reactslowly at low pH, and at higher pH they may leave rusty deposits in glassware.
A large excess of sodium pyrosulfite should be used. Oxygen is being absorbedat the air-liquid interfaces, the two menisci. A large excess of solute shouldreduce the slowing of the reaction caused by depletion of pyrosulfite reagentand accumulation of reaction products at the interfaces. Assuming a bubblesize of 0.700 mL, then the content of oxygen should be a little over 0.140mL, which represents approx 6 micromoles of oxygen gas. This amount wouldbe absorbed by the same amount of pyrosulfite ions. This amount would becontained by about 6 microlitres of 1.0M solution. About 0.60 mL of 1.0Msodium pyrosulfite represents, therefore, an excess of approximately onehundredfold.
The volume of a gas changes with temperature, so it is necessary that temperaturebe recorded as well as volume. It will be assumed that the temperature ofair in the bubble is the same as the temperature of surrounding air. Therecorded gas volumes can be corrected for temperature, so that the initialand final volumes of gas in the bubble will be at equal temperatures. Athermometer that can read to 0.1 degrees C should be used.
The reactions involved are slow, so preparations should be made to gatherobservations over an extended period of time.
Using a filler bulb, carefully draw sodium pyrosulfite solution into theone millilitre graduated pipette, up to about the 0.70 to 0.65 mL graduation.
Withdraw the pipette from the sodium pyrosulfite solution, and draw in about0.7 mL of air. Draw in a further volume of 0.25 to 0.3 mL of solution, sothat there is a clear bubble of air trapped between two volumes of solution,and the two menisci at the ends of the bubble lie both within the range ofthe graduations.
Hold the pipette vertically for about a minute to allow any film of liquidaround the bubble to run down. Then holding the pipette in a horizontal position,work quickly to remove the filler bulb, and tilt the pipette slightly andgently so that solution runs into the tip without dripping out. Clamp thepipette with the tip tilted downwards at a very small angle, and promptlyread the positions of the two menisci to the nearest tenth of a graduation,that is, to 0.001 mL.
Record the time, and the room temperature to 0.1 degrees C.
The pipette should be monitored for about forty-eight hours: the positionsof the meniscus and the volume of the bubble should be recorded periodically.As the rate of shrinkage of the bubble slows down, the temperature shouldalso be recorded. This must continue until the volume of the bubble ceasesto shrink.
During the reaction, if a small volume of solution flows down into the spaceoccupied by the bubble of air, the pipette should be rocked carefully beforea volume is read, allowing the liquid in the pipette to move slowly to andfro until the liquid is swept up, and the gas space is free of droplets ofsolution. During this process, the bubble must remain intact and no solutionmay be spilt from the pipette.
Records should be kept of the following observations:
**At each observation: time of observation, the positions of the menisciand thus the volume of the bubble.
**At setting-up, and at observations made towards the end of the reactiontime: temperature.
ALL VALUES SHOULD BE RECORDED TO THREE SIGNIFICANT FIGURES:
The "volume of bubble" is the difference between the values at the two meniscibounding the air bubble. All pipette readings should have three digits: forexample, a reading that is exactly 0.71 should be recorded as 0.710, whilea reading that is exactly 0.1 should be recorded as 0.100.
Temperatures should also be recorded to the nearest 0.1oC, soa reading that is exactly 18oC should be recorded as18.0oC.
As the reaction approaches its end, values for the "volume of bubble" isthe volume of the bubble should be corrected to the initial temperature ofthe experiment, using the Charlesí Law equation.
The "volume of oxygen absorbed" is the difference between the initial volumeof the air bubble and the corrected final volume of the bubble.
It may be observed that the rate of shrinkage of the bubble is quite rapidat first, but becomes slow after twelve to fifteen hours, as the partialpressure of oxygen drops.
INTERPRETATIONS AND EVALUATION
The percentage of oxygen in the sample can be calculated from the volumeof oxygen absorbed compared with the initial volume of the bubble.
The final estimate of oxygen percentage in the air should be presented,therefore, to one decimal place, as, for example, 21.2%, or 21.0%.
EVALUATION OF ERRORS
There are some sources of error in this measurement:
**an error of 0.001mL in estimating the position of meniscus may have a smalleffect on final estimates of oxygen percentage.
**errors of as little as ±0.1oC in reading temperature mayalso affect volume readings, and therefore estimates of oxygen percentage.
**it is assumed in this procedure that oxygen is the only component of airthat can vary and so affect the volume of the bubble. No account is takenof the water-vapour content of the initial air sample or of the final bubble,nor of the small partial pressure of sulfur dioxide in the airspace overthe pyrosulfite solution.
**accuracy of results may be affected slightly by changes in atmosphericpressure over the time of the experiment.
This procedure should still be able to yield a proportion of oxygen inan air sample of between 20.0% and 22.0%. If the procedure is replicatedseveral times, an average estimate could be made for percentage oxygen contentof air.