5.2.2: Oxygen in air

Lab #2: Oxygen content of an air sample

We need to breathe. When we take up air in our lungs, oxygen travels to all cells in the body and reacts with nutrients in our food such as fats and carbohydrates, providing the energy that keeps us alive. In the air we breathe out, part of the oxygen is replaced by carbon dioxide and water, two of the products of oxidizing nutrients. When we are in regions where oxygen levels are low (e.g. at high altitude), our bodies adopt after a couple of days. In a lecture hall where oxygen levels drop and carbon dioxide levels rise, we get tired faster and start to yawn.

Today, we will measure the oxygen content of an air sample.

You will need equipment from one of your drawers (the technician should provide the equipment and will be responsible for collecting it at the end of lab). The record keeper is responsible for labeling samples so that they don’t get confused with one another.

A chemical reaction that uses up the oxygen in air trapped in a 15 mL tube

In order to measure the oxygen content of your air sample, you will trap a defined volume of air in a 15 mL tube and run a chemical reaction that uses up the oxygen within about 40 minutes. During that time, the air has to stay trapped so you can monitor by how much the volume of your sample decreases, and to prevent fresh air from replenishing the oxygen in you sample.

The chemical reaction is the formation of rust. Oxygen in the gas state reacts with iron to form a solid product (rust), leading to a reduction in volume we can observe (the chemical equation is a simplification, but captures the important result that dioxygen is removed from air and becomes part of a solid):

\[\ce{4Fe(s) + 3O2(g) -> 2Fe2O3(s)}\]

If we run the reaction long enough to remove all of the oxygen, the volume change will indicate how much oxygen was present in the sample at the beginning. The trick is to make the reaction fast enough so that most of the oxygen is used up within our lab session, and to run it in a way that we can observe the volume changes.

We monitor the volume changes by running the reaction in an upside-down graduated tube submerged in water. To make the reaction sufficiently fast, we will use a loosely packed sample of steel wool (the high surface area is advantageous in the reaction with a gas) that has been soaked vinegar (which is an acetic acid solution with additional molecules that give it flavor). The vinegar acts as a catalyst – similar to salty wet conditions that will make your car rust faster in a New England winter than, say, in the Arizona desert.

Procedure

1. Divide the steel wool in your kit into four parts, “fluff it up”, and place it in a 15 ml tube. Pour vinegar into the cap and transfer it into the tube. Close it with the cap and invert the cylinder a couple of times. Then, open the tube and discard the vinegar in the sink or in your waste container. Make sure the steel wool does not fall out (if it does, fluff it up more and put it back). Take a picture if you can.
2. Invert the cylinder and submerge it in water (in a drinking glass or in a shallow bowl, as illustrated in the video below). Start the timer (ideally as soon as the air is trapped, but as you are working alone with your hands full, once you have assembled everything. Alternatively, make note of the time). Carefully record the volume to a tenth of a mL (lines are 0.5 mL, so you have to estimate), taking a picture sufficiently clear to check the reading later, if you can. It is confusing to make readings while the tube is upside down, so make sure you are not off by one (e.g. if you look at the still image of the video below, the value is between 14 mL and 15 mL, maybe 14.9 mL. Common incorrect readings would be 14.1 mL or 15.1 mL).

   trapping_air from Karsten Theis on Vimeo.

    

3. Every ten minutes, read off the volume. To do so, hold the tube vertically, and lift it until the inside and outside level of water approximately match. During this manipulation, you have to make sure the lip of the tube never is above the water level in the glass or bowl, otherwise your gas volume and composition will change, invalidating the experiment. In addition to the readings at regular intervals, also read off the volume and note the time when the level is exactly at one of the graduations (i.e. at 14.5 mL, 14.0 mL, etc). These are more accurate because you don't have to estimate the volume between graduations (and timers nowadays are very accurate and show time to the second or even smaller intervals).
4. Plot the volume of air over time. Start once you have two data points – this will help to decide when the reaction is almost complete. Using a spreadsheet (e.g. Google docs, software on your computer, chart-studio.plotly.com etc), plot the volume (along the y-axis) against the time (x-axis). This type of graph is called scatter plot or (if you connect the dots) line graph.
5. Once you don’t see any change in volume (i.e. less than 0.1 mL difference in 10 minutes), take one more time/volume point. Then, remove the tube from the glass or bowl, remove the rusted steel wool and discard it with the household waste. (The steel wool might be difficult to get out. You can try to pry it out with a toothpick, or bang the tube against a hard surface that is easy to clean, e.g. bottom of a stainless steel sink). Clean the tube for future use with water (if there are some rust stains, it is fine).
6. Calculate the oxygen content: Discuss within your group how you would calculate the content of oxygen in your air sample, and compare your results. As an analogy, imaging you want to know what percentage of M&Ms are red by first counting all of them, eating the red ones and counting the remaining ones. How can you use the count before and after eating the red ones to calculate the percentage of red ones. Discuss what factors might result in systematic or random errors in your determination (consider the gas laws, the way you measured volumes, the time passing between steps 2) and 3), the volume of the steel wool), and how you might change the experiment to improve it.

Question for lab report

"What is the percentage of oxygen in (fresh) air?"

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Lab #4: Experimental controls

For most experiments, the interpretation of results depends on a conceptual model of what is happening during the experiment. Controls are a strategy to check whether our model is correct.

For the oxygen content of air experiment, I made the following statements, some qualitative and some quantitative, of why the experiment gives us an estimate of the amount of oxygen in air.

The reactants and products of the chemical reaction

• The only component in air that reacts is oxygen
• The reaction partner is the iron in steel wool
• No gases are formed in the reaction
• Oxygen reacts fully because there is sufficient iron to react with

Behavior of gases

• The amount of a gas is proportional to its volume (at constant pressure and temperature)
• When gases mix, their volumes add up exactly

Role of inverted cylinder and water

• Water does not take part in the reaction
• No gases can get into the cylinder or get out of the cylinder because water acts as a barrier

Rate of the chemical reaction

• The reaction is sufficiently fast to remove all of the oxygen within the lab period
• The reaction is sufficiently slow that no oxygen is used up before the first volume measurement
• Acetic acid acts as a catalyst (makes the reaction faster, but is not part of the reaction)

For lab 4, choose one of these statements and design an experimental control (either
negative or positive) that generates experimental support for it and disproves an alternate
model of the behavior seen in the original experiment.

Do not start your experiment until you have received permission from your instructor.