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Just for fun, I would like to see if I could make my own concentrated $\ce{CO2}$ by separating it from my own exhaled breath. I would also love to see if I could pressurize it so I could make my own carbonated water!

When you exhale, the gas that comes out of your mouth contains 78% nitrogen, 17% oxygen, 4% carbon dioxide and 1% other gases (source).

Is there any existing procedure for separating carbon dioxide from breathing air? I have done some research, but I have only found (industrial-scale) methods for separating nitrogen.

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    $\begingroup$ One can chemically trap one's carbon dioxide as calcium carbonate. An old school experiment to was bubble one's breath in a dilute solution of limewater. The solution would become cloudy, and further breathing into limewater would make the solution clear. The cloudiness was due to calcium carbonate. I will leave the chemistry for you to search yourself. $\endgroup$
    – AChem
    Feb 15 at 0:39
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    $\begingroup$ If you want to isolate it, you can capture CO2 in Na2CO3 solution, evaporate it and calcinate NaHCO3 to release captured CO2. $\endgroup$
    – Poutnik
    Feb 15 at 6:08
  • $\begingroup$ You got it all wrong. Concentrated CO2 is produced from Coca-Cola. Your exhale is just too diluted. $\endgroup$ Feb 15 at 7:38
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    $\begingroup$ In my biology class they mentioned that in a spirometer there is a canister of 'soda lime' that removes CO2 from the air tank when the user exhales. $\endgroup$
    – user129756
    Feb 15 at 14:32
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    $\begingroup$ Always write clear questions with included relevant details, purpose, context or background info to receive useful answers. Otherwise the answers may be just guesses. $\endgroup$
    – Poutnik
    Feb 15 at 19:57

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As mentioned in the comments, the easiest method is to bubble your breath through an alkaline solution. Use a bubbler that produces small volume bubbles and a solution in a tall vertical column to get the best capture. The choice of alkaline salt depends on your intended recovery method. Calcium ions will lead to precipitation of most of the carbon dioxide as calcium carbonate, which can be collected as a solid and heated to release the carbon dioxide as gas (calcination). With sodium ions, the carbonate will stay in solution, and you can recover much of the carbon dioxide by acidifying the solution and collecting the released gas.

If your goal is to get relatively pure carbon dioxide directly, as opposed to a solution containing carbonates, the common method is to use a cold trap. Flow the gas over an absorbent to remove water vapor and then cool the gas. The carbon dioxide will freeze while the other gases remain gaseous. This of course requires a very cold trap. The deposition temperature is $-79.5$ $^\circ C$.

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  • $\begingroup$ I am afraid the statement "Calcium ions will lead to precipitation of most of the carbon dioxide as calcium carbonate, which can be collected as a solid and heated to release the carbon dioxide as gas (calcination) " is not entirely correct. If we continue to bubble CO2 in limewater, the initial precipitate redissolves forming calcium bicarbonate. $\endgroup$
    – AChem
    Feb 16 at 1:30
  • $\begingroup$ @AChem - Good point. I am assuming that initial pH is high enough to avoid bicarbonate formation, but certainly it would be possible to add enough CO2 to overcome that. $\endgroup$
    – Andrew
    Feb 16 at 13:42

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