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I want to run a Lego Pneumatic Engine on the CO2 that is produced, but before I do, I want to be sure that that's the ONLY gaseous product from a real-world reaction. Everyone seems to assume that it is, just by omission, but no one actually says it.

I'm sure it would run, but I'd rather not damage the ABS plastic or probably-steel parts (shiny metal piston shaft), or a rubber seal or the factory lubricant or something else that I can't see. CO2 seems inert enough by itself, but is that really ALL I'll get?
(assuming of course, that I keep the foam out)

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  • $\begingroup$ Vinegar contains about 50 - 60 g acetic acid per liter. The other acids are tantric acids and lactic acid, which are present at 1 to 3 g/L. All acids react with baking soda with production of $\ce{CO_2}$ No other substances can produce a gas, with reaction with baking soda. $\endgroup$
    – Maurice
    May 29 '20 at 9:53
  • $\begingroup$ You won't get pure $\ce{CO2}$ out of an aqueous solution. There will be water vapor in the $\ce{CO2}$. Over a period of time that will almost certainly cause any steel Lego parts to corrode. $\endgroup$
    – MaxW
    May 29 '20 at 10:17
  • $\begingroup$ @MaxW That makes sense. If there's water involved, then inevitably some will evaporate. So how does that compare to normal humidity? These parts are about 20 years old, with no special care except possibly by accident (open air and regular use for the first 10 years or so, then in sealed plastic bags for maybe another 10 years, all climate-controlled), and no discernible damage so far. $\endgroup$
    – AaronD
    May 29 '20 at 10:54
  • $\begingroup$ Acetic acid and dissolved salts will promote corrosion, unless it is stainless steel. And even in that case, the shiny polished surface may got not so shiny over time. $\endgroup$
    – Poutnik
    May 29 '20 at 15:12
  • $\begingroup$ So far, the biggest concern seems to be the metal shaft. Plastic, rubber, and a bit of grease are all okay by omission then? $\endgroup$
    – AaronD
    May 29 '20 at 18:12
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It seems to me, according to the comments on the question, that there are NO additional reactions or products to worry about.

However, because of the foam, some of the original reactants can be tossed up, and find their way into the later parts of the apparatus as a haze/fog/cloud that is not completely inert.

Is that more-or-less accurate? I'm trying to get an answer here, and not just a string of comments.

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  • $\begingroup$ Your answer is an accurate summary and is fine. So afterwards, rinse the LPE with water, dry it and maybe put a small amount of oil on the steel shaft. $\endgroup$
    – Ed V
    Jun 2 '20 at 1:23
  • $\begingroup$ @EdV I guess the easiest way to do that is to run it on baking soda and vinegar as planned, then run it on water to rinse it out, then run it on air to dry it. Then the question becomes, "How to hook up a garden hose to an LPE?", and is probably better answered on bricks.stackexchange. A sports inflator needle fits wonderfully, so I can use that to hook up a shop air compressor. (and dial the regulator down a bit!) I've been using a bicycle pump for testing, but I don't think I want to pump long enough to dry it! $\endgroup$
    – AaronD
    Jun 2 '20 at 2:44
  • $\begingroup$ I have to admit iI have no idea how big the LPE is, but getting all the reaction residue (and whatever might be is excess), washed away, is good. Just reasonably dry is OK: the rusting of iron and steel requires both oxygen and moisture, so dryer is better. Best of success! BTW, have an upvote: upvotes here are easier than at the EE stack exchange, where they are tossed around like manhole covers. ;-) $\endgroup$
    – Ed V
    Jun 2 '20 at 3:27

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