As the question title says, I want to know how to measure the partial pressure of a gas (in a vessel containing a mixture of many gases). I wonder how to measure it? Please help me answer this question
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1$\begingroup$ There are a variety of gas measurements possible, in various ways. One set of options is the many ways of doing mass spec. $\endgroup$– Jon CusterCommented Apr 23, 2023 at 15:57
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1$\begingroup$ I'm not sure if you can directly measure the partial pressure of each gas in the mixture. But the partial pressure of a given gas in the mixture is the pressure that it would exert if it alone were present (i.e., if all the other gases were removed). The partial pressure of each gas follows Dalton's law of partial pressures. $\endgroup$– Bob DCommented Apr 23, 2023 at 16:01
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2$\begingroup$ What's wrong with getting a total gas pressure, a gas analysis, and multiplying concentration by total pressure? $\endgroup$– David WhiteCommented Apr 23, 2023 at 17:21
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$\begingroup$ I feel like the historical approach may be covered in the pop-science book “An Ocean Of Air,” but I don’t have my copy nearby to check. A good read anyway. $\endgroup$– rob ♦Commented Apr 23, 2023 at 19:57
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2 Answers
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Residual gas analyzers (RGA) do exactly this, but they typically work on much lower pressures ($\leq 10^{-4}\text{ mbar}$). They often are therefore used to measure unwanted gasses in vacuum systems.
For pressures closer to atmosphere ($10^{3}\text{ mbar}$), there may be a way to use a mass spectrometer - a similar device but I think you usually somehow load in a sample instead of having it continuously connected to your system. But I'm just not very knowledgeable about this solution.
Both of these somehow work on the principle of ionizing the gas and seeing how the ions respond to an electric field.
Certainly you could use an RGA if you just leaked your gas slowly into a vacuum chamber. But depending on how you do the leaking the process of leaking might preferentially let through certain components of the gas (ex helium passes through small leaks better than bigger atoms/molecules).
I know that originally the content of the atmosphere was determined by cooling things down and seeing at which points different gasses became liquids. For example, as you cool, at some point (~80K) all the oxygen turns into a liquid and you can determine how much oxygen was present from the mass of the liquid.
All of these solutions are expensive for a citizen scientist.
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$\begingroup$ I wanted to give this an upvote, but then I noticed that chemical analysis measures composition, not partial pressure. For ideal gases this works out, of course, but it won't for non-ideal gases and for gases that are interacting with each other. Water vapor, for instance, at normal conditions, is actually a two-phase system (droplets of water in equilibrium with a gas mixture). I am not sure how to measure an equivalent quantity to partial pressure in that case. $\endgroup$ Commented Apr 23, 2023 at 18:31
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$\begingroup$ @FlatterMann I think partial pressure only makes sense as a concept in the case of ideal gasses. So for ideal gasses you can measure the mass or number of moles and infer the partial pressure. For example, in a real gas, there will be "cross terms," where the pressure due to oxygen is reduced when the amount of nitrogen is decreased. So it's not clear what pressure is due to O2 and whats due to N2. And in the case of water vapor of course only the parts in the gaseous state contribute. Any water trapped in liquid droplets does not contribute. The question was about a mixture of gasses. $\endgroup$– AXensenCommented Apr 23, 2023 at 18:45
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$\begingroup$ I noticed that this was discussed before: physics.stackexchange.com/questions/747688/…. I tried to find a generalization of the concept of partial pressure for real gases, but I don't seem to find more than concepts like vapor pressure (for a two phase system) or ad-hoc attempts for special systems. If you have a link to a general definition (a pressure matrix kind of approach?), maybe that should be mentioned in the answer. $\endgroup$ Commented Apr 23, 2023 at 18:55
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$\begingroup$ @FlatterMann to be clearer - my opinion is that a discussion of the definition partial pressure in the case of real gasses is beyond the scope of the question and an unimportant caveat that the asker was probably not interested in. I also don't have any idea how to formulate a reasonable definition $\endgroup$– AXensenCommented Apr 23, 2023 at 19:05
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2$\begingroup$ Using a small orifice, one can use an RGA to sample at atmospheric pressure just fine. You just need the conductance of the orifice to be much smaller than the pumping speed of the RGA. $\endgroup$ Commented Apr 23, 2023 at 19:30
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As AXensen points out correctly in his comments to his answer, the concept of partial pressure gets problematic for real gases and for ideal gases it is identical to chemical composition.
I think that in the general case of real gases we would need to have something like a species-specific semipermeable membrane (molecular sieve) between the vessel with the gas mixture and a test vessel containing just a pure test gas at variable pressure. This membrane can only let the test gas pass and has to block all others. If the pressure in the test gas vessel is the same as the partial pressure of that species in the gas mixture, then the pressure in the test gas vessel won't change. The difficulty here is that such ideal molecular sieves do not exist in reality. I am not even sure that it is thermodynamically possible for such a material or system to exist, but I don't have evidence either way. Somebody who understands thermodynamics better than I do can probably give a trivial argument for why the concept of ideal molecular sieves is not feasible.
