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Practical use of Henry's Law (Experimentalists please advise)

  1. Aug 28, 2015 #1
    Hi,

    I need to find out practically how much gas to bubble into a liquid to make it fully saturated. I've found web pages on Henrys Law but can seem to find one on how you would do this experimentally.

    For example this youtube video shows you how to calculate it:



    But how would I experimentally measure the partial pressure of the gas? what equipment would I need? etc etc

    Thanks for any help with this.
     
  2. jcsd
  3. Aug 28, 2015 #2

    Bystander

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    What specifically are you trying to measure?
     
  4. Aug 29, 2015 #3
    Hi, I need to fully saturate a liquid with gas, then it will be exposed to ultrasound. We will be measuring the sonochemical yield via KI reaction.
     
  5. Aug 29, 2015 #4

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    At what pressure?
     
  6. Aug 31, 2015 #5
    Just standard room pressure, unless I'm missing something. It will just be water in a container that will have gas bubbled through it to saturate it.
     
  7. Aug 31, 2015 #6

    DrDu

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    If you bubble the gas through the liquid at normal pressure, its partial pressure will equal the ambient pressure. Which gas are you considering to use? the saturation concentration of many gasses is quite small but it takes a long time to reach equilibrium, so you have to have the gas for a long time in contact with the liquid. If the gas is expensive, maybe you can dissolve it shaking a container filled with both gas and liquid.
     
  8. Aug 31, 2015 #7
    Thanks for that. We will be using various gases and gaseous mixtures and funds are kind of limited hence the reason for attempting to use Henrys Law. It would be much easier to just bubble it through over time but waste is an issue so will perhaps try the shaking method you describe. Can I just confirm you are saying the partial pressure would be the ambient pressure of the water?

    Also if I use a mixture of gases, say Ar/30%O2 how would I deal with the Henry's law constant for each gas? (note the water would be degased first)

    Is there any other simpler way (experimentally) to determine if a gas has fully saturated a liquid?

    Thanks again for any suggestions / help.
     
  9. Aug 31, 2015 #8

    DrDu

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    If your ambient pressure is, say, 1 bar, then for a Ar/ 30% O2 mixture, the partial pressures of Ar and O2 at saturation would be 700 and 300 mbar, respectively. To measure the concentration of oxygen in water, maybe you can use a Clark electrode or the like?
    I don't think that Henry's law is very useful here, as the amount of Argon and oxygen which desolve in water is really tiny. So in principle you don't need much of the gasses. The bigger problem is the long time to reach saturation.
     
  10. Aug 31, 2015 #9
    Appreciate the reply but I'm a bit confused (chemistry is really not my strong point).

    So the partial pressures referred to using Henry's law are those when the gases are fully saturated? How does this help practically? i.e. how would you measure this?

    Is that similar to the Winkler method? edit I see it's not, but would that mean I have to test for different gases seperately?

    I have use of a Gas Chromatograph, so I could take samples as I bubble the gas through, do you think this could be used to determine if the gas is saturated?

    Any further suggestions you (or anyone else may) have to solve this problem would really be appreciated.
     
  11. Aug 31, 2015 #10

    DrDu

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    It is not the gasses which can be saturated but the liquid. Henry's law lets you calculate the concentration of O2 in water which is in equilibrium with the partial pressure of the gas you bubble through. The use of an electrode has the advantage over Winkler or gas chromatography that you don't have to take samples but can continuously measure the oxygen concentration of your solution and easily judge when it becomes constant.
    But I am not an experimentalist.
     
  12. Aug 31, 2015 #11

    DrDu

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  13. Aug 31, 2015 #12
    Ahhh, if I know the oxygen content of the liquid then for a two componant gas mixture of course I will then know the content of the other gas also! I get it. That's very helpful, I will look into the electrode method. Thanks again!
     
  14. Aug 31, 2015 #13

    DrDu

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    Not quite. It may be that the oxygen reaches equilibrium more rapidly than argon. But I don't know how to measure the argon concentration in water (as I said, I am not an experimentalist).
     
  15. Aug 31, 2015 #14

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    In an earlier threadhttps://[URL="http://www.physicsforums.com/threads/dissolving-a-gas-in-water.822256/"]www.physicsforums.com/threads/dissolving-a-gas-in-water.822256/[/URL] [Broken]
    Let's see if we can sort things out a bit: you want to look at reactions between (among) two or more gases dissolved in water?
     
    Last edited by a moderator: May 7, 2017
  16. Aug 31, 2015 #15
    oh dear, thought I had it.

     
    Last edited by a moderator: May 7, 2017
  17. Aug 31, 2015 #16

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    Okay. Gas, or gas mixtures, at atmospheric pressure, presumably moderate or room (25 C) temperature, and some way of establishing that the dissolved gas(es) are in equilibrium; with the (wet) vapor phase --- reproducible control of composition for your measurements/experiments --- takes me a while sometimes.

    For single gases, "over-saturate" at higher pressure (1.5-2 atm. total), agitate (sonicate) at the one atmosphere pressure you are interested in measuring, and excess gas will ex-solve from the solution. For most mixtures, you should be able to get away with the same trick.

    Holler if you've got anything "unusual" you're planning to measure.
     
  18. Sep 1, 2015 #17
    hmm, ok you've lost me. I see what you are saying, bubble the gas through and then sonicate the liquid and it will remove the excess gas. BUT the purpose of the sonication in the experiment is not to remove excess gas, I'm not using ultrasound as a method for removing excess gas, I'm measuring the effects of ultrasound for a liquid (water) that is saturated with different gases (that is I'm measuring the sonochemical yield, it varies with different gases that are dissolved in the liquid). Are you suggesting I could use the ultrasound to somehow ensure the gas is fully saturated?

    Also I'm not sure what you mean by "over saturate", the purpose of trying to use Henry's law was to not oversaturate the liquid (is that possible?) and waste the gas. I just need to saturate a liquid with gas mixtures with as little gas wasted as possible.

    If I've missed your point (quite likely) please could you reemphasise it?

    The help is really appreciated though! :-)
     
  19. Sep 1, 2015 #18

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    Your use of the expression "Sonochemical yield" suggests you anticipate a permanent change in the state of the saturated system rather than a temporary perturbation; this would be a yield of "what particular effect on or product of the system?"
     
  20. Sep 2, 2015 #19
    I'm not sure what you mean, there is a change in the system as radical production is effected by the gas content of the sonicated fluid. The idea of saturating the gas is simply to maximise the gas effects on the reaction. Or did I miss what you are saying?

    Thanks again
     
  21. Sep 2, 2015 #20

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    You can approach saturation from two directions, under and over saturated conditions; when the two approaches yield the same solubility results, you've got as good a measure of gas solubility as you are going to get. You are, by the nature of the experiment you are planning, able to agitate solutions that are under or over saturated at whatever conditions you choose for measurement. Neither the under nor the over saturated solutions should require large volumes of gases.
     
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