Science question: Is dissolved O2 displaced by CO2?

[Broken Crow]

If you know my posts, you'll likely know I have brain-damage. I used to let the science lead in all things and I tried to look this up, but can't grok the results, so I'm asking for your brains:
I've always had a minor concern in the back of my head over the amount of dissolved O2 in the Star San I mix up...since the science became incomprehensible with the injury I tend to go with the empirical and 45 years of broad knowledge, but gas content in fluids is not a subject I ever gave more than a superficial study of, but I seem to suspect that under pressure and refridgeration; CO2 will displace O2 dissolved in liquid and so I've intended to get a couple of oxbar kegs to store my star san in for my keg-sanitizing routine.... I do wonder though.
Does CO2 displace dissolved O2 in star san in a keg?

 

[hotbeer]

Like you, I'm more into the empirical evidence and what I think sounds like reasonable scientific argument one way or the other. Hoping not to get caught up in the half truths that start out with a scientific fact and then with circular logic arrive at the position they want to pass off as true.

But I'd ask, how much StarSan do you think is going to be left in that stuff to get into your beer. If a cup, you might have a issue for O2 in five gallons of beer. If less than a ¼ cup, then probably no issue. Even if there was a much a 6.5ml/litre of O2 in the water/StarSan solution.

 

[hotbeer]

Oh, I found this a interesting read when I was looking up information about displacement of dissolved O2 by CO2. Which I think is a interesting question in itself that you posed.

https://fivestarchemicals.com/blog/...ext=Even 1 ml (or cm,better used as a coaster.

 

[sibelman]

@Broken Crow, my intuition is that the imagined displacement will not occur. But in a closed container with CO2 and almost no oxygen above the liquid, some oxygen will leave the liquid and equilibrate with the gas above the liquid. Just as in osmosis across a membrane.

Any actual chemists here to check me on this?

 

[jerrylotto]

There's nothing wrong with oxygen dissolved in starsan. You don't leave any of this behind when you Brew so all dissolved oxygen is expelled when you push the starsan out of whatever you're cleaning.

 

[Broken Crow]

It's mostly idle curiousity, but it's still in the back of my head and I can't stand unanswered problems
In my own sanitizing and purge routine I fill my kegs with the lid hanging inside by the bail, the PRV pulled open and turned to lock it, and a gas disconnect to nowhere and once star san starts running out the top I pull the lid up; then it starts squirting out the prv and gas post, so I pull the gas diconnect off and turn the PRV shut... Thing is; I always start with about 7 and a half gallons of star san.. Usually when I'm done, I'll save about 5G in a bucket for next time, when I'll make up a fresh 5G to go with it. I don't like to waste star san (and also I'm not concerned with the miniscule amount left in the keg) so I'd like to get a pair of 5G oxbars so that I can store a whole 10G....because I always ovefill my kegs, I need more than 5G to do it so this seems a reasonable option to me. I've just been wondering though; Storing a couple kegs of star san under pressure... just how much dissolved O2 there will be in the kegs and will it end up in the headspace?
I figure it's likely to be an insignificant amount, but it does highlight a lack of knowledge on my part and for me that's the bigger deal that's compelled me to ask on here.
Thanks for all your thoughts so far...Having a chemist or one of our members who likes an excuse to use thier expense tools for measuring O2 could be fun and educational though.

 

[Bassman2003]

My hunch is that CO2 & O2 will coexist. Gas theory and practice is kind of a black box to me. Every question I ask seems to have an answer I do not like Basically, gases go everywhere and you can't stop them.

 

[tracer bullet]

I'd agree they probably coexist - you can dissolve "air" into water and air is a mixture of a variety of things.

I think there will be air in any water used to make a star-san mixture. If the keg is half full of star san, and there's CO2 above it, I'd expect some of the oxygen in the air to come out and mix with the CO2 headspace to sort of "balance out". Maybe not a good way to store something if you want it oxygen free.

If it's a quick thing, which is how I do it, star san goes in, CO2, pushes it out, and then kegging commences, I doubt enough air is coming out over the course of that 15 or so minutes to even be concerned about.

If Star san is pushed out and the keg is full of CO2, and it's held that way - yeah some O2 will come out of the star-san solution (if it's in the water used to start with). But probably very little... O2 is like 21% of air, there won't be much air dissolved in the couple ounces remaining in the keg, and there's just a few ounces (if that) so... sure it's a thing but probably not too concerning. Especially if you store a keg somewhat pressurized, and then release that pressure just before kegging. You'd likely let half or 2/3 or more of any O2 out when you depressurized.

Seems like a good question, seems like something to not worry about.

 

[AlexKay]

To a very good approximation, any time you have a gas in contact with the liquid and they are allowed to equilibrate, the amount of gas dissolved in the liquid will be proportional to the pressure of that gas in the headspace. The constant of proportionality is the Henry's Law constant, which depends only on the identity of the gas (and liquid) and the temperature. The presence of other gases either in the headspace or in solution does not matter at all (again, to a very good approximation.)

For oxygen in water, the Henry's Law constant is ~0.042 g/(L atm). Oxygen is 20.9% of the atmosphere, so when the total gas pressure is 1 atm, the partial pressure of oxygen is 0.209 atm. At equilibrium,
[O2] = k_H * p_O2
[O2] = 0.042 g/(L atm) * (0.209 atm) = 0.0088 g/L

Since a liter of water (or beer) weighs (approximately) a kg, this means if you leave liquid sitting out long enough, it will have 9 ppm of O2 in solution.

If you remove the oxygen from the headspace, some of the dissolved oxygen will come out of solution, in order to reestablish the Henry's Law proportionality.

But in all these cases, adding CO2 does nothing.

Also, none of this says anything about how long it's going to take to reach equilibrium. The process can be quite slow, as those of us who set-and-forget force-carbonate know.

One thing that may or may not be in the back of people's heads (and may or may not be causing confusion) is the practice of gas stripping. If you bubble CO2 (or any other inert gas) through a liquid, you will pull other dissolved gases out of solution. This is because the bubbles create a new gas/surface interface (and depending on the number of bubbles, the surface area can be quite large), and the CO2 inside the bubble is (hopefully) pure, inert gas, so any other gas in solution will tend to come out into the bubble. Gas stripping can be very fast, but in terms of what's dissolved, you're just getting the solution into equilibrium with your pure CO2 (which hopefully does not have oxygen in it.) There is no physical/chemical/whatever interaction between the O2 and CO2.

In terms of oxygen in Star San, it's best to assume that the Star San is saturated, with 9 ppm O2. If you leave 50 mL of that Star San in the keg and then fill it with beer, it has 50 g * 9 ppm = 450 ug of oxygen in it that will go right into your beer. Assuming 20 kg of beer, that's 22 ug of O2/kg of beer = 22 ppb. That's not nothing, but it's very unlikely to be your biggest problem.

 

[Broken Crow]

Thanks really! I don't think I've been entirely clear though (and that's on me); I have no concern whatsover about residual star san in my purged kegs that I'm gonna put my beer in... It's the headspace in kegs for storing star san for next time that I'm wondering about.
Again though, it is mainly an idle curiousity and if it really bugs me: Thank You @AlexKay for the 'gas-stripping'! I suppose I could always break out my 'quick-carb' stone, tee and pump. (provided I understood that correctly )