Does the “Fizz-Keeper” Work?

The Fizz-Keeper is a gadget (U.S. Patent 4,723,670 [PDF], 1988-02-09) that screws on to the top of a carbonated beverage bottle and contains a pump that allows re-pressurising the bottle after it has been opened. Marketing for the gizmo claims it “preserves carbonation and keeps soda fresh for week” (interestingly, the patent makes no such claim). But Henry’s law states that “the amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid”. So how can pumping air, which is mostly nitrogen and oxygen, into the bottle possibly affect diffusion of carbon dioxide gas from the remaining liquid until the partial pressure of CO₂ above it rises to equilibrium?

Well, let’s see…. Here are two articles from the Journal of Chemical Education, jailbreak courtesy of Sci-Hub, discussing the Fizz-Keeper.


It works because the pressure is pressure. Doesn’t matter if the dissolved gas is CO2, O2, or N. Yes, the CO2 will dissolve out of the liquid, but O2 and N will dissolve into it.

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But according to Henry’s law, pressure isn’t pressure—it is the partial pressure of each individual gas in the liquid and gas above it that matters. According to Henry’s law, the solubility of any given gas in a liquid is:

S_{\rm gas}=K P_{\rm gas}

where S_{\rm gas} is the concentration of gas in the liquid, K is the Henry’s law constant for the solubility of that specific gas and liquid pair, and P_{\rm gas} is the partial pressure of the gas above the liquid. Hence, it doesn’t matter what the pressure of the nitrogen and oxygen pumped into the bottle may be, the carbon dioxide dissolved in the pop will continue to come out of solution until the partial pressure of CO₂ in the gas rises to the equilibrium point with that dissolved in the liquid.

The solubility K of different gases in water (which I’ll assume is the same as the liquid in the bottle) varies widely, so the behaviour of the gases involved is very different. Here is the solubility of the three main gases we’re dealing with here, all for 5° C, the temperature of a typical refrigerator, and all in units of grams of gas per kilogram of water.

  • Nitrogen 0.275
  • Oxygen 0.65
  • Carbon dioxide 2.9

So CO₂ is 10.5 times as soluble in water as nitrogen and 4.4 times as soluble as oxygen. Consequently, after the bottle is pressurised with air, the main flow of gas will still be CO₂ from the liquid into the gas above it. Nitrogen and oxygen will dissolve into the liquid, but since their solubility is so much lower, a far smaller volume than the flow of CO₂ in the opposite direction. As a result, the CO₂ coming out of solution will add to the pressure of the nitrogen and oxygen in the bottle, raising the pressure and resulting in the “explosion” when the Fizz-Keeper was removed at the end.


Interesting. So then, if the air is pumped into a separated area inside the bottle, preventing the diffusion between CO2 into N2+O2, such modified Fizz-Keeper would work, correct?


I’m not sure how that would work. The problem with the pop going flat is that every time you open the cap to pour some out, the CO₂ above the liquid escapes and is replaced by air and then, after you replace the cap, more CO₂ comes out of solution to replace it. So to keep that from happening, you’d like to find a way to pour out the liquid without losing the CO₂ gas. One way to do this might be to have a cap with a valve in it you could open to pour the liquid. To fill your glass, you’d invert the bottle, open the valve, and then squeeze the bottle to dispense the liquid. When you inverted the bottle, the CO₂ would go to the top, and since you were squeezing the bottle, no air would enter. (These bottles are made from polyethylene terephthalate (PET), which is very flexible, so squeezing should be no problem.)

As you proceeded to crush the bottle, it might not stand up straight, but if you store it on its side as many modern refrigerators permit, that should be no problem.


Of course, there would be some loss of CO2 on each use, but as long as the area above the remaining liquid is small, the pressure builds up quickly, minimizing the loss of CO2 from the liquid.

From: “The Fizz Keeper, a Case Study in Chemical Education,
Equilibrium, and Kinetics
” [PDF]:

One simple and easy way to shift the equilibrium so that
less carbonation is lost on storage of resealed pop is to reduce
the gas volume. Thus the suggestion made on the ChemEd-L
list of pouring the leftover pop into a smaller container, one
that it nearly fills, should be much more effective than use
of the Fizz Keeper device, provided one is very careful in pour-
ing so as not to lose even more CO2 .

The actual device could be implemented ether as:

  1. a membrane that separates the compartment with the liquid from the compartment with the pumped air; or
  2. a piston on the bottom that reduces the volume of the bottle as the liquid is poured out;

Yeah, I was wondering if that was the case. If they were equally soluble, then the fizzkeeper would work perfectly. Or, if you could fill the space at the top of the bottle with CO2, which is heavier than air. Trouble is that you would have to fill it with the same pressure that was there, which would either mean having your fizzkeeper pulling from a store of CO2, or having pressurized CO2 to fill the top of the bottle.

Or you could do what I do, which is buy smaller quantities and drink the whole thing. Problem solved!

I wonder what the climate hysterics are going to do when they find out that every molecule of CO2 has a CARBON!!! atom in it, and that soda bottles are CARBONated.


Sorry to keep bashing on this, but I I don’t think it would. (This is counter-intuitive and what I would have guessed before reading up on Henry’s law today, but this is what I think it means.) If oxygen and nitrogen were as soluble in water as CO₂, then more would dissolve in the liquid, but you’d still have as much CO₂ migrate from the liquid into the gaseous phase until the partial pressure of CO₂ in the gas reached equilibrium. This might make the pop fizzier with dissolved nitrogen and oxygen, but you’d still lose CO₂ every time you opened the bottle to pour out some pop, since you’d lose the CO₂, which would have to be replenished from the liquid. This would make the pop go increasingly flat as you emptied the bottle.

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I’m a bit lost here. Fizz is merely gaseous effluent. ?So why does it matter which gas you use. There are commercial “pop makers” that basically fizz a liquid you put into the bottle. Surely they don’t use CO2 to power the machine, yet the “pop” produced appears to be adequate.

Isn’t the simple answer that, in the gas/liquid/bottle system, each gas acts independently of the other. That is what partial pressure means.

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Surely they do use CO₂. Home carbonation devices such as SodaStream and the humble and vintage soda siphon so beloved in slapstick comedy use cartridges of compressed carbon dioxide to force the gas into solution. They call it “carbonation” because the gas dissolved is carbon dioxide which, in addition to making the water fizz, adds carbonic acid, reducing its pH to a between 5 and 6 and imparting a tart flavour.

As I noted in comment #2 above, carbon dioxide is more than ten times as soluble in water than nitrogen and four times as soluble as oxygen, so far more of it can be placed in an aqueous solution. Since much less of other gases can be dissolved, they would impart only a very weak fizz and would not add the flavour of carbonic acid.

Big Clive has a running series on YouTube, “Will it Carbonate”, where he tries various beverages in a SodaStream machine: here is a playlist.

Especially, check out the one with Bailey’s Irish Cream.


Ouch! Now there was a sight one can’t really unsee!!!