Good news for dark room fans

Nitrogen is inert relative to our needs, but being very slightly lighter than air (MW 28 vs. 29 for air), you have to fully exclude the air from the bottle for it to be fully effective and there's no way to know for certain you've done so. Unless you already use nitrogen (for nitrogen burst agitation, for instance) it's probably more expensive than butane in the quantities we'd use, or you'd wind up with a multi-century supply (plus, you need to buy or rent-and-never-return a high pressure bottle and a regulator to bring the 3000 psi down to a few pounds suitable for not blowing all your chemical onto the ceiling).

Thanks for your reply. I use nitrogen for nitrogen burst agitation so it is something I have on hand and had always thought it was heavier then air so that is why I use it. A lot of different darkroom printers have commented in the past that it was a good inert gas for topping of chemical bottles but I guess now that is not the case.
Gord

So, air is made up largely of N2, O2, Ar, CO2, H2O, and trace amounts of Ne, He, CH4, Kr, and tiny amounts of other stuff. The molecular weights of N2, O2, Ar, CO2, and H2O are 14, 18, 40, 44, and 18 respectively. These are all relatively evenly mixed in the atmosphere up to about 80-100 km. Except water vapor, whose concentration varies as we all know, because it can condense out or evaporate.

The ~1 percent of argon and CO2 have ~twice the molecular weight of the other gases. Why don't they sink to the bottom, creating a 100m thick layer of suffocating gas at sea level so that oxygen-breathing life only exists on hilltops? Diffusion. The kinetic energy of the gas molecules at ~ room temp is large enough that it overcomes the potential energy difference that would come from stratification.

If you have an entirely sealed and still container, the gas can stratify within it, but in an open container or room, volatile gas will diffuse quickly unless it is really near condensation. (I suspect that the difference here is the "open top" of the container, which provides a slightly lower-pressure upper boundary condition for hydrostatic equilibrium, vs the sealed container, but I haven't tried working that out.)

Here's a webpage that discusses this in a different but similar context - comparing CO2 in sealed wine bottles vs why CO2 diffuses into the upper atmosphere despite its high molecular weight: https://news.climate.columbia.edu/2020/09/23/carbon-dioxide-distribution-atmosphere/ (They don't give a technical answer to why diffusion is more important in the atmosphere than a sealed container, but it is a non-technical page.)

Anyway, for photochemicals, I agree that a heavy gas would be preferred for topping off a sealed container, since you probably can't evacuate all the oxygen; you want something heavier than oxygen to settle. (Although decreasing the oxygen content by a lot may itself be helpful.) But for an open container, a heavy gas isn't going to sit around above the fluid for long, unless it is near condensation.

gordrob said:

I use nitrogen for nitrogen burst agitation so it is something I have on hand and had always thought it was heavier then air so that is why I use it.

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The air around us is ca. 70% nitrogen. That's why the weights of air and nitrogen are so close. They're largely the same thing.
Nitrogen will be OK in your bottles of developer if you have a way to fill the bottles with nitrogen while keeping all air out during and after the fact.
What has gone lost in the discussion following my mention of the Coravin system is that said system actually consists of a device that keeps the wine bottle hermetically sealed and while doing so, replaces any liquid drained with an equal volume of inert gas. The discussion focused on the inert gas, but the interesting thing about the Coravin system is its hermetic seal, which is fundamentally different (better, as well as vastly more expensive) than a Vacuvin system - and it's arguably more effective than simply topping off a bottle with gas (with an unknown pattern of diffusion of air into the mix while doing so).

Wasn't really the answer I was hoping for - so using the nitrogen was basically a waste of time. I guess my other option is to go back to using wine or wood finishing perseveres that contain argon to form a blanket to prevent the oxygen from coming in contact with chemicals. Thanks for your input.

gordrob said:

I guess my other option is to go back to using wine or wood finishing perseveres that contain argon to form a blanket to prevent the oxygen from coming in contact with chemicals

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Argon, carbon dioxide (for some chemicals, not those with hydroxide alkali), or cheapest and easiest (IMO) butane. Butane is only hazardous if you have an ignition source in the darkroom while you're putting it in the bottles. Butane has a high enough boiling point that (in a cool darkroom) you can often see a small amount of liquid on the surface of your developer after shooting the butane into the bottle; this will push any remaining air out as you're closing the cap. It also has a high enough molecular weight to stratify more readily than the other reasonably economical candidates, making it a better air exclusion gas.

Another option, at least for replenishers or stock solution that hasn't been diluted to working strength yet, is wine or woodworking preservation systems that rely on simple debubbling of a flexible container. Wine boxes are inexpensive, include multiple bladders in each purchase, and can be had in various sizes; it's easy to dispense small amounts of chemical without introducing any air. Not very helpful for a working solution that you'll reuse a few times or replenish, but very good for that 5L of Xtol that you'll be drawing from for the next six months...

I am about to try XTOL replenishment in a nitrogen burst system here shortly. I will mix up a second 5L batch of XTOL for replenishment and store it in an old 5L Kodak Carboy with a dispensing tube. I used this type of storage when I was processing E6 and using Kodak E6 chemicals. I am hoping that Kodak is going to make the 5L kits available in the near future as I have about 30 rolls plus some sheet film waiting to be processed. I will probably go the Butane route based on the information that has been provided in this thread. I will use up my wine preserver first as it doesn't seem to get used on any wine bottles that we open.