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.