Nitrogen asphyxiation should be effective and painless, but not if the person fights it. Which means it'll be hard to use as an execution method (unless you surprise them with it somehow)
Hell, regular suffocation isn't that bad if you stay calm and relaxed and accept it. It's all about the state of mind, and someone being murdered isn't exactly going to be meditative about it.
My understanding is that we can't detect oxygen deprivation, but we can detect CO² buildup which is the idea behind nitrogen asphyxiation. Wouldn't regular suffocation (like, something obstructing your airflow) be quite agonizing then in comparison due to CO² buildup?
Complete tangent, but I don't think superscript numbers are a good choice in chemical notation. You're trying to use them here to stand-in for subscript numbers, but superscript numbers have a meaning in chemistry as well. They denote ions.
So, I think CO2 is more accurate than CO², since it could be confused as carbon monoxide with some sort of ion of charge 2 (unclear of positive or negative).
It's certainly not pleasant, but not outright painful. The bad part of it is you feel like you should be panicking and struggling and fighting to breathe. If you're rebreathing your air, so you're not prevented from inhaling, and you take long, slow, breaths and keep calm and relaxed, it's not nearly as unpleasant. I had a near suicide attempt where I gave it a try to see how bad it would actually be and it was scarily not as bad as I thought it would be. Granted I didn't take it to unconsciousness or I wouldn't be here to talk about it, but I got to the point my vision was getting fuzzy so I think it's far enough to draw this conclusion.
I want to warn anyone thinking of trying this: don't.
Obviously there's the don't commit suicide part, and that's the most important part. But also, as someone who has unfortunately spent time considering various methods, I can tell you: don't even consider doing it this way.
Genuinely sorry to be contradictive, but you absolutely would have been in a painful situation if you'd continued. The only explanation is that you didn't get to the point that your body 100% takes over from you and forces a desperate, painful, writhing attempt to get air.
You would die of increased CO2 concentration in your blood long before you actually ran out of oxygen. That increased CO2 would be very painful. Like, lizard brain stem absolutely taking over, full panicking levels of painful. Don't try it!
You DON'T GET the CO2 concentration in proper nitrogen suffocation. Most people that are in oxygen-free environments don't even realize that it's happened; they get lightheaded, short of breathe, then black out and die. Any gas that's heavier than air and isn't CO2 can cause that kind of suffocation in an enclosed space, which is why SCBA equipment gets used in a lot of industrial applications. For instance, welding in enclosed spaces? The argon or nitrogen can easily kill you before you realize you're in any danger.
If you, for instance, connected your CPAP mask to 50# nitrogen tank, and cracked it open, you'd be breathing in straight nitrogen, and exhaling CO2. There would be zero CO2 buildup. As a result, zero panic. The problem is when you try to displace oxygen, but not CO2; you need to be displacing both so that you're not rebreathing your own CO2.
You could give them a sedative. They pass out and can't hold their breath, then you administer nitrogen. You could probably even find an acceptable oral medication so you wouldn't require a doctor to administer it. I'm in no way saying this is acceptable, but it isn't that difficult.
Put them in a small sealed room with airflow you control. Using a randomly generated number to pick the start time (within some maximum that's deemed appropriate), you then throttle up the nitrogen and throttle down the oxygen.
The time spent in the room would still be pretty terrifying so I doubt it'd be much better than the mask... Maybe the room is their cell and it's a randomly chosen day within an execution "week" where you make sure they're asleep first?
Probably best just to not kill people of course, but if you're gonna do it, do it the most humane way possible.
Tbh I would love nothing more than for my friends to throw me a surprise euthanasia party. Reminiscing, having a good time, then they suffocate me or something.