In order to know if it's painful or not, you just have to understand how it works:
I've written a small essay of what that happens in our body when we ingest sodium nitrite. I will do my best to describe the pathology and biochemistry of methemoglobinemia and nitrite poisoning. This is from my own research and I will post the sources from scientific articles and journals...
sanctioned-suicide.net
TLDR it limits oxygen transport and delivery in the body through the formation of methemoglobin.
You'd
start dying with hypoxia because there will be a decrease in oxygen supply to organ tissues with some oxygen still being received, until there is no oxygen anymore and then die from anoxia.
Loss of consciousness is supposed to occur fast enough to barely feel discomfort anyway. You'd probably have time to feel the heart pumping, then pass out, and see the light.
It's not as fast as inert gas, where you'd blackout in a few breaths, but it looks like a peaceful way to go.
Here's a thread where someone wanted to try a small dose and described how it went (with 1,5G):
Needless to remind not to try that; it's poison, so don't fuck with it. Also, the regular dose to CTB being 16 times that, things would probably go faster and/or stronger on d-day.
Hi I tried a small amount of SN tody to make sure it was actually as advertised and pure enough. 09:20 AM | I took 1.5 grams at fasted, but had a few glasses of water. 09:35 | I start to feel my heart beating faster and feeling weak 09:55 | Heart is beating fast and I'm getting tired and...
sanctioned-suicide.net