This articlesays phosgene “persists” for half an hour at temperatures of 21-32 Celsius. Presumably, that isn’t its decomposition temperature, since it remains stable at somewhat high temperatures. So what happens when the half an hour has gone by - does it transform into another chemical?
Half-life in the specific environment. Normally the term “persists” is used in reference to halflifes in ecosystems or living beings.
Depending on the environment and the substance, what happens is different. Example: the persistence of an ingested substance in the body of an animal that can dispose of it by breathing will be relatively short, and what will happen to the half of the initial amount which is not in the body anymore is that it has been breathed out. Your example: phosgene; it will dissipate (if it’s in an open system) and react with any of many susbtances including lots of possible biologicals. From your own first link:
It affects those metabolic pathways by reacting with the chemicals that are part of them.
Ok, but presumably, if one enters a 30-degree-Celsius room with phosgene in it (phosgene vapor that has been sitting there in the air in excess of two hours,) one will still be poisoned, even if the phosgene no longer “persists” since it’s surpassed its “persist date?” That’s what’s confusing me.
Phosgene is one of the most dangerous gases known and it’s poisonous at a very low concentration; the combination of both is what made it so popular in WWI. It’s also relatively heavy, so it tended to concentrate in the trenches.
If you have 1kg in a room, after the persistence period has passed you still have half a kilo. Half a kilo of phosgene is still more than enough to kill a person. That’s what “half-life” means: the time it takes for the initial amount/concetration to be halved, not to disappear completely.
I see. Then what happens when more half-lives have gone by? After two hours, you have 1/4 of a kilo, and then after three hours, you have 1/8 of a kilo, etc. etc.?
Yep. And so on and so on… Now, since a real system will never be closed, there will be events that break that nice exponential, but so long as you have an undisturbed, closed system, and enough of whatever mechanism is getting rid of the phosgene (unlike radiactive materials, phosgene doesn’t get rid of itself), it keeps halving.
Phosgene reacts with water to form carbon dioxide and hydrochloric acid. I’m guessing the reaction with water vapor is the primary decomposition mechanism in air.
Yeah, I´d expect the rate of degradation to depend greatly on humidity. According to this site the half-life of phosgene in the troposphere is estimated to be more like a few weeks…!