So I want to have an alien creature on a planet with a halogen atmosphere. It will have to be at a temperature where the hydrogen halide is liquid or solid.
This hydrogen halide I would have as the main useable component of the creature's blood as a substitute for water along with some halogen gas carried in the blood cells as a substitute for oxygen. So now I need to figure out which hydrogen halide, in other words which acid to have in the creature's blood.
A body temperature of 37°C completely rules out hydroflouric acid. Plus, flourine is very reactive, in fact it is the most reactive of all the halogens. So every molecule would end up flourinated and the creature would die just from the flourine not letting go, not to mention that hydrogen flouride would boil off and make the entire creature swell up and burst. The only thing that doesn't react with flourine or hydrogen flouride that I know of is teflon.
Highest survived fever for a human is 46°C, just 2 degrees below the boiling point of concentrated hydrochloric acid. Hydrochloric acid though is the most common hydrogen halide and is known to be in living organisms as stomach acid. Chlorine is the most common halogen as well so this would be the most practical. A heavy gas that absorbs UV light and forms a strong acid is a pretty high contender as far as acidic blood is concerned. And it is pretty easy to form a buffer solution to prevent tissue from being corroded by hydrochloric acid by just producing a lot of sodium bicarbonate. But the fact that the boiling point is just 2 degrees above the highest survived fever in humans makes it not ideal in that aspect.
Hydrobromic acid and hydroiodic acid both have boiling points above that of water. But bromine would be a fuming liquid at body temperature and iodine would be a sublimating solid. No chance of getting a significant amount out with each breath so the lungs(or whatever breathing mechanism the creatures use) would just get flooded with liquid bromine or iodine powder and the creature would asphyxiate and die.
This really only leaves me with the option of hydrochloric acid. And I have been told to find out if any organic molecules would be resistant or not react with hydrochloric acid and the only ones I can think of are those already saturated with chlorine or flourine. Because of the lack of hydrogens it wouldn't be acidic in the same way hydrochloric acid is acidic. But it wouldn't be alkaline either. So it wouldn't protect tissues from being corroded unless it was like in a mucus or other viscous substance. And mucus if there is too much of it, can make you susceptible to infections. Also it would probably lead to the death of the creature because the chlorine and the acid would go through it way too slowly.
So now I'm kind of stuck, mucus isn't the ideal protectant in terms of the speed the chlorine and the acid will get into the cells where they are needed, sodium bicarbonate is basic so the blood wouldn't be acidic, other hydrogen halides aren't really an option for reasons already described, and hydrochloric acid boils at 48°C.
So how can I make this hydrogen halide idea work besides decreasing the creature's body temperature?