The Mars Phoenix mission has recently discovered signs of perchlorate salts in the Martian soil. So what are perchlorates?
Perchlorates are salts of perchloric acid, HClO4. In the perchlorate ion, Cl has a valence of +7, significantly higher than the -1 valence state in chlorides (like table salt). This makes it a powerful oxidizer. In the lab setting, perchloric acid is a notorious safety hazard, because it is a powerful oxidizer (ammonium perchlorate is used in solid rocket fuel), and because some perchlorate salts are shock sensitive, and will detonate if struck, dropped, or banged.
The main issue with perchlorate use is that perchloric acid needs to be used in specially designed fume hoods. The reason is that the acid fumes will react with metal ductwork and explosive perchlorate salts can accumulate in the ducts. The Oak Ridge National Laboratory describes their perchlorate decontamination program here, and mentioned another incident where
“a maintenance worker on an Atomic Energy Commission-related project was killed and two others were seriously injured in an explosion touched off by routine use of a small ball peen hammer and 6-inch chisel. The workers were dismantling a perchloric acid fume vent system when the explosion--violent enough to be heard 4 miles away--occurred.”
Marwan Bader, the OHS manager responsible for the decontamination, even deserves the quote of the day:
‘The highest concentration of perchlorates found ranged from 140,000 ppm at an elbow in a duct to 800,000 ppm on the inlet side of a filter housing. "Those are very high concentrations," Bader said’
Some of us like to refer to 800,000 ppm as 80%.
The Oak Ridge clean up has been published by Bader here.
The understatement of the week belongs to Michael Hecht of the Mars Phoenix lander mission, who said, “different types of perchlorate salts have interesting properties that may bear on the way things work on Mars”
On the other hand, this part of the press release was downright irresponsible:
“It is an oxidant, that is, it can release oxygen, but it is not a powerful one…. The compounds are quite stable and do not destroy organic material under normal circumstances.”
Compare this to the University of Kentucky OHS site, which warns that
“Perchloric acid is destructive to human tissue as well as very reactive.”
Now, folks at NASA may have a different safety culture than we had in my lab, but I don’t refer to compounds that spontaneously combust or explode due to impact as “stable”.