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”
I’ll say.
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”.
I should also add that in most labs where perchlorates are used, there is a bucket of water (or dilute baking soda solution) into which kim-wipes used for wiping up drops are placed. While soggy tissues are unsightly, they are also less likely to burst into flame.
ReplyDeletePerhaps we can now finally solve the mystery of what happened to the Beagle 2 lander - it hit a patch of perchlorate!
ReplyDeleteMore seriously, could it be that the perchlorate salts found on Mars are relatively unreactive and/or insoluble?
Perchlorates in general are more stable wet than dry- keep them wet is safety generalization number one. And organics aren't an issue because it is Mars. Aside from that, though, I have no idea. The only predictable thing about planetary science is that discoveries will be outside the frame of human imagination. Just like Lunar anorthosites, Io volcanoes, Triton's gysers, and hot extrasolar Jupiters, this falls into that category.
ReplyDeleteVery interesting post, as usual. I wouldn't describe perchlorate as "stable" either - I wonder about those nutty NASA people sometimes.
ReplyDeleteAh, NASA culture. I remember after the spacewalk where both astronauts were outside the station at the same time... this was controversial and they weren't sure it was going to be safe... and the astronauts almost got blasted by maneuvering rockets, and had to come in early because of a suit malfunction... they got in safely... and a high official of NASA said, "This proves it's safe."
ReplyDeleteI knew NASA was in trouble before then, but that really drove home just how broken NASA is.
Chris
Well, umm... if you're trying to imply that Cl(+7) is a strong oxidizer, I'll have to politely disagree. Strong reducer, yup. The oxidizer is the oxygen. Did you know that oxygen is involved in almost every naturally occuring combustion event on this planet? Oxygen is dangerous! NASA uses whole tanks of it in liquid form, nearly every time they launch a rocket! Crazy jerks! If they thought about safety, they'd use a trebuchet to send probes to Mars. Every single lab I've ever worked in had a fire extinguisher just in case the oxygen got out of hand. OK, no more snark, but come on.
ReplyDeleteI don't remember seeing in any of the reports an estimate of concentrations, which is the real issue. Of course, since NASA's hand was forced by a poorly researched and worded article in Aviation Week that went viral (I actually posted on the issue myself), they're really just guessing: the evidence isn't really conclusive that perchlorate is, in fact, present. They had hoped to make an announcement in a few weeks after they had a chance to check on a variety of issues (possible contamination, did it show up in TEGA if they looked for there, wet lab replication, etc.), but us web'uns made that unfeasable.
In the announcement of the hastily organized press conference the previous day, the statement was made that there were results from Phoenix "...suggesting one of the soil constituents may be perchlorate, a highly oxidizing substance." In the actual press release, in fairness, NASA did state, "It [perchlorate] is an oxidant, that is, it can release oxygen, but it is not a powerful one." That is clearly incorrect, but the statement might have been made in the sense that gastric juices are not strong compared to many acids. Again, the latter is technically incorrect (HCl is a strong acid), but in the stomach, the H+ activity is fairly low compared to many lab situations. Kjeldahl digests are kind of scary to think about until they become routine. Again, context and concentration are everything.
A passage from this article captures in simple language the general gist of the NASA press release. "According to Phoenix scientists, oxidizing chemicals are not always 'bad news' for life. 'It does not preclude life on Mars. In fact it is a potential energy source,' said William Boynton of the University of Arizona. Indeed, perchlorates have been found in Chile's highly arid Atacama Desert, a location often used as an analogue for the Martian landscape." (I have read elsewhere that some bacteria do use perchlorate as an oxygen source, just as other bacteria use sulfate- sulfuric acid is dangerous!)
I haven't read this blog before today. You've got lots of interesting things to say, but I get a little impatient with this kind of overreaction.
Oops. I just thought it through (again) and realized I'd just made the same mistake I've made way too many times... the Cl+7 is the electron acceptor, therefore the reducer. Glad I disagreed politely. Sorry about that.
ReplyDelete