Posted by: Green Knight | January 18, 2011

Strawberry Letter 23

During my research into methyl bromide and methyl iodide, another chemical name kept coming up that nagged at my hindbrain, but since I was specifically asked to focus on methyl iodide, I ignored it for the time being.  It eventually it hit me; I’d seen it before in other circumstances, something called chloropicrin.

Then I remembered: Green Cross!  No, not the symbol for the National Safety Council (which is kind of ironic given what it refers to here), but the name of a chemical warfare agent used by the Germans in World War I in 1917, on the Italian front.  There were a few ingredients, and the composition varied, but chloropicrin was one of the main ones, and was known as “vomiting gas.”  The stuff was able to get past the primitive masks being used at the time, and soldiers would have to take them off to throw up, which would expose them to even deadlier constituents of the shell.

So what the heck is this stuff doing in something we fumigate our strawberry fields with?  Well, we actually got a lot of modern products used today as solvents, bug sprays, and many more, from chemical warfare research…after the Geneva Conventions began looking down on battlefield use, other applications were discovered for many of the compounds.

I figured I’d better look into this additive, and find out how bad IT was compared to the ones I’d already looked at, how much was being used and for what purpose, what its environmental behavior is like, etc.  Some very interesting facts came to light.

I already knew, just from the name, that it’d be nasty.   Chloro- means chlorine, which means it’s gonna be toxic, and  -picrin means it’s related to picric acid and the various picrates, meaning it has nitrogen in a reactive form and will be shock-sensitive, and my guesses turned out to be correct.  This stuff is less toxic to our poor lab rats in an oral dose than methyl bromide or methyl iodide, but much more toxic via the inhalation route.  It gets a “4” rating from the NFPA for health effects.  However, it’s much heavier than air, non-volatile, so whence the airborne exposure?  Well, it’s not the gas, due to the non-volatility, but it’s the mist when a shell of Green Cross explodes near your foxhole.  Therefore, not an inhalation hazard for applicators or nearby critters, especially with the use of the plastic soil-cover tarps.

So what are the problems with using chloropicrin?  Mainly that it’s insoluble in, and heavier than water, and is acutely toxic to aquatic life, both plants and animals, although it doesn’t bioaccumulate.  If it gets into a body of water, that pond will not be coming back to life anytime soon.  Chemicals that are heavier than water can be pumped out, but the stuff that gets into the bottom sediment will live there for a long time, doing its evil work.  Chloropicrin degrades relatively rapidly in soil, air, and shallow water, but since most of it sinks, that’s what we must deal with.

Then there’s the reactivity (gets a “3” on the NFPA diamond).  The nitrate component (the “picrin” part of the compound) renders it shock-sensitive in mixtures where chloropicrin is at 45% or more, and the literature shows that in methyl bromide mixtures, CP has been used at anywhere from 1.4% to 99%, and from 2-75% with methyl iodide.  Bulk containers may detonate on severe impact or with enough heat…a highway accident or fire involving a tanker of this stuff could result in a BLEVE (a boiling-liquid expanding-vapor explosion).  Picture the knights in Monty Python and the Holy Grail yelling “run away!”  In some emergency situations, the only thing you can do is evacuate everybody to a safe distance uphill and upwind and wait for the pyrotechnics to finish, then go clean up the aftermath when it cools down enough.  And if an old container of picrate-related material has leaked and dried out, forming yellowish crystals around the cap, even a tap from a fingernail can cause it to explode, about 25 times more vigorously than TNT (firefighters call that a BFK, or “big f***in’ kaboom”).

So, when it comes down to brass tacks, I’m more concerned about chloropicrin than I am about methyl bromide or methyl iodide.  To save wine grapes from phylloxera aphids and associated fungi in the late 1800s, European grapevines were grafted to rootstocks of the American grape…not a permanent solution, as the bugs are developing a resistance in California, but what if we tried something similar with wild strawberries?  Hike on up to Strawberry Valley, CA, in Yuba County, and do some experimentation.  The UC Davis folks could investigate this, as it’s a similar problem, and while I can survive without lovely strawberries, without wine I’d be crying into my empty krater.



  1. I forgot to mention that this stuff, known as PS gas to NATO’s acronym-happy chemical warfare bunch (couldn’t find out what it stood for, try as I might), is not only added for bug-killing properties but also as a “warning agent.” It’s a severe lachrymator in very small amounts, meaning that, like its close relative phosgene, it makes a great tear gas. Added to the mix, the chloropicrin gives you an early warning that you’re being exposed. Kind of like why they add methyl mercaptan to your natural gas (which is odorless). The mercaptan has that rotten-egg smell to let you know your house is about to blow up. When i worked for the State, I’d get calls from people who lived near landfills, saying “I can smell the methane!” I’d politely say “no, you can’t. Methane has no odor. You’re smelling the hydrogen sulfide.” Mercaptan is another sulfur-based compound. Sulfur is also what makes poop smell the way it does. Not my fault, I’m just telling the truth.

    I must revise what I said about applicators being exposed…when mixed with methyl bromide or methyl iodide, the resultant brew is more volatile, so airborne exposure IS a possibility. I may just have to start living on plankton from now on. But spirulina shortcake just doesn’t have the verve there.

  2. Actually, beta methyl indole is one of the major components of “poop” smell. Not to show you up but picric acid is the chemical which is supposed to be very sensitive to shock and explode when dry. My own experience with it would say that is unlikely. I have handled it dry in glass and plastic bottles without explosion. One of the bad ones is diethyl ether peroxide which is said to be extremely shock sensitive when dry as in an old ether container where the ether has evaporated.

    A few years ago there was a great fear reaction about picric acid exploding in many high school and college storerooms and bomb squads were called out for show and tell. I have looked at the shock sensitivity of dry picric acid and found it is not very sensitive. It takes a detonator to set it off in military ordance. In world war I the troops used to open shells and burn picric acid to keep warm. Obviously it has explosive potential but the danger may be over blown.

    I have never seen anything about cholropicrin exploding so I guess that the warnings have to do with the nitrogen content guidlines and not actual explosions. I have not searched this but maybe you can do that.

  3. Yeah, Jim, a friend of mine who did lab-pack cleanups said that lead picrate was far worse than picric acid. I did have to call the bomb squad once, though, back in ’88. I got the detonation info from the DOT and an MSDS.

    C9H9N, 3-methylindole, scatological scatole, that’s interesting now that I’ve read about it, esp. considering it has no sulfur in it. When I was an archaeology major, I read about a guy in Utah, I think, who rehydrated human coprolites from cave dwellings and sniffed them to determine diet. His nose could have gotten him a job in oenology, but he liked his sh!tty job. All I can say is, if you’ve smelled the mudpots at Yellowstone, H2S certainly maintains a presence.

    Ether, oh, YEAH, brother! The Emergency Film Group out of Oakland has some great videos, including “Fun With Chemistry,” showing an ether thermos go POP, and the University of Nottingham has some cool ones as well, if you look up the Periodic Table of Videos on Youtube. And “Discover” magazine’s Joe Genius videos also feature some BFKs. Perhaps picric is not as bad as its rep, but I advise my students to learn lab-pack work from people who still have most of their fingers. Not from a guy nicknamed “Lefty,” unless he really learned his lesson that time.

    And soldiers warming their hands over cracked shells of Green Cross, that’s priceless! Makes me want to watch Bill Murray in “The Razor’s Edge” again.

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