[EMAIL PROTECTED] spaketh thusly: >The active ingredient in bleach is not gaseous chlorine but >hypochlorite ion, which is not volatile.
If hypochlorite in the form of bleach is not volatile, there would be no reason to cap the bottle, and one would not smell chlorine from an open container. Hypochlorite reverts to diatomic chlorine when permitted, reducing the potency of bleach solutions. Bleach is created through a process in which the chlorine is retained in solution by its very high pH; Clorox used to have a very nice web page on the subject, but it seems to have disappeared. In any event, lye (sodium hydroxide) is dissolved in water to produce a solution with a very high pH. The high pH allows chlorine gas which is bubbled up through it to be retained. The chlorine is held in the form of the hypochlorite ion, which is surprisingly ineffective at disinfection. More important is hypochlorous acid, which has ~80x the strength of the hypochlorite ion to deactivate biologicals. Forming hypochlorous acid: Cl2 + H2O -->HOCl + HCl (low pH) Forming bleach solution: Cl2 + H2O + 2Na+ + OH- --> 2Na+ + 2OCl- (high pH, equation not balanced) But- if one were to drop the pH of the second solution (with the hypochlorite ion, the OCl-), it goes a little something like this: Equilibrium between hypochlorite and hypochlorous acid: OCl- + H+ <--> HOCl (hypochlorous acid) While the hypochlorous acid one is marked as reversible, they're all pretty much reversible, except when gas is released. Plus, it's early in the morning, and I'm caffeine-deprived. Someone will check me on these equations, I trust. However, this last one is very dangerous: too much acid, and the gas chlorine comes out a lot faster than it does when in alkaline solution (bleach). Chlorine is very toxic, of course, in elemental form to us air-breathers. >Accordingly, bleach needs no confinement to work properly. >It destroys the amino acid tyrosine, a constitute of all >complex proteins, on immediate contact; the rest of the >organic matter follows close behind. Perhaps it seems too >easy to accept without performing traditional orchid >rituals, but bleach is really that effective and can be >used with confidence to clean pots and sterilize cutting >edges. While this may be true under certain conditions, field tests have demonstrated that bleach does not serve to deactivate plant viruses all the time. If memory serves, one of the oldest tests was at Epcot, where tomato plants were tested with bleach-treated cutting edges, and infectivity remained at the 20-30% level with relatively brief exposure (a few minutes). The problem is that cutting edges are not perfectly smooth, and a thin layer of plant sap may protect viruses from deactivation. Scrubbing in conjunction with soaking is required for the best results. It is not at all clear that chlorine is the effective component in bleach for disinfection of tools in any event; trisodium phosphate works as well or better, even with no chlorine in it. The reason is fairly simple: the very high pH of a saturated TSP solution is sufficient to denature viruses. In a similar manner, the very high pH of the bleach solution- from the lye used to hold the chlorine there in the first place- is probably responsible for most of the activity. Virkon S, a chlorine-based disinfectant that does *not* have a high pH, failed to deactivate pepper mild mottle virus (PMMV) in fresh and dried leaf and root material at exposure times of less than 24 hours; even at 24 hours, some material retained infectivity. Similarly, blades treated with Virkon S retained infectivity at a low level at 1, 5, and (in one case) 10 minutes of exposure. These experiments were run with large sample sizes (in the hundreds and even thousands of samples). >I cannot supply a time table of the rapidity of destruction >of every chemical entity, but it does not take much to >destroy functional protoplasm. Twenty-four seconds should >suffice, 24 hours gives one a safety margin. I would >sooner be suspect of the oven than an exposure to good >strong bleach. Bert Pressman The Plant Viruses Online index lists several viruses that have been recorded in refereed literature as infecting orchids. Some data are supplied as follows. Turnip mosaic potyvirus (known from Calanthe): thermal inactivation point (TIP) of 62 degrees C. http://image.fs.uidaho.edu/vide/descr855.htm Cymbidium mosaic potexvirus (from Cattleya): TIP of "60 to 70" degrees C. http://image.fs.uidaho.edu/vide/descr274.htm Orchid fleck(?) rhabdovirus (from Cymbidium and many more): longevity of the sap in vitro (LIV) of "1 days (at 6 degrees C)". http://image.fs.uidaho.edu/vide/descr541.htm Cymbidium ringspot tombusvirus (Cymbidium and others): TIP of "85 to 90" degrees C. http://image.fs.uidaho.edu/vide/descr275.htm Odontoglossum ringspot tobamovirus (many genera): TIP of 90 degrees C. This was once known as tobacco mosaic virus - orchid strain. http://image.fs.uidaho.edu/vide/descr531.htm Habenaria mosaic (?) potyvirus (from Habenaria radiata): TIP of 65 C. http://image.fs.uidaho.edu/vide/descr383.htm Vanilla necrosis potyvirus (from Vanilla fragrans): TIP of 58-60 C. http://image.fs.uidaho.edu/vide/descr865.htm In short, the refereed literature on plant viruses affecting orchids indicates that the highest TIP (10 minute exposure, for those that were wondering) is 90 degrees. If anyone has any data that contradict this, I'd be interested in hearing about it. People continue to claim the efficacy of solution disinfection for prevention of spread of plant viruses, but there are few hard data to substantiate these claims. The claims for Physan and other quats, for example, are based on a single, non-refereed study in a small number of cucumber seedlings with cucumber mosaic virus done >35 years ago and apparently never followed up. Such tests are expensive, tedious, and threaten to undermine conventional wisdom. The only way to reliably, repeatedly, and consistently deactivate viruses in this context (plants, sap, dirt, and pot scale) is with heat. Failing that, TSP and bleach will do a satisfactory job when done correctly- but few people are sufficiently patient to allow a sufficiently long duration of exposure before cutting into the next plant when disinfecting tools, or to scrub the blade in solution to facilitate exposure. Of course, there are plenty of growers with far more awards (cultural and other) than I'll ever have that don't pay nearly the same attention to disinfection that I do, and grow far better plants than I could ever hope to. I forget if it was here or r.g.o. that Dr. Neptune (a retired cardiac surgeon that one might safely suppose knows more sterile technique than I do) noted his absence of concern for extreme measures with regards to controlling viruses, and he continues to rack up AOS CCMs the same way I collect scorpions around the house (6 in the past two months for me- I can't speak for Wilford on CCMs). So, I'll cheerfully say that it's all largely theoretical. For those that are interested, I don't reuse blades; I buy new sterile scalpels on "eBay." As for pots, virtually all of my orchids are minis; the clay pots go into the large pressure cooker I normally use for orchid media, and they get autoclaved. Plastic pots get thrown into the back yard and exposed to the elements- sunlight kills all, including pots. Eventually they embrittle, so I throw them away and buy new ones rather than reuse 'em. :-) Cheers, -AJHicks Chandler, AZ _______________________________________________ the OrchidGuide Digest (OGD) [EMAIL PROTECTED] http://lists.orchidguide.com/mailman/listinfo/orchids
