hodges
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Preventing Mold Growth in Organic Solutions
I had about a gram of picric acid that I didn't need so decided to convert it to ammonium picrate. I added dilute ammonium hydroxide solution in
excess. The picric acid dissolved and produced an orange solution. I left this solution exposed to the air so the ammonium picrate would
crystallize. Thin needle-like crystals started forming at the bottom of the container after a few days. However, after about a week, I found some
mold growing on the top of the solution.
I previously had a similar problem when evaporating a sodium acetate solution. In that case, I decided to go ahead and dry in the oven to avoid the
problem. But in this case, drying in the oven might not be such a wise idea.
I removed the mold - how can I prevent it from recurring? For now I added a couple drops of 5% NaOCl solution.
Hodges
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DJF90
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Generally use deionised or distilled water, and make sure you cover your beakers of solutions with a watch glass or filter paper - this generally
keeps stuff out.
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Picric-A
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deionised water still contains bacteria and other living matter which could lead to mould growth so you should stick to distilled water or even
simpler boil the water before use and yes covering them with a watch glass helps.
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kclo4
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It is amazing it can grow, surely it is able to live off of the impurities since I can't imagine an organism that can survive purely off of ammonium
picrate or just sodium acetate. I'd imagine it would need to get calcium, magnesium, etc from somewhere.
So it is probably also important to keep your solutions as pure as possible if you want to avoid this, which includes using distilled water, but other
factors would play a roll in it as well.
Alternatively maybe you could store it in some place where it is exposed to significant amounts of UV light? Sounds like a PITA, but its one method.
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Picric-A
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Well how pure was the sodium acetate? if it was made from vinegar it probably contained plenty of organic particles and bacteria...
Most organic compounds are damaged by UV so i would not advise this. alternativly buy a UVC lamp and treat your water with this to remove bacteria.
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kclo4
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Good point about the UV damaging the compounds.
I don't really think it is about removing bacteria, or killing them it is about preventing them from growing. In order to keep the bacteria out you'd
need to seal it and pressure cook it or something like that. This would prevent it from evaporating unless you, after boiling it, put a clean tyvek
sheet over it, which would still signficantly slow down evaporation.
If you want to get into all that stuff I'd say look over at the shroomery on how to make and keep sterile solutions. They have DIYs on everything from
pressure cooking a mason bottle to making a laminar flow hood from what I remember.
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hodges
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I'm sure there are impurities present in the solution. Probably a trace is all that is needed (in addition to the carbon) for the mold to grow.
Certainly being exposed to the air it is going to pick up mold spores and bacteria. It just seems pretty amazing that microorganisms are adapted to
decompose such unnaturally occurring substances as picrates. Of course, I've read that some roaches have been known live on household plastics so I
guess this is not any more unusual.
Hodges
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panziandi
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The bacteria can utilise acetate as a carbon source and assimilate it into various organic compounds. In fact the minerals required needent be that
much to permit bacterial, fungal or protozoan growth. Generally distilled or deionised water is fine most of the time. I have only experienced one
case of growth and that was in a bottle of dilute magnesium sulphate solution. Probably fixing CO2 from the air in a poorly stoppered bottle.
Generally don't leave your bottles of water undone for longer than is required, ratehr keep a larger stock and then dispense into smaller bottle as
needed.
Don't leave solutions uncovered to evaporate, cover with cotton wool in a conical, or filter paper over a beaker. You can make your own drying cabinet
to aid evaporation of small volumes. Boil the solution to reduce it's volume before setting aside to crystalise (this helps concentrate the solution,
getting a more saturated solution as it cools.
I don't know whether they are consuming the picrates, likely the ammonium ions are being uptaken and assimilated, picrate is highly negatively charged
and aromatic, environmental organisms have evolved to expell these from their cells.
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Picric-A
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Deionised water has just had ions removed from normal tap water NOT bacteria, therfore it is not usefull in preventing bacteria growth!
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DJF90
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And how many bottles of deionised water have you opened to find bacteria growing inside? And panziandi mentioned boiling before crystallising...
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Eclectic
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As I recall, NaN3 is a very effective sterilant for organic solutions. 0.02% or 200ppm.
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entropy51
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Quote: Originally posted by hodges  | I removed the mold - how can I prevent it from recurring? For now I added a couple drops of 5% NaOCl solution.
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Don't overdo it with the NaOCl. Picric acid + NaOCl -> chloropicrin. 
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Rich_Insane
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Yea, try not to make chemical weapons at home please.
Bacteria and fungi are extremely resilient. I work with them every day. many can survive mainly off of trace metal impurities. If your ammonium
picrate was prepared from non-ACS reagents, it probably can harbor bacteria and some fungi. Distilled water, extremely pure, will cause osmosis to
destroy cells. Deionized would work, but I prefer ultra-pure water when I want a pure sample. You could also use a UV sterilizer, or even better
dissolve the ammonium picrate in ethanol, which denatures cell walls.
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kclo4
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Oh yeah there is an obvious answer! Just add alcohol. Amazing that wasn't the first post, it is an obvious choice. Alcohol doesn't react with much
either and is easily taken care of by a bit of heating 
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panziandi
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Deionised water is what is used as a standard in labs, including micro labs! These solutions, if required sterile are then autoclaved! You can run
deionised water through a micron filter to remove bacteria. Eclectic is right about sodium azide, it is quite often included in buffers etc to
maintain sterility.
Picric-A I quote myself as saying in my last post "distilled or deionised water is fine most of the time" and indeed I guarantee you that almost every
chemist will routinely employ distilled or deionised water (depending on the cost benefit) and will not employ sterile water, however the water used
is almost always essentially free of bacteria. My deionised water contains less than 1 colony forming unit per mL (that is it contains less than 1
bacterium capable of forming a colony when 1mL of water is plated onto nutrient agar and incubated at 37.5*C for 24hrs).
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monoceros4
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I can tell you a couple things I've tried over time for preserving buffers and starch solutions that, while they may work for a time, don't hold off
decomposition forever. One method I've tried is adding toluene dropwise with shaking until there's a visible excess, then allowing the solution to
stand until the excess dissolves (and it will.) But for your purpose a substantial excess of toluene might be the best idea. It would protect the
surface of the solution while not hindering evaporation and crystallization.
Also I've tried 0.2% salicylic acid for buffers, but a phosphate buffer solution I prepared using salicylic acid still spoiled eventually after some
weeks. I've also found that even substantial additions of denatured alcohol don't necessary guarantee against decomposition, at least with starch;
nor did near-saturation with NaCl. Toluene was more effective.
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entropy51
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Two things used in the olden days to preserve buffers and so forth were CHCl3 or thymol. Standard solutions of dextrose were made up using saturated
benzoic acid instead of water. For what it's worth.
Ethanol doesn't kill spores, as in mold spores.
[Edited on 30-8-2009 by entropy51]
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Rich_Insane
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Think of this: Yeast produce ethanol from carbohydrates: they will not die. Unless you add very very pure ethanol, they will die, and spores will
still remain.
Someone I knew told me that some labs ad a very small drop of CHCl3 to kill all bacteria and mold growing in solvents and reagents. Most chlorinated
solvents unleash hell upon fungi and single-cellular organisms.
In our lab (micro lab) we sterilize everything. If we want to clean our PCR hood and our Inoculating hood, we do an ethanol wash and sterilize the
hood with far UV light. I haven't had a single contamination of my plates or broth since I started working in the lab! Just be careful. Some
gram-positive bacteria and molds will form very resistant spores. I assume you're not going to experience anthrax around your lab, but be aware that
some fungi and bacteria have adapted very well.
dH2O will pop cells via osmosis, so this is why we use tap water when we want to create live cultures that do not grow. In our fuel cells, we prepare
a media with salts so they do not pop.
Water can help, but perhaps a small amount of CHCl3 may be better.
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hodges
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Here is how my ammonium picrate crystals turned out.
These crystals burn fairly intensely, but nothing compared to a primary explosive such as mercury fulminate or TATP.
Hodges
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Jor
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I have heard, but I'm not sure, that HgCl2 is highly effective even at low concentrations. If used in trace amounts, the Hg wouldn't be of much
concern. But I'm not sure about the effective concentration.
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