mr.crow
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Why do chemicals cake together?
Here is a n00b question. Most of my reagents have caked together inside their bottles. I have to give it a good shake to get it back to a
powder/crystals and there is still some indestructable lumps.
Why does it do this?
Double, double toil and trouble; Fire burn, and caldron bubble
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Picric-A
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due to moisture. This can be resolved by using air tight bottles for your reagents.
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mr.crow
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Moisture is mildly annoying
Double, double toil and trouble; Fire burn, and caldron bubble
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Hoveland
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A better question would be, why is it that after breaking a crystal of sodium chloride, which fractures cleanly along the crystal plain, the two
pieces cannot simply stick back together?
I thought perhaps dust or even molecules of gas fill in the gap and prevent close enough contact for ionic forces to do their work. I do not think
exact alignment would be too much of an issue.
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hissingnoise
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It's doable - keep trying. . .
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peach
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Yep, if you dry your materials to an anhydrous form, they won't cake.
The seals on bottles are, bluntly, shite.
I have gotten bottles of anhydrous salts from big name chemical suppliers, sealed them in a ziplock bag without opening the tops, and come back to
find the label rotting off, indicating the seal is leaking. Or I've had bottles of iodine turn their labels black (or extremely purple if you want to
get technical).
There is a very simple and very cheap way of solving this (tens of seconds per bottle, tens of pence for tens or hundreds of bottles). Buy a roll of
PTFE tape and wrap the threads of the bottle. It will then be next to impervious to both water ingression and chemical attack. Wrap in the direction
that the thread does up, so the tape doesn't unwrap it's self. Gas fitter rated PTFE tape is thicker, and less likely to tear. It comes in a yellow
pack in the UK and is a few tens of pence more expensive. I can also buy liquid PTFE for thread sealing from hardware and tool stores. I'm not sure if
it forms a solid layer. If it does, that'd work even better than the tape (no unraveling or tearing).
Caking is a major issue for the food producers, who'll often blow the products through transfer pipes between stations, which allows it to clog and so
on.
One of the reasons it does it in the pipes is static, which can also cause it to explode if it's a fine, combustion supporting powder (like flour).
Another is moisture.
They also don't want the product caking in the packing as it sits in storage or on the shelf. An example of this is icing sugar (frosting if you're in
the US). It will form a solid lump if left in a cardboard pack. They purposefully add moisture absorbers, called anti-caking agents, to draw the
moisture away from the cake problem. This caused my own major issues when I attempted to use icing sugar to dust my homemade Turkish Delight (which is
actually from what is now called Iran, or Persian if you hate the idea of 'rag-heads' creating something good), as the agent would continually suck
all the moisture out of the gel, turning the sugar to slush. I spent a long time tweaking the recipe it's self to try and stop it, adding a heap of
gelatin or cooking the sweeties gel for six hours or more. Even with a kilo of icing sugar over it, slush. So, if SWIM's pet racoon is cooking his
recipe for this sweety, make your own icing sugar in a high speed coffee blender (works great).
In terms of crystals 'sticking back together', that can be done.
If you read into Gravity Probe B, you'll discover the entire structure of the analytical section is made from high purity Quartz, to keep it
dimensionally stable. There are no bolts, screws or glue involved, the components stick together through the molecular level attractions.
The probe is designed to look for space-time frame shift. Einstein's theorizing said space could be viewed as a sheet, which dips when mass is
present, like a marble on a sheet of silk held at it's edges. This creates 'wells' that things fall down, as they move closer to the source of the
gravitational field. If that is correct, he also thought it would be true of time.
GP-B watches a specific, stable position star through a telescope and uses it as it's fixed point of reference. It orbits earth and is equipped with
ultra-high precision gyroscopes, containing high speed rotating spheres that are ground to within about 40 atoms deviation from a perfect sphere.
Superconducting Quantum Interference Detectors detect variations in the gyroscopes spin by sensing changes in the spin of the spheres that are next to
the absolute limits of what is physically possible. It requires this level of precision as the effect of gravity is pathetically weak by comparison
with the other forces. If the gyroscopes fall out of alignment, the space-time frames the probe is passing through are being warped by the mass and
rotation of Earth; like pinching the sheet of silk, pressing down on it and then turning your hand, bowing and twisting the fabric around that point.
[Edited on 4-8-2010 by peach]
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mr.crow
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I love your posts peach.
I keep my Iodine in a glass test tube with a clear plastic screw cap. The cap turned brown instantly so I glued a penny on top. The threads haven't
been discolored at all after years. The cap has a plastic ring type thing inside.
I tired using NoSalt as a source of KCl and the anti caking agents were the most horrible sticky slime.
I will give the teflon gas tape a shot. Always use the teflon paste for pipes, tape sucks.
Double, double toil and trouble; Fire burn, and caldron bubble
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peach
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Why thank yars very much!
As I say, I've never used the liquidy form of it, so I don't know how gooy it is. I'd be worried about it moving with regards to pressure differences.
Anhydrous material will pull moisture from the atmosphere through the tapers of normal bottles. Then it'll react with it and release things like
HCl(g), which will try to get back through the taper. If the liquid PTFE isn't thick enough, the differences may be enough that moisture will
eventually push it's way through and the resulting gases will push their way back out. The tape is solid, so it's not going anywhere between undoing
the cap (which can easily end up with it trying to undo it's self, yes).
Another 'trick' to using PTFE tape is to pinch it before screwing things together. Hold the screwthread in your fingers, grip it and turn it in the
tightening direction, which will force it into all the tiny gaps and encourage it to sit still.
If you liked that, you might also like this discussion of how Aldrich protection atmosphere sensitive compounds and how to handle them.
As I keep going on about, note that the labels on the container are the same of the ones on the solvents I can hold a burning lighter to and not
ignite. Yet these will ignite or explode on contact with the atmosphere. Which is why I also keep going on about NFPA 704, the 'fire diamond' hazard
system.
[Edited on 5-8-2010 by peach]
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entropy51
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The use of PFTE tape is hardly novel, having been described in a multitude of
threads on this very forum. No need to take a bow for the audience.
Save the pirate affectations for a forum that might appreciate them.
[Edited on 5-8-2010 by entropy51]
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mr.crow
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Avast ye scurvy sea wenches!
So the sodium bottle had only one 'row' of threads and some cheesy plastic insert, no wonder. I don't know how much good the teflon tape will do
Double, double toil and trouble; Fire burn, and caldron bubble
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peach
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Quote: Originally posted by entropy51  | The use of PFTE tape is hardly novel, having been described in a multitude of
threads on this very forum. No need to take a bow for the audience.
Save the pirate affectations for a forum that might appreciate them.
[Edited on 5-8-2010 by entropy51] |
Here we go, again.
Where did I say that was a novel trick? I didn't claim to have invented the idea. I even put the word 'trick' in quote marks to illustrate it's not a
trick. He asked about things caking, people replied saying it was moisture, I replied telling him how to keep things dry.
Reference the dry ice nozzle reply I made just now, the section in particular regarding not being able to please everyone, and then mr.crows replies.
Then compare them to your own and have a think.
You seem to have taken a remarkable dislike to me for whatever reason.
I can't say that makes me happy. I can say I'm reporting the post because I'm now tired of the personal mission you seem to have that means posting
multiple complaints whilst adding absolutely zero to the threads.
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mr.crow
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Ok that didn't help, my sodium is as bubbly as ever.
Now I put metal tape on top of the lid, put electrical tape around the threads and cap and sealed the whole thing in a ziplock bag. Should throw one
of those silica gel packs inside too.
Double, double toil and trouble; Fire burn, and caldron bubble
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peach
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| Quote: Originally posted by mr.crow | Ok that didn't help, my sodium is as bubbly as ever.
Now I put metal tape on top of the lid, put electrical tape around the threads and cap and sealed the whole thing in a ziplock bag. Should throw one
of those silica gel packs inside too. |
Did you wrap it up really well? It can take a lot of wrapping to get a good seal; one or two wraps won't be enough because the thread is so coarse and
the tolerances are so poor (compared to a machine pipe thread). You can wrap until the threads are all rounded off and disappearing, it'll still
thread on and it'll seal better with more there.
If the sample may react with the seat in the cap, you can wrap over the top of the bottle as well, which plumbers will often do by mistake and have to
poke out of the flow path. Or, take the seat out and wrap it separately.
Also, if you don't purge the atmosphere within the bottle with dry gas, it'll still react with what's left in the bottle. Which is why choosing a
bottle that's about the same size as the sample is a good idea.
If silica gel isn't enough, you could add a little solid NaOH or KOH to a bigger container and store the more sensitive one within that. Or mock up a
drying tube with a bit of tubing or pipe siliconed to the container or cap. The base doesn't have to be in mechanical contact with the sensitive
bottle, so long as the atmosphere around it can contact the base, it'll work. You could simply drop a little bottle of it in there with a hole drilled
in the cap, so you don't end up with slippery, annoying base all over the sensitive bottle.
I'd go with prevention over cure. Wrap until the thread is at it's limits of being screw-on-able. If it's still getting through that, I'd be
impressed. The failure of PTFE tape and you having to move to electrical tape will be the thickness of the wrap. PTFE is, of coarse, far less likely
to degrade over time and will produce a very tight seal, provided it's wrapped thick enough. This is why I suggested gas fitter PTFE tape, as it's
specified for thick, gas tight wraps; the tape it's self is physically thicker.
If you're a hands on type DIY guy and used to using PTFE to seal water pipes (where one or two wraps will work fine 'forever'), ignore a lot of that,
keep wrapping. The bottle threads are piss poor by comparison.
[Edited on 8-8-2010 by peach]
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mr.crow
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Thanks for the advice!
I think the bottle just sucks, if the tape doesn't work I am going to get one with a better lid.
Double, double toil and trouble; Fire burn, and caldron bubble
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condennnsa
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I don't think moisture is the only reason chemicals cake together. I have samples that were dried to 300 C +, kept in a airtight jar, which also
caked.
I think it's sound which keeps moving the little particles of a fine powder so that they end up interlocking a bit. I haven't tried storing something
in a soundproof place to see if it's true.
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peach
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It's not the only issue no, they will settle as you described. In fact, finely ground anhydrous powders are even better at doing so than big lumpy,
damp stuff.
Airtight is a spectrum for a lot of seals. As has been coming up thus far, others and I have been mentioning how bottles that should be sealed,
aren't; even when they're anhydrous material direct from brand name chemical suppliers.
If the threads aren't taped to buggery and the bottle isn't stored somewhere dry, you may still get enough moisture to cake the contents. Which,
again, will happen even worse for finely ground powders as they're already in such close contact. In terms of moisture, choosing a bottle that you can
almost fill is also a good idea, as the atmosphere remaining in the bottle will be damp as well if you don't purge it with some dry gas; the less dead
space there is, the less damp atmosphere will be present.
Laser inkjet printer refills are super fine carbon dust essentially. Before using them, the suppliers recommend violently shaking the bottle for a
minute before hand. That gets it pouring like a liquid again, due to the 'liquid powder bath' effect; the same thing that makes you sink in sinking
sand (something less dense between the particles, e.g. water / air). The powder also electrostatically charges, repelling the particles from each
other. People who powder coat item for baking sometimes use powder baths, I think. There's compressed air blowing up through a big bin of the powder
and they simply dunk the item to coat it. There, they'd charge the bin to one polarity and the item to the other (to suck the powder onto the
surface). The same effect can sink an oil tanker provided enough air rises around the hull.
As I say, when Aldrich want a sample to be oxygen / moisture free, they blanket it with dry, inert gas and crimp a septum seal to the top (which they
call 'sure seal'). That's about the only realistic way to still get access to the material but provide it with the highest level of protection;
dissolve it in something, poke a none coring syringe in.
[Edited on 20-8-2010 by peach]
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