RogueRose
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Proper way to use a dessicator
I have a few 12" glass dessicators with the ceramic plate with pores in it. When I got it there was this really nasty grease on the sealing part that
had dirt and gunk in it so I cleaned it all off. I know that this stuff is probably what helps make the seal and also keeps the lid from sliding off.
Does anyone know what this greasy stuff is? I'm guessing it is some kind of silicone based grease. Is it easy to get or are there alternatives?
I ended up using mineral oil and just smeared a very thin coating on the base and the lid and it seems to be sealing well but there is some running
down the inside of the container, very little bit, that will be caught in newspaper that is on top of the ceramic plate.
I'm trying to dry CuSO4 using CaCl2 (anhydrous). I just want the water out of the sulfate, I'm not trying to dehydrate it, just get excess moisture
off the surface.
I ran the vacuum for about 10 mins then turned the knob/hose connector to seal the chamber. IDK if I need to run the vacuum every once in a while or
not. I'm drying about 1kg of sulfate (it was fairly dry to begin with, and I have about 2kg of CaCl2 in the bottom, maybe a little more. Do I need
to use the vacuum every 30 mins, or 2 hours, or 6 hours or whatever?
Would putting the container on a heating pad be of any help (the kind people use on your body)?
Finally what do people use to hold the material to be dried? I figured a plastic plate would keep everything at the bottom from drying so I used 2
ceramic plates with a layer of paper grocery bag (brown kraft paper) in between and then 2 layers of newspaper on top of the top ceramic plate then
the sulfate.
I thought that moisture might be sucked into the paper and it being closest to the bottom, the drying agent would pull the moisture out of it and the
cycle would repeat. It would also keep anything from dripping of falling down into the CaCl2.
Well, this should be interesting to see how it works out in the end but I think it should work but I'm guessing there are much better ways of doing
this.
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Deathunter88
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Come on man, start learning to UTFSE. Sciencemadness isn't a personal question and answer platform. A quick google search:
https://www.belart.com/corporate/instructions/942402021.pdf
https://www.youtube.com/watch?v=Y57KvfGsB6w
When the Wikipedia page on desiccators already answers several of the questions you posted, it really shows how little thought and effort you put into
them.
https://en.wikipedia.org/wiki/Desiccator
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RogueRose
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None of what you listed is different than what I said except for having to use something other than "vacuum grease" - which is why I asked about that.
Then I wanted to know if anyone had ever used something else than a glass/plastic plate to hold the material to be dried. None of that is covered in
anything you posted - nor where I looked online.
finally I was wondering about re applying vacuum after X time because the amount of H2O, if it would fill the chamber as a gas/vapor and if it would
be beneficial to apply vacuum again. Again, none of what you listed covers any of this.
Why would you assume that I didn't search before posting? It is possible that topics can be similar but not cover the issues at hand.
The internet gets more hostile by the day.
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Ubya
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if you apply a vacuum, you are removing all the air (mostly), water will start to evaporate and fill the dessicator. you don't need to apply vacuum
again and again because the pressure in the container now depends on the vapour pressure of water at that tempeature. at 20°C the vapour pressure is
17,5 mmHg, so you can't go lower, but don't worry, the dessicant will remove the water vapour, thus driving out even more water from your sample
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feel free to correct my grammar, or any mistakes i make
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Herr Haber
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It's the same kind of grease you use for labware joints.
So there's quite a range to chose from... High thermal resistance, fluorinated, you name it.
It is my understanding that the ceramic plate at the bottom of the dessicator is made of ceramic precisely so you can put something that you just took
out of the oven.
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RogueRose
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Thank you for the replies. I really need to find some of that grease. The mineral oil works alright for a seal but it is messy and it dripps a little
down the sides b/c the fit is so tight. It is amazing that they can get the two pieces so perfectly flat like that it almost seems like there isn't a
need for any grease or oil.
In the video linked above, they used some kind of plastic netting that was placed around the unit, I guess in case it imploded. There was no top to
the netting just a vertical "wall" that wrapped around the unit. Are these things prone to imploding or exploding? Mine is extremely thick and heavy
probably close to 20-30 lbs and that would be very dangerous if something like that happened. I could't even imagine what would go through my mind if
that happened I was injured, talk about being in shock...
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macckone
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Vacuum desiccators are not prone to imploding in my experience.
Obviously damage could cause a problem.
Vacuum grease is readily available.
Try sciencecompany.com or carolina.com or elementalscientific.net if you are in the US. I don't have any affiliation with any of these.
Yes, a heating pad will work for low heat. I would not try to apply a lot of heat to thick glass.
There are two primary methods of using a desiccator.
1) with a drying agent
2) with continuous vacuum.
Combining the two doesn't seem to have a benefit as the partial pressure of water is going to be determined by the drying agent or by the vacuum if
applied continuously. The partial pressures of air and water are independent for non-boiling situations. Continuous vacuum will remove both water
and air. While intermittent vacuum will result in the two agents achieving a steady state partial pressure which will decrease as the desiccant
absorbs water.
With a drying agent you shouldn't need a vacuum.
If you use vacuum, you should not need a drying agent.
But if you use a vacuum, you will need to keep the pump running for it to have any impact.
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OldNubbins
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I use silicone dielectric grease from the auto parts or home improvement stores.
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Sulaiman
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I agree with the use of automotive silicone grease for glass-glass fittings.
It is attacked by chlorine gas etc.
and can melt out of a ground glass joint at higher temperatures,
but it is good for most everything else,
and as only small quantities are required each time I use it,
a small tube has lasted years.
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wg48
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A sealed evacuated desiccator will dry the item more quickly than than one that was not evacuated as the water vapour does not have to defuse through
the air. Depending on the depth of the vacuum, the size of the desiccator and the item it can reduce the drying time to 25% of the no vacuum drying
time.
If the item is a dead mouse or a block of plastic the vacuum has only minor improvements in drying time as diffusion through the item is the
limiting factor.
PS: covering the desiccant with anything that restricts the flow of water vapour will increase the drying time.
[Edited on 5-10-2018 by wg48]
Borosilicate glass:
Good temperature resistance and good thermal shock resistance but finite.
For normal, standard service typically 200-230°C, for short-term (minutes) service max 400°C
Maximum thermal shock resistance is 160°C
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macckone
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wg98 is correct about diffusion. But I have never found it to be that significant. Transport into the desiccant and out of the substance being dried
have always been limiting factors for my experiments. It is impossible to achieve a non-zero depth and the bottom doesn't interact much without
stirring/scraping
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happyfooddance
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Quote: Originally posted by macckone  | | Transport into the desiccant and out of the substance being dried have always been limiting factors for my experiments. It is impossible to achieve a
non-zero depth and the bottom doesn't interact much without stirring/scraping |
Sometimes I drop a stirbar in with the dessicant and manipulate it with a small stack of magnets that I keep handy when I want to stir/scrape.
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JScott
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I have a few newbie questions. I have owned two desiccators, most recently I broke one. I had assumed, incorrectly, that they could be heated. In my
case I found that was not true.
On to my question. Neither (with proper grease) have held a vacuum for very long. I draw a good vacuum, shut off the petcock on the top of the
desiccator, and when I return a few minutes later and turn the petcock to open the valve I see a small amount of vacuum indicated on the pump (which
slowly drops as the vacuum in the desiccator pulls air through the pump). At that point it seems to have held a vacuum.
However, if I check it after a few hours, I can not see this indication on the pumps gauge nor can I hear the vacuum being released when I pull the
petcock.
Both desiccators are nearly identical. I have used various quantities of grease, I have worked the top on with the twisting motion I've seen
suggested, but I don't seem to hold a vacuum.
Any suggestions, or thoughts on what I might be doing wrong?
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Sulaiman
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I based this comment on my use of silicone grease for ground glass joints,
it recently occurred to me that silicone grease is so slippery
that even a small tilt could cause the lid to slowly slide off.
Maybe that is why the old grease that you removed was so sticky. ?
If you do use a 'slippery' grease then consider adding mechanical retention of the lid on the desiccator.
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macckone
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Jscott: one trick is to apply a slight pressure to the desiccator with the lid weighted and use soapy water to determine the source of the leak. You
can't use a lot of pressure because the lid isn't truly secured. I use an aquarium air pump with a splitter and put one end in a cup of water to get
exactly 8" of h2o pressure.
More often than not, the problem is around the valve. Some of them don't seal well.
Which is fine if you are using a chemical desiccant but not continuous vacuum.
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beerwiz
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If you don't have vacuum grease laying around then you can use vaseline.
Thanks to OP for asking this question. Now I no longer need to keep the vacuum pump running for 24 hours to dry my materials, just put in a
dessicant, pull the vacuum, cut it off and leave it like that until dry.
I understand I can use anhydrous epsom salt for water.
But what if I'm trying to dry DMSO wet compounds?
[Edited on 7-10-2018 by beerwiz]
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