Heptylene
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O-ring for dessicator ground joint
I finally got a 200 mm dessicator but I find it annoying to use grease on the large ground joint between the lid and base. The grease tends to trap
dirt which then damages the flat surfaces. It's also annoying to open, you have to slide it slowly to the side.
Does anyone use an o-ring on their dessicator? I found large o-rings on ebay, but I'm wondering if it would be as airtight as grease. Perhaps a rubber
gasket cut to shape could work instead , or several concentric o-rings?
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Texium
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Unfortunately some desiccators are just made to be greased. You can post a picture for clarity, but if it has a flat, ground surface as you described,
an O-ring isn’t gonna cut it. There is a different “dry-seal” design that has a groove around the rim for a large, loose-fitting O-ring to fit
in. These work well, but only fully seal under vacuum.
The “annoyances” that you describe are honestly really minor. If you want to effectively use your desiccator, you just have to get used to them
and, uh, don’t get dirt on your grease. It shouldn’t be that hard.
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Tsjerk
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I often saw people use way too much grease, only a really thin layer is needed. Too much grease traps dirt, usually when you wipe off as much grease
as you can with some paper it is already enough grease.
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Dr.Bob
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I second Textium, O-rings won't work except on ones made for them, and under vacuum. Use a clean razor blade to scrap off the grease and dirt, then
put a minimal amount of grease on. I have used these for years, and they stay dry. The ones with doors, gaskets, and other tricks all leak like a
sieve in real life. If you are using something a lot, then using a small container or desiccator for it makes sense, but large glass ones are best
for long term storage, and work best under vacuum, with a container of drying agent in a container in the bottom.
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Heptylene
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It's what I feared, it won't seal well with an o-ring. I might be putting too much grease, I'll try with less. I know this is only a minor annoyance,
but an upgrade as simple as an o-ring seemed worth considering. Thanks for the input!
Oh Dr. Bob about the vacuum in the dessicator, I always assumed it was required to facilitate diffusion of water from the surface of the liquid to the
dessicant. Is this correct or is there something else at play here?
This must also mean that to operate under vacuum, you want a very strong dessicant that doesn't dehydrate under vacuum correct? I'm thinking CaO,
P2O5, are suitable, but silica gel not so much.
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Mateo_swe
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Isnt Magnesium sulfate (epsom salt) that has been oven dried at over 200°C for about an hour usable in the desiccator if using vacuum?
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Dr.Bob
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Vacuum use depends on what you are using the desiccator for. If you have a wet compound and are trying to dry it, (like material crystallized from
water or a hydrate salt) then you will want to use almost any drying agent (or none) and keep the desiccator connected to the vacuum (with a good cold
trap to protect the vacuum from the water/solvents). I have seen well over 100 ml of water removed from solids that way. But this will often
cause water to condense on the inside of the desiccator until the material is completely dry. Once that is done, I often repeat the process with
fresh drying agent and break up the material to allow more surface area. The vacuum simply helps to lower the bp of water and increase its vapor
pressure. But much of the water goes away to the pump and trap, so the drying agent is mostly to trap vapor before it condenses on the glass in my
opinion.
If, instead, you are trying to store water sensitive material for a long time, the drying agent should be stable to vacuum and not able to outgas
(otherwise hard to maintain a vacuum - eg, sieves and carbonates are bad for this, they hold/generate gas well), and the desiccator can be put under
vacuum for a short time, the valve closed, and then the desiccator can be stored for months without the need for a constant vacuum.
If you don't have a vacuum, the desiccator can still work, you just need to slide the lid on so that you don't build up pressure inside the
desiccator. If you just sit the lid on it, you can create a pressure inside it, that keeps the lid from sealing. If you use a drying agent that
absorbs a little CO2, water vapor and/or oxygen, it helps to create a slight vacuum and hold the lid tight. Same for putting a hot drying agent into
the desiccator, as it cools, it will create a slight vacuum as well.
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