Sciencemadness Discussion Board

Why don't drying tubes have two joints?

DavidJR - 18-2-2018 at 20:06

Here's something i've never been able to figure out:

Why do drying tubes commonly have only one ground glass joint, with the expectation that you'll use a rubber bung at the other end?

I've seen pictures of ones with two joints but haven't been able to find where I can buy one (24/29).

j_sum1 - 18-2-2018 at 20:23

I asked much the same question recently.
I don't know that we got a complete answer but zed gave some details of the history of the device.

UC235 - 18-2-2018 at 20:29

I only use them to provide an inlet/outlet for pressure without allowing moisture in.

If you need to dry a gas stream, get a gas drying tower or make one from an empty hempel column with a wad of cotton or glass wool at the bottom. Fill the column with dessicant, plug at top if desired.

wg48 - 18-2-2018 at 22:48

Quote: Originally posted by UC235  
I only use them to provide an inlet/outlet for pressure without allowing moisture in.

If you need to dry a gas stream, get a gas drying tower or make one from an empty hempel column with a wad of cotton or glass wool at the bottom. Fill the column with dessicant, plug at top if desired.


0r a Chromatography Reservoir
aaa225.jpg - 6kB

DavidJR - 19-2-2018 at 00:30

Hmm, a chromatography reservoir would be a bit bulky for my liking.

LearnedAmateur - 19-2-2018 at 02:46

The only times I’ve seen them used are during moisture sensitive reflux/distillation where they’re placed at the top of the condenser or the receiving end respectively, hence no need for a second ground joint (or from my personal experience, no joint at all, just a narrow tube to slot into a holed bung). The bung/cotton wool is placed at the end to keep the desiccant in the tube, especially when you have a bent design to save vertical space since a reflux arrangement can be pretty tall.

Like a lot of glassware, having extra bits like a second joint is just to increase versatility - following along the same lines, it would be like a single jointed condenser versus a double jointed one, where the latter can be more easily configured into other QuickFit arrangements and is better suited for a wider variety of applications, especially when you’re dealing with vapours that will corrode rubber. The reason you’re having trouble finding a double jointed drying tubes is because there isn’t really the need or demand for one outside of niche applications, and because other glassware does the job.

IMO, half the fun of glassware is coming up with ingenious applications for pieces that typically have unrelated uses. For example, a Liebig or air condenser, usually with two joints, can be used as a drying tube if packed with a desiccant between cotton or glass wool. Saves the issue of buying a specialised drying tube and you have some extra length if you need to dry a large amount of gas or to ensure that it is absolutely dry.

Sulaiman - 19-2-2018 at 03:16

I have found fractionating columns / air condensers to be very useful;

Fractionating column
- rare and fragile glass spirals are required for loose column packing, but internal operation can be observed

Air condenser
really useful for products with b.p. higher than water
either as the main condenser
or as a pre-condenser to put less thermal stress on a following water/glass condenser.

I've not done any anhydrous chemistry yet but as above, an air condenser would make an excellend air-drier.

OR if you are likely to venture into chromatography
then a column with a glass fritt would also make an excellent air drier - I assume.

LearnedAmateur - 19-2-2018 at 06:11

Quote: Originally posted by Sulaiman  

I've not done any anhydrous chemistry yet but as above, an air condenser would make an excellend air-drier.

OR if you are likely to venture into chromatography
then a column with a glass fritt would also make an excellent air drier - I assume.


Nor have I to be honest, pretty much every single reaction I’ve done involves water to some degree, the one exception being the synthesis of aspirin using acetic anhydride under reflux (obviously water needs to be kept away to prevent hydrolysis but it doesn’t require absolute atmospheric dryness). I think an even better option would be a chromatography column like you said or even an addition funnel, a bonus of this could be that a gas line can be easily slipped over the exit tube and the stopcock can be used to regulate gas flow if required, something which a simple tube lacks. The glass frit should be fine for most drying applications also, with the exception of a few gases (namely HF if a brave amateur wished to dry it) which can corrode glass, rendering it useless for any future applications.

unionised - 19-2-2018 at 12:17

Quote: Originally posted by DavidJR  
... with the expectation that you'll use a rubber bung at the other end?
. (24/29).


That's not the expectation.
It's also a very bad idea.

LearnedAmateur - 20-2-2018 at 00:00

You can use an intubated bung to keep the air flowing, it’s what I, and I assume DavidJR, were referring to.

DavidJR - 20-2-2018 at 05:18

Yes that's exactly what I meant.

Anyway thanks for the suggestions, I'll probably buy an air condenser which could double as a drying tube.