Sciencemadness Discussion Board

Loose ground glass?

Elrik - 31-10-2018 at 16:01

Not exactly living in Paris, I went to a small college where we muddled through with obsolete equipment. As a result I have essentially no experience with jointed glass.
I got a vacuum distillation set from china. I immediately observed that the joints are not a perfect fit, most of them wobble just a tiny bit. When I put a quality ground glass stopper in and put a keck clamp on and pour in 20 ml of water most joints slowly leak. For instance the leibig and vacuum adapter leak 2 drops a minute and the vigreux leaks at one third that rate.
My jointed bottles dont do this.

Just how normal is this and how big of a problem is this?
I was planning to use it at pressures down to 15 torr, will it work below atmospheric pressure? I dont have my pump yet so I can not test it.
Thanks :)

JJay - 31-10-2018 at 18:18

I have heard of others running into this problem, but I have not. I can usually get ground glass joints to wobble slightly by flexing the glass, but I am always worried that something will break, so I generally only do it when joints are seized. I don't think that is the issue you are running into here.

If you can't get a mated pair of standard taper joints to twist together so that they hold without Keck clamps, one or both joints is defective. Keck clamps don't actually serve a functional purpose in holding ground glass joints together except to keep glassware from falling and breaking if the apparatus comes apart. If, after twisting, the joints hold firmly with a few kg of force (don't drop anything), the joints are fine. If not, you should probably get a refund.

Elrik - 31-10-2018 at 21:40

When I twisted them together firmly [but within reason] it took several kilos of straight pull to get them apart, it was if I wiggled them side to side that they came apart fairly easily. In some cases I had to rock them to get them apart without too much force. The keck clips do serve to keep them from rocking that fraction of a degree back and forth so that may help if I give one last twist when clipped.
I'll distill some used xylene in it tonight. I expect that the unit will work just fine at atmospheric pressure, its under vacuum that I'm concerned about. I've seen mention of people using silicone grease when working with vacuum but I don't want all my experiments contaminated by traces of silicone.
And I wouldn't want to request a refund if this is within tolerances people actually do have success with.
My mistake was not having a vacuum pump here and assembled already.

CobaltChloride - 31-10-2018 at 23:00

Did you grease the joints with silicone grease? This usually makes them much less susceptible to leaking.

JJay - 31-10-2018 at 23:45

Some sort of lubricant is required to get ground glass joints to hold a strong vacuum. I don't usually grease the joints for vacuum filtrations, but I certainly do grease them before vacuum distillations.

Sulaiman - 1-11-2018 at 04:32

My Quickfit kit fits together so nicely that hardly any grease is required.

My generic/cheap Chinese glassware needs many times more grease,
so much that at higher temperatures it liquefies and causes a mess in the glassware.

I have most recently used plumbers ptfe tape instead of grease,
a few neatly wound turns as a single tape width ring works well.

Elrik - 1-11-2018 at 09:10

Thanks :) It sounds like the set I got actually are in the area of current chinese production standards so I wont bother them with complaint, as I would be unlikely to get anything better. At least I'll have less worries about irreparably stuck glassware ;) The RBFs, at least, are actually quite well ground and I had a stopper get temporarily stuck in one when testing.
Quote: Originally posted by Sulaiman  
...I have most recently used plumbers ptfe tape instead of grease, a few neatly wound turns as a single tape width ring works well.

I'm glad you said that, that was one of my ideas as well! lol

vmelkon - 2-11-2018 at 07:38

I have mostly Laboy glassware. It is from China.
Also also bought some no name 250 mL 2 neck RBF. It is also good.
I bought some solid glass stoppers and although they fit well into my china glassware, the grounding is too rough. The pits are too deep.

The joints are pretty good. There is no wobble.

As for performing a vacuum, you have no choice. A solid to solid joint never offers good contact. You either need grease, PTFE tape, some other soft thing like silicone or rubber or indium tape.

As a test, I put some grease on my joint and pumped out the air to 10 to 15 mm Hg using a hand pump. The vacuum held for 24 h and then it started to slowly go up.

morganbw - 2-11-2018 at 09:38

You really need to grease, (needed for a true seal).
You will lose some glassware in the future due to stuck joints but proper greasing will greatly limit this.
The Teflon tape is also okay, but joining dry ground joints is (in my opinion, not idea.).

Elrik - 2-11-2018 at 14:45

Yes, I was planning to use a little grease when working with vacuum. My fear was just that chinese joints might not seal even with grease, or might draw it inside the still.
I suspect it will be okay.

BromicAcid - 2-11-2018 at 19:09

No, there should not be any wobble in your joints, joints should fit snugly. Always use grease to prevent frozen joints, it's cheap insurance. Under vacuum it's a necessity. Making a properly fitted joint from scratch is a huge waste of time for a glass blower so usually they buy their joints pre-made and affix them to whatever they are working on, sounds like whatever company made your piece may have been using culled joints from the process (cheaper) or maybe they're just not the best to begin with. A small amount of grease should render the joint transparent, look for areas, especially around the bottom where the grease is broken/uneven which would show where the issue is. So long as you have a band of transparent grease somewhere on the joint though you should be good.

Really though, before I load up a flask with material to distill I usually do a vacuum check on the system and I would recommend you do the same. That's really the only thing that matters, does the system hold vacuum? Set it up for the distillation, grease and all, and connect to your pump with a manometer inline. Pull down the setup for a good 20 minutes and take a reading on the pressure. Have a valve inline or a hose clamp and seal off the setup from the pump for another 20 minutes then open it up, did the pressure jump up indicating a leak? (Similar to what vmelkon mentioned above) Can you pull it down as low as you need it to go even with the leak? These are the important answers you'll determine.