I need to heat a fluid for seven days at a time. The fluid goes in round bottom flask that is suspended in a water bath. The test fluid will decompose and generate a small amount of gas over time.

Gas generation is variable and dependent on what material I am testing in this material compatibility experiment.

I have experience in plumbing and fluid systems but not chemistry. In a plumbing system I would add a relief valve to relieve the pressure generated over time, without allowing potentially dirty air to contaminate the experiment.

I have a ground glass joint. I found a source that says the ground glass plug will lift to relieve pressure, even if I put a plastic clip on it, and the plastic clip would ensure the plug would not go flying.

I still have concern since this is an environment I am not familiar with, and I am not keen on exploding glassware in the facility. Past experimenters have placed small beakers upside down on top of the flask, and have also used aluminum foil as a cover (I will not do this so the vapor contact with foil does not affect the experiment results). I tried it with a beaker but a moderate amount of fluid condensed on the surface, which was not ideal.

Will I be fine securing a ground glass joint with a plastic clip, should I use an upside down beaker, or is there a more obvious solution?

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    $\begingroup$ What about sticking a balloon on an outlet of your setup, allowing the gas to expand into it? $\endgroup$ – electronpusher Oct 24 at 7:10
  • $\begingroup$ Made clear if you want to protection the material inside or if you are interested in collecting gss. In the first case simply let the system to atmospheric P by connecting a simple trap with cotton or calcium chloride. Beware calcium chloride can clog on long run. In line with Tgr_Vinz answer. Otherwise DrMoishe Pippik $\endgroup$ – Alchimista Oct 24 at 9:56
  • $\begingroup$ What gas is generated? If it's nothing corrosive etc. then the balloon suggestion is a good one. Place a rubber septum into the ground glass joint to seal it, and pierce it with a wide-bore needle attached to a balloon filled with an inert gas (N2 or Ar). $\endgroup$ – PCK Oct 24 at 9:56
  • $\begingroup$ I am testing hydrogen peroxide for compatibility with various materials, so there will be gaseous oxygen, water vapor, and hydrogen peroxide vapor. If I use balloons, the hydrogen peroxide vapor will react with it and show a higher rate of decomposition than reality. Going to think on the reflux condenser a bit but that seems like a good solution. Not sure if I am allowed to link here, but "Long Term Storability of Hydrogen Peroxide" pages 3-4 (there is a legal pre-published copy online) describe the test, although I am trying to improve it. $\endgroup$ – user2139103 Oct 24 at 18:28
  • $\begingroup$ A balloon can be connected to the flask with a long, thin polyethylene tube, say ~1 mm diameter (you can draw it out with gentle flame heating). Pressure can be relieved by slow gas diffusion rather than a free-flowing connection between the flask and the balloon. $\endgroup$ – James Gaidis Oct 25 at 12:48

If I didn't miss any point, you want to make sure that: -the system is not in direct contact with the environment, in order to keep it clean -pressure doesn't build up dangerously -the non-gaseous product is not lost

What about a reflux condenser? Even unstoppered, it makes loss of material very unlikely. If you are concerned about contamination, you can add to it an inert atmosphere or a tube packed with cotton, according to your needs

  • $\begingroup$ I think this is a good solution for one of the two steps of the test process. For the other step, I have some 50 mL volumetric flasks on a hot plate with temperature feedback. They have a 12/21 joint. I have to heat the 90% hydrogen peroxide to 100C for 24 hours, so water bath was not a good option, various oils were fire hazards, etc. Open to suggestions on other setups, but what is the proper chemistry tool to act as a mini-condenser in a joint that size? $\endgroup$ – user2139103 Oct 24 at 19:25

Ground-glass joints are notorious for seizing at the wrong time, particularly if reactants are alkaline.

If pressure might change, I'd add some relief mechanism, such as a water-trap or rupture disk. Without knowing your setup, something as simple as a balloon might work, if not attacked by reactants.


I routinely use a positive pressure of argon with a balloon in an otherwise sealed reaction setup where all joints are (greased) ground glass joints, potentially secured with clips. It is rather rare for them to give in to the balloon pressure but at least once the weakest link popped open (a single stopper that was not secured properly). Depending on your setup, the weakest link may be one that gives way non-catastrophically or it may require enough force to turn catastrophic—it is not something I would want to bet on if I have a precious experiment running. Especially since heating a fluid to me almost automatically implies reflux condensor.

If exchange of air with the outside atmosphere is not a problem, your options include:

  • any glass tube with a joint and a $90^\circ$ angle that you can put on top to prevent contaminants falling down from above
  • a classical drying tube, filled e.g. with calcium chloride to prevent significant water drawing
  • a plastic stopper with a large disc-like top that is one size smaller than the ground joint

If gas exchange is not desired (i.e. you have some kind of inert gas in your vessel) then I would opt for one of many possible relief valves

  • the type commonly used for alcoholic fermentation, filled with a suitable liquid
  • special pressure-relief glassware that fits on joints exists
  • a balloon can help relieve the pressure but it has limits

Neither list is complete and with a little MacGyver skill I am sure you can build an appropriate device with whatever you have in your laboratory.


If I understand the set up and purpose, just use a rubber stopper with a needle (any) through it. I do expect that nothing you put in hydrogen peroxide shall react violently, as for you are doing compatibility tests.


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