Separatory flasks that I use have a ground glass joint, but it is not clear how that joint is to be used. Videos I have seen just show people pouring stuff into the throat of the funnel.

Normally the situation I have is that the solution is in a large flask, maybe a 1000ml flask, and I need to separate a thin layer of the solution that is at the bottom of the flask using a 500ml separatory funnel. The question is how to get the bottom of the flask to the funnel.

Saying "oh just use a bigger funnel" is not a solution, because the flasks, reactors and carboys I have tend to be significantly larger than any normal separatory funnel. For example, if I have a 3000ml reactor or 5 gallon carboy with a solution in it, I am not going to go looking around to spend hundreds of dollars on a gigantic separatory funnel.

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    $\begingroup$ I'm not sure I understand the question, but why not carry out separation in batches? Or, if the liquids are highly immiscible, why not to try to get rid of some volume of the upper layer by decantation? // Also, I'm not sure how to interpret the part about the ground glass joint — are you asking what it is for in general, or whether it can be helpful for processing separation of large quantities of liquids? $\endgroup$ – andselisk May 21 '19 at 11:21
  • $\begingroup$ Have you considered syphoning? It can be very useful at times, especially in large scales. $\endgroup$ – Nicolau Saker Neto May 21 '19 at 11:45
  • $\begingroup$ @NicolauSakerNeto Right, I would think the natural use of the ground joint of the separatory funnel is to attach it to some kind of siphoning system, but I don't know the proper way to set that up, so that is why I am asking. $\endgroup$ – Shaka Boom May 21 '19 at 11:51
  • $\begingroup$ @andselisk Part of my question is to ask what the purpose of the ground glass joint is on a separatory funnel. Every single separation setup I have seen, either in books or on YouTube makes no use of this joint (except for attaching a wide mouth funnel), so what is its purpose? $\endgroup$ – Shaka Boom May 21 '19 at 11:54
  • $\begingroup$ @ShakaBoom From what I know, the ones with the joint can not only be used as separatory funnels, but are also usually used as dropping funnels, e.g. by attaching it to the round-bottom flasks directly or via Claisen adapter to carry out reflux or inert atmosphere synthesis. $\endgroup$ – andselisk May 21 '19 at 11:59

You should proceed as follows:

  1. Charge the funnel with the mixture until it is almost full.
  2. Let the content settle.
  3. Separate the lower phase.
  4. Separate the upper phase.
  5. Repeat until you have processed all the mixture.

The ground joint is to close the funnel hermetically. This is necessary when you need to perform an extraction. A liquid with a mixture of components is charged in the funnel and an immiscible liquid is added. The funnel is closed with a stopper and it is vigorously shaken.


It is better to shake a bit first, take off the pressure, shake again, etc. To release the pressure, put the funnel upside down and open the valve. Put the funnel upright and let settle. When there are two clean phases, release the stopper and take them out separately. The components will be distributed between the phases in different quantities, so it is a way to purify those substances.

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    $\begingroup$ does not answer the question $\endgroup$ – Shaka Boom May 21 '19 at 17:32
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    $\begingroup$ It answers some of your questions. I read it and learned something. $\endgroup$ – Karsten Theis May 21 '19 at 20:39
  • $\begingroup$ I did too. I would instinctively have removed the plug at the top joint to let out some pressure, thought after reading this I'll always use the valve, which is more practical and almost seems self-evident now. $\endgroup$ – Veritas Jul 21 '19 at 19:04

The ground glass joint at the top of the separatory funnel is for a ground glass stopper. Occasions arise where you have a homogeneous liquid which you wish to extract. You put this liquid into the funnel, add the extraction liquid, stopper the funnel, shake (carefully...), then separate the layers.

The pressure development is frequently a result of using a volatile solvent like ether to extract organics from an aqueous emulsion or possibly of gas production (like CO2).

In your case, where you already have two layers (let's say, 800 mL over 100 mL in a 1 L flask), it should be relatively easy to slowly pour off 400-500 mL into another vessel, leaving much less liquid to put in the separatory funnel. This is comparable to siphoning, but may be easier. If any of your desired underlayer gets carried over, you can do the separation on this portion of the batch also.


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