[Cukitas2001]

Hey all,
     I recently did an extraction and im curious as to why there is a buildup of pressure in the sepratory funnel even though no reaction to create gas takes place. Anyone know?

[beheada]

It would help knowing what you're using?

But, my guess would be if there is any type of exothermic reaction (i.e. sodium hydroxide into water) the heating and therefore expansion of the solution may force the air out faster than it would normally seep out. Hope that helps.

Always a good idea to vent when using a sep funnel, irregardless.

[Cukitas2001]

It was a separation of p-Dimethoxybenzene and benzoic acid. It was first dissolved with diethyl ether and placed into the seperatory funnel. There it was shaken and when vented a lot of gas would come out and the shaking and venting was repeated until no more pressure came out. Theres no reaction so whats causing this pressure?

[Ψ×Ψ]

You're right that it has nothing to do with whether or not a reaction is taking place. 
It has to do with nonideal solutions.  (I want to say it has something to do with fugacity, but I slept through thermo, so I'm not entirely sure.)

[firewall]

The buildup of pressure occurs because a portion of the solvent vaporizes when the separatory funnel is shaken. An attentive student will notice that when the solvent is water very little pressure buildup occurs. That is because water has a relatively high vapor pressure at room temperature, and it is less willing to became a vapor. In your case, however, the solvent was diethyl ether. As you know, diethyl ether is extremely volatile at room temperature. By putting it in a separatory funnel and shaking it around, you are releasizing a significant portion of it inside the container. Eventually, after a certain amount of ventilation, the buildup of pressure stops, which may be because the most energetic molecules have escaped, leaving behind those that lack the same propensity to leave the liquid state.

[Cukitas2001]

Thanks it all makes sense now, i completely forgot about diethyl ether being volatile and having to do with that but it makes embarassing sense now thanks again sirs.