washing and extracting

How to Perform a Liquid-Liquid Extraction (Generally) A separatory funnel is used to perform liquid-liquid extractions. Tips: Always remember to close the stopcock before pouring in any solutions! Always remember to have something underneath the spout of the separatory funnel in case (when) you accidentally forget to close the stopcock! Failure to do these means you may get to enjoy experiencing a bench-top extraction. The general steps to a liquid-liquid extraction with a clean separatory funnel are: Pour in the organic (or aqueous) layer that contains the desired organic compound(s) through the top of the separatory funnel. Add the aqueous (or organic) solvent that will be used to extract (or wash) the organic compound(s). This will result in a biphasic mixture (Before extraction, Figure C). Cap, shake, and vent (see procedure for details) carefully! Molecules traverse between two phases only at the interface. Vigorous shaking maximizes the surface area between the two phases so that the organic compound(s) can move from one phase to the other. Most of the organic compound(s) will end up in the organic layer (after extraction). However, some will be stuck behind in the aqueous layer still. This is why we always perform successive extractions (usually in triplicate). See below for some basic math to support this (no, you do not need to know the math, it's just to illustrate a point). Collect the aqueous and organic layers into two separate flasks. The extract is the layer that you used to pull the organic compound of interest into. This is usually the organic layer for a neutrally changed organic compound, but it can be the aqueous layer if we are exploiting protonation states and are working with a charged compound (charged organic compounds are usually salts, and salts tend to prefer aqueous layers). The organic layer is shown at the top in Figure C, but this is not always the case! Be sure to know when the organic layer floats on top or sinks to the bottom. Return the layer that you are extracting from to the separatory funnel & repeat Steps 2–5 two more times using fresh extracting solvent. Note, if you are extracting with an organic solvent, then you put the aqueous layer back in and extract with a second portion of fresh organic solvent (and vice versa). The reason for this is that you are successively pulling the target compound(s) out of the old aqueous layer, and into the new organic layers. The first extraction usually does a good job of getting about 70–90% of the desired compound(s) out if performed properly. The subsequent 2 extractions ensure that everything is extracted. There is usually no need to perform more than three extractions. Reinforcing the Concept of Successive Extractions Successive extractions is a concept that confuses a lot of students, but it's ultimately very straightforward. We will reinforce this concept now so you can confidently perform this technique in lab without being confused. Whenever you do a liquid-liquid extraction you: Add one liquid phase to another (you should see two phases), mix, and let them separate out. Then take both layers out of the separatory funnel. You can either take the bottom layer out of the bottom and into one Erlenmeyer flask, and the top layer can then be taken out the bottom into a different Erlenmeyer flask, OR... You can take the bottom layer out of the bottom and into one Erlenmeyer flask, and the top layer can then be taken out the top into a different Erlenmeyer flask Some people will say it matters which method you choose, and yes, it sometimes does. However, let's keep it simple in our course. You can choose whichever method is more natural to you. So long as you are doing everything correctly, we are fine either way. Then place the layer you are extracting from back into the separatory funnel. You should see two immiscible phases again. If only one huge layer forms, you just added organic to organic or aqueous to aqueous! Talk to your TA on how to correct this! The solution is usually simple, but it is somewhat situation-dependent, so we cannot offer a "one-size-fits-all" solution here. Then add a new portion of the liquid phase you are pulling the analyte(s) into, in the separatory funnel. Repeat the above steps, starting with the "mix and letting them separate out." This is what makes it successive, because you are adding a portion of extracting phase, taking it out, adding another portion, taking it out, adding another portion, taking it out, etc. Notice that you do not add the extracting phase portion, then immediately add another extracting phase portion before taking the first out. That's just one massive extraction and is not successive extractions. Once you are done, collect all the organic phases together, and collect all the aqueous phases together separately. DO NOT discard your aqueous phases until the END of your experiment.