APO
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Washing Procedure
This is a possibly dumb or uninformed question, but I'll go ahead with it. I've been pondering the usual procedures of organic synthesis, and I wonder
if using separate portions when washing a product is really necessary, but I'm not sure. For example, to me, it seems like 3x washes of 50ml could be
supplemented with one wash of 150ml just fine as long as it was mixed or shaken 3x as long. One drawback I can foresee is that if you are neutralizing
a product, you cannot tell as easily if it needs further neutralization. But if you are working with a procedure known to be reliable and successful,
would replacing a few separate washes with one larger wash affect it adversely if at all?
"Damn it George! I told you not to drop me!"
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Etaoin Shrdlu
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Multiple washes are better. They build on each other, because successive washes only need to remove what was left behind by the ones before them.
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Amos
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Dissolution or extractions from solvent to solvent operate on an equilibrium, like pretty much everything else. Each successive wash has less total
solute to work with and therefore an easier time dissolving more of what's left behind. Kind of like how a compounded interest rate accumulates more
cash in an account than a simple interest rate.
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APO
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Ahh, ok, I see how that can make a difference. How much of a difference does it make though?
"Damn it George! I told you not to drop me!"
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DraconicAcid
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As an example, let's imagine a compound for which the eq'm constant for organic = aqueous is 4. If you extract 50 mL of the organic solution with 50
mL of water, the concentration in the aqueous phase will be four times that in the organic layer, so you've extracted 80% of it (leaving 20% behind).
Doing this three times means you've left behind 20% of 20% of 20%, which is 0.8% of the original.
If you extract once with 150 mL of water, the concentration in the aqueous phase will be four times that in the organic layer, but since there's three
times as much aqueous solution, you'll have twelve times as much substance in the water. You'll leave behind one-thirteenth of the original compound
in the organic phase, which is 7.7%.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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APO
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Thank you very much, I understand now.
"Damn it George! I told you not to drop me!"
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