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Author: Subject: Using enormous excess of solvents for extractions?
Fyndium
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[*] posted on 24-3-2021 at 06:30
Using enormous excess of solvents for extractions?


I have noted that many amateurs seem to use huge excesses of solvents and reagents, for example 1 liter of 50% NaOH to neutralize 50 grams of reagent that has minority of acid impurity, or extracting a 1L volume of aqueous layer 3 times with 500mL of solvent that contains mostly traces of non-water soluble product.

Meanwhile, I've mostly done these by doing some molar calculations and common sense and gotten yields what I was looking for with a fraction of solvents and reagents. In one reaction, I actually just crashed out the substance with water and extracted once just in case with 200mL with solvent to get full yield. The text told to extract thrice with 500mL.

I understand that a test reaction with volume of 50mL can be extracted with amount of solvent that is convenient, aka 3x50mL because 150mL is absolutely a small volume, but it seems that if people scale things up, they scale everything else directly.

I've discussed about this earlier already, but I think process economics can be one aspect of this hobby.
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outer_limits
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[*] posted on 24-3-2021 at 13:49


It's just a lack of basic knowledge like molarity, equilibrium and partition coefficient (and why it is better to extract 3 times with smaller amount of solvent than 1 with large).

Many want just to follow the recipe - dump reagents to the flask and do all the work without any theoretical background about the process that is happening in reaction flask or even sep funnel. But it's not a chemistry, there is no science.

[Edited on 24-3-2021 by outer_limits]
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24-3-2021 at 14:16
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[*] posted on 24-3-2021 at 15:16


A simple form of reasoning can show you what can be done with carefully adjusting the use of reagents.

Just an example to demonstrate such a form of reasoning.

Imagine you have some nice salt, fine crystals, but there is a adhering water and you want to get rid of it quickly, because it slowly deteriorates your product. Imagine that appr. 1 ml of water sticks to the salt and you decide to use acetone to mix adhering water with the solid and let it settle and decant the acetone and then let evaporate quickly. In this way, you can get rid of a lot of water quickly. After decanting, appr. 1 ml of liquid again sticks to the solid.

Now imagine that you add 50 ml of acetone and swirl the beaker with your solid in the acetone. Your 1 ml of adhering water is diluted over 50 ml of acetone. After settling and decanting, you are left with 1 ml of liquid, sticking to your solid. Appr. 1/50 part of the water still is there.

Now imagine that you take three rinses of 15 ml. Each rinse dilutes the water with a factor of appr. 15. After three rinses only 1/(15x15x15) part of the water remains. More likely, the water content of the acetone will be larger than this value (even high purity acetone may still have 0.1% to 0.2% of water in it). So, with using a little less acetone you get a much better result.

Using a few small rinses after each other is MUCH more effective than a single large rinse.

For extractions another type of reasoning must be used, but in many cases, especially if the compound to be extracted dissolves very well in the solvent, using extraction with a small amount is equally effective. You then can use a second extraction with a small amount to get the last little amount.




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[*] posted on 25-3-2021 at 10:34


I had a huge problem with that at first! It wasn't mechanically intuitive how much of any reagent a solvent could take up so I would really go overboard and then workups took significantly longer.

Now, I think I went to an extreme in the other direction. Suppose an extraction calls for 3x100mL washes--I opt for maybe 6x50mL washes instead. It's overkill, but I think it is probably the stingy part of my brain exerting itself.
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Fyndium
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[*] posted on 26-3-2021 at 01:02


Not sure if this is part of the topic, but I constantly get different readings from boiler and stillhead, and sometimes overheating of flask walls or bath can cause misreadings to thermoprobe even when everything is in check.

Only one mystery has happened, when stillhead reported higher temps than the boiler. Only explanation might be that the overheated boiler walls heated the vapor even more and caused it to be actually hotter at the head than in the boiler.
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[*] posted on 26-3-2021 at 03:28


Quote: Originally posted by Fyndium  
Not sure if this is part of the topic, but I constantly get different readings from boiler and stillhead, and sometimes overheating of flask walls or bath can cause misreadings to thermoprobe even when everything is in check.

Only one mystery has happened, when stillhead reported higher temps than the boiler. Only explanation might be that the overheated boiler walls heated the vapor even more and caused it to be actually hotter at the head than in the boiler.


You had super heated vapor, this is a common thing in refrigeration.
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