akimhaneul

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In the kaplan book, it says that you use gravity filtration when the product of interest is in the filtrate and you use vacuum filtration when the solid is the desired product. Why is it that you cannot use vacuum filtration when the product of interest is in the filtrate?
 

bobeanie95

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Assume your desired compound A is dissolved in a solution and you are trying to extract it from solid contaminant B. To maximize separation, you would heat the solution so that the most of A is dissolved. You would use gravity filtration because vacuum filtration uses a vacuum, as the name suggests, and as a result it cools the solution. When a solution becomes cooled, the solubility of A in the solution will decrease, and more will precipitate out. As a result, you get a lower percent yield and so you use the gravity filtration.

In another scenario, if the solid B is your desired product, you can use vacuum filtration because prior to filtration you should have cooled your test tube in ice to maximize precipitation. So using a vacuum filtration would not only be faster and more efficient, but would keep the solution cooled, so that B doesn't redissolve into the solution.
 
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akimhaneul

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Assume your desired compound A is dissolved in a solution and you are trying to extract it from solid contaminant B. You would use gravity filtration because vacuum filtration uses a vacuum, as the name suggests, and as a result it cools the solution. When a solution becomes cooled, the solubility of A in the solution will decrease, and more will precipitate out. As a result, you get a lower percent yield and so you use the gravity filtration.

In another scenario, if the solid B is your desired product, you can use vacuum filtration because prior to filtration you should have cooled your test tube in ice to maximize precipitation. So using a vacuum filtration would not only be faster and more efficient, but would keep the solution cooled, so that B doesn't redissolve into the solution.

Why would vacuum lower the temperature?


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bobeanie95

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It sucks out air so it creates a light air current and allows for better heat transfer since molecules that have reached thermodynamic equilibrium are replaced with other molecules. Its similar to blowing air over a hot cup of coffee so that you can cool it.
 
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wizzed101

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lol I think it's more simple than that. If you use vacuum filtration and the desired product is in the filtrate, you are basically trashing your product.

Just google how vacuum filtration is set up!
 
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akimhaneul

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lol I think it's more simple than that. If you use vacuum filtration and the desired product is in the filtrate, you are basically trashing your product.

Just google how vacuum filtration is set up!
Can't you keep it in the flask?


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aldol16

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lol I think it's more simple than that. If you use vacuum filtration and the desired product is in the filtrate, you are basically trashing your product.
How do the think the vacuum is obtained?
Not sure I'm understanding what you're getting at either. As long as you ensure that the receiving flask is clean and that the vacuum line itself is not disturbing the fluid coming out of the filter, your filtrate should be enriched in whatever is not caught by the filter.
 

wizzed101

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There is a risk that some of the filtrate and its products will be trashed into the vacuum line. What if your product is volatile? I think of vacuum filtrate is just a mean to speed up the process esp. when the filter is very fine.
 

bobeanie95

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There is a risk that some of the filtrate and its products will be trashed into the vacuum line. What if your product is volatile? I think of vacuum filtrate is just a mean to speed up the process esp. when the filter is very fine.
Both solutions could be reacquired into a receiver vial, regardless of the method of filtration. Even if you're doing vacuum filtration on the macro scale the amount left in the tube is minimal. The whole purpose of the experiment is to extract compounds of different solubilities via recrystallization. Which is why having the solution at certain temperatures (cold vs. hot) is desired for efficient recrystallization. You should maybe read up on the theory behind the experiment
 

aldol16

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There is a risk that some of the filtrate and its products will be trashed into the vacuum line. What if your product is volatile? I think of vacuum filtrate is just a mean to speed up the process esp. when the filter is very fine.
Yes, so if you set it up poorly, the vacuum could suck up some of the solvent into the vacuum line and you'll end up with very dirty plastic tubing. But that's a fault of set-up, not a fault of design. I always tell students to gradually ease up the vacuum and never have it on max if it can be avoided - this prevents the filtrate coming out of the filter from being sucked into the line.

If your product is volatile, then it is possible that vacuum filtration could cause your product to evaporate. However, this is only with a small class of molecules - otherwise, rotary evaporation simply would not work. Most organic compounds (except low-weight hydrocarbons) have boiling points that are too high for any significant amount of them to vaporize and disappear under vacuum. Again, if this were the case, the rotary evaporator would be a pointless instrument.
 

wizzed101

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@bobeanie95 honestly, what you wrote just reeked of post-hoc justification. It may be true, but I have no interest in that lol. And I don't think this set of experiment is designed to "extract compounds of different solubilities via recrystallization."

@aldol16 but then if it is the solute we want to recover we wouldn't use filtration. Would it be much better to just filter the solid out and use something else for the liquid?
 

aldol16

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@aldol16 but then if it is the solute we want to recover we wouldn't use filtration. Would it be much better to just filter the solid out and use something else for the liquid?
You just contradicted yourself there. The process of filtering the solid out is indeed filtration. If you have a complex reaction mixture and you're trying to purify it, there are a few things you can do before going to the column. The easiest thing is to see if you can get anything to crash out and then filter it - vacuum or gravity, doesn't really matter although vacuum will probably be faster so you'd use that in lab. Now you have a solution with various things dissolved in it. One way to separate the stuff is then to distill it. Another way is to pass the stuff through a column packed with silica or alumina.
 

wizzed101

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You just contradicted yourself there. The process of filtering the solid out is indeed filtration. If you have a complex reaction mixture and you're trying to purify it, there are a few things you can do before going to the column. The easiest thing is to see if you can get anything to crash out and then filter it - vacuum or gravity, doesn't really matter although vacuum will probably be faster so you'd use that in lab. Now you have a solution with various things dissolved in it. One way to separate the stuff is then to distill it. Another way is to pass the stuff through a column packed with silica or alumina.
I thought the question at hand is that if I am handed a solution of unknown origin and some solid in it and asked to "filter," I would definitely use vacuum filtration to get the solid because I don't really care about the liquid, and NOT use vacuum to collect the liquid because I don't know what it is. I mean, it's just common sense! Ex: sand and ethanol.

But you are right. It doesn't matter either way. The yield and purity will suck regardless, with just filtration, that is.
 

aldol16

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I thought the question at hand is that if I am handed a solution of unknown origin and some solid in it and asked to "filter," I would definitely use vacuum filtration to get the solid because I don't really care about the liquid, and NOT use vacuum to collect the liquid because I don't know what it is. I mean, it's just common sense! Ex: sand and ethanol.
The question at hand is what do you do when the product of interest is in the solid phase vs. the liquid phase (refer to OP's original post). Common sense here fails because of that misunderstanding - if you're trying to get the solid, then of course you just filter it and take the solid. But that's not all that a filter is used for. Say you have a solution with various solutes dissolved in it. You add in some ions and cause a solid to crash out. That solid is an impurity. So you filter your material and collect the filtrate. Dispose of the solid and then continue to purify the filtrate. That's how it's normally done in chemistry research - precipitation of impurities and subsequent filtration is one of the first steps we usually take to remove impurities.