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BenZeen
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[*] posted on 20-4-2010 at 04:54
Pear shaped flasks


Hi, This is something that has been bothering me for quiet some time, and i am sure that i cant be the only person who has wondered about this before, so i thought i'd ask the clever chemists at sciencemadness for an explanation.

What is the purpose of a pear shaped flask?

any enlightenment on this subject will be very much appreciated, cheers
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DJF90
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[*] posted on 20-4-2010 at 05:12


Pear shaped flasks are generally used on the smaller scale, as when distilling from them you can recover much more material than in a round bottom flask (remember you never distill to dryness), and hence higher isolated yields. This is sort of why you'll never see large pear shaped flasks, only to 100mls capacity or so.
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[*] posted on 20-4-2010 at 05:22


Ahhh of course, thankyou

so i guess that they need to be heated on a bath?


[Edited on 20-4-2010 by BenZeen]
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Ephoton
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[*] posted on 20-4-2010 at 05:58


you can get pear mantles.




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DJF90
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[*] posted on 20-4-2010 at 07:06


Or you can make them...
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bquirky
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[*] posted on 20-4-2010 at 08:31


Pear shaped flasks are generally used on the smaller scale, as when distilling from them you can recover much more material than in a round bottom flask (remember you never distill to dryness), and hence higher isolated yields. This is sort of why you'll never see large pear shaped flasks, only to 100mls capacity or so.


im curious why is this the case ?
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matei
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[*] posted on 20-4-2010 at 08:42



Larger (500 or 1000 mL) pear-shaped flask are useful for evaporating solutions to dryness using a rotavap - you can scrape all the solid materials on the wall with a spatula.
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mr.crow
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[*] posted on 20-4-2010 at 08:58


Quote: Originally posted by matei  

Larger (500 or 1000 mL) pear-shaped flask are useful for evaporating solutions to dryness using a rotavap - you can scrape all the solid materials on the wall with a spatula.


Would that be a recovery flask? Conical at the top and round at the bottom.

I think pear shaped flasks are round at the top but pointy at the bottom so a pipette can remove all the liquid.




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DJF90
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[*] posted on 20-4-2010 at 13:26


That is a florentine flask, used on the rotovap, and is as mr.crow describes. It has straight walls and a round bottom so that recovery of crystalline material once all of the solvent has been removed (unless you have an oil) is facile.

bquirky: Why what is the case?

[Edited on 21-4-2010 by DJF90]
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JohnWW
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[*] posted on 20-4-2010 at 13:27


Quote: Originally posted by mr.crow  
I think pear shaped flasks are round at the top but pointy at the bottom so a pipette can remove all the liquid.
That description is the shape of a separating funnel, with an inlet tube and joint at the top, and an outlet tube with a tap at the bottom to allow decanting out of the different immiscible liquid fractions into separate flasks or beakers.
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bahamuth
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[*] posted on 20-4-2010 at 15:47


A pear shaped flask is used when recovery of small amounts of product, distilling with "never distill to dryness" in mind, rotational evaporation with e.g. product recovery in mind or any other good use you can find for it.

Have only used it as a recovery flask or for rotavapping(is this a word?).

269317498_tp.jpg - 24kB




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[*] posted on 20-4-2010 at 21:30


thats the one its big in the wine industry.

you can use flame as well on them but still I would go with a mantle.

they dont tend to get the hot spots as easy with flame as RB flasks.

I have seen quite a few wine makers use them for distilling with a flame.

but then again wine makers tend to do things a little old school as there normaly not chemists as such.




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DJF90
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[*] posted on 21-4-2010 at 02:25


I used pear shaped flasks whilst I was back at school with a microburner (essentially a mini bunsen). Never once had an issue with combustion, but we did always grease the joints and to the best of my recollection, never used organic solvents in a reaction (generally oxidations with acid dichromate, nitration of methyl benzoate, preparation of aspirin, etc etc). Solvents were of course used for work-up, but flames had been extinguished by then.

JohnWW: The description is consistent with a pear shape flask, the item we were describing funnily enough. Separating funnels are a similar shape in some/most cases, but indeed have a tap at the bottom to allow take off.

Bahamuth: That's not the right type of flask for a rotovap. As I mentioned before, such flasks are of a shape "inverse" to a pear shaped flask, and are called "florentine flasks". I've attached a picture.

Florentine.JPG - 13kB
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bahamuth
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[*] posted on 21-4-2010 at 07:51


DJF90, I know it is not the "right" flask, considering that it has a to large flat surface for pressure to be exerted with possible failure as consequence, but the pear shaped ones are neat to work with when one are working with e.g. chlorophylls and you need to dry them with e.g. benzene before finally dissolving them in the correct solvent for HPLC.

Just mentioned that I have used them for just that, was not my intention to label them as to such use.

I like these kinda threads, since even people at my faculty, lab techs and chemists sometime do not know the correct use for some of the equipment we have, including myself...




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DJF90
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[*] posted on 21-4-2010 at 09:31


Sorry I wasn't implying you was wrong, just that typically the florentine flask was essentially *designed* for a rotovap. Even at my institution, RBF's are consistently used for rotovapping, despite dificulty recovering all the material as some of it gets where a spatula can't reach. The solution I use is to remove as much material from the flask as possible, then add a little more solvent and make sure I get the stuff that the spatula can't reach into solution. The flask is placed back on the rotovap, this time with no rotation... the solvent evaporates and the crystals are left in a position easily accessible with a spatula.
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[*] posted on 21-4-2010 at 15:35


Small pear shaped flasks are brilliant for first year student labs.

-They're hard to break :P




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peach
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[*] posted on 13-5-2010 at 13:08


I just had an ingenious thought! Or one I think is anyway.

The shape of a pear flask will also heat more smoothly in a flame, since the heat will lick up the sides evenly as opposed to creating a hot spot.

Probably less important with the advent of hotplates. And I'll admit my ingenuity came to me when I saw one with an alcohol burner under it and remembered your post.

I'm a massive fan of the erlenmeyer. Stands up on it's own, doesn't fall over very easily, cheap, tricky to break, fits a huge stir bar in the bottom and gives a much bigger surface area for reactions involving different phases. Never really any too many problems with heating them. You can even get your swirl on with your hand alone. Go erlenmeyer!

Don't like trying to separate things with them though. Pears are better for that, although my friend said they all taste like apples to him. They're also designed to purposefully spill your labor of love all over the bench. I'm sure inside every pear there's a tiny heart that laughs at me when it happens. They even look like hearts. Solid evidence.

I was also the only student who managed to smash her conical flask.

"You'll need to shake those solutions really hard! Be careful around the benches."
[SMASH]
"That was predictable. $5, it's going in the book."

Good question though! ;)

[Edited on 13-5-2010 by peach]
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[*] posted on 13-5-2010 at 13:39


This is what I've always thought of as a pear-shaped flask:

https://www.vwrsp.com/catalog/product/index.cgi?catalog_numb...

I've never used one, however, they look like the right idea for a pot for minimizing residual volume during a distillation.




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DJF90
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[*] posted on 14-5-2010 at 09:03


Yes Magpie that is indeed a pear shaped flask :) Very nice for small scale work!
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MagicJigPipe
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[*] posted on 15-5-2010 at 10:04


But that's hardly shaped like a pear. They should be called "inverse water drop" flasks. I have a flask that is exactly shaped like a pear. The "stem" is just a small open glass tube. I don't know exactly what it's for but I assume maybe collecting gases?

[Edited on 5-15-2010 by MagicJigPipe]




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