ManyInterests
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2 vs 3 neck boiling flask?
I was in the market for a multi-neck 2000ml boiling flask (flat bottomed) and I looked over my options. I noticed the 2 necked and the 3 necked flasks
all the cost the same. I was wondering if there was an advantage of a 2 necked flask vs a 3 necked? I have plenty of stoppers and can plug the
unneeded neck when it is not in use, but I was just wondering if there is an advantage or disadvantage.
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igxlabs
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I'm a fan of three necks. One for condenser, one for thermowell, one for addition. It's the thermowell that's most important to me. The three-necks
always seem to have the neck at the perfect position for that purpose. At least for me.
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RustyShackleford
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Get the 3 neck since theyre the same price, the only disadvantage i can think of is its more likely to leak when pulling vacuum, but with good grease
and stoppers thats no issue.
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Rainwater
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I have trouble getting the 500ml, 1l and 2l, 3 necked rbf into the sized heating mantles. 2 necked flask are not in issue. Probley doesnt apply to
flat bottom flask.
Always have to use 1 size up.
Cant recall where i heard it, but you should be careful if your using a vacuum on flat glass. It has an increased implosion risk
"You can't do that" - challenge accepted
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teodor
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According to my observations there could be only one disatvantage of 3 necks vs 2 necks: when you use a complex setup (condensers, bends etc) the 3rd
unused and stoppered neck can interfere. So, generally, it somehow limits all possible joints and apparatus. But as a first multineck flask of some
particular volume I always prefer more necks.
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Texium
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At work, where I do air free reactions frequently, I prefer 2-neck flasks usually: one for the Schlenk line and the other for a rubber septum. An
extra neck is just another point that air can potentially infiltrate. At home, I tend to prefer 3-neck flasks for their versatility. I’d suggest you
go with the 3-neck flask if you can only get one, because you can always stopper a joint, but you can’t add one if you don’t have enough.
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Dr.Bob
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I rarely use 2 necked flasks. One necks have the advantage that you can use them for rotovaps, high vacuum work, and many other functions, where a 2
or 3 neck flask is not as good. But once you add a second neck, the third neck does not add any less function, and often I need to add a
thermometer, condenser, addition funnel, or such and don;t have the space. However, you can often add a Claisen adapter to add an neck to a flask.
That is how I often use a one neck flask for things that could use a 2 or 3 neck flask, but I don't have enough of them.
Also, I would skip the flat bottomed flask, it is much less safe for vacuum work (round is much stronger than flat under vacuum), and while they do
sit upright (I use a cork ring with round bottom flasks to solve that issue), they are much less practical for many things, and harder to find as
well. If you want to heat a flask, use a water or oil bath, heating mantle, or heat gun, rather than trying to directly heat on a stir plate. Much
safer.
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Sulaiman
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Do any members here actually pull a vacuum in 2 litre or more flasks?
(in a home/hobbyy/amateur lab)
I am still a beginner and even I have used all three necks of my 500ml 3-neck flasks.
Claisen adapters add, in descending order of my personal dislike:
height, weight, joints and cost.
So I consider more necks = better
But
I was gifted a 5 litre rbf with mantle, the rbf has one long NS35 neck
(I added an adapter for my NS24 glassware) and one NS19 neck for a thermometer.
good for distilling, refluxing, extracting etc in batch runs that don't need an overhead stirrer.
But another neck would be nice to have 
CAUTION : Hobby Chemist, not Professional or even Amateur
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Fery
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Sulaiman I used at most 1 L RBF. Like you I read too that 2 L RBF is a limit for glass RBF for vacuum distillations. If I distill from 2 L in the
future - at first step I would distill just water from the 2 L RBF and used water aspirator level vacuum to test the assembled apparatus - if the
flask implodes then the heating mantle with stirrer could be very likely easier to clean and dry. The force applied to the glass would be not too much
different from later usage of oil rotary vane vacuum pump with stronger vacuum, like force to glass walls at -0,975 atm of water aspirator is
practically almost the same as -0,999 atm at oil pump.
You can add more necks using still head plain parallel adapter, but it is not much suitable for thermometer immersed into RBF.
Sometimes a condenser could be mounted at the top of pressure equalized dropping funnel thus saving one neck of flask.
Few years ago I switched from flat into round bottomed flasks due to frequent vacuum distillations. For RBF I use small tin cans (from sweet corn,
canned tomato paste etc) which would otherwise end in a trash can for metallic recyclable waste. Also glass beakers could be used. I have bought few
sets of cork and plastic ring stands but I did not yet open any of the packages as I'm still satisfied with tin cans.
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BromicAcid
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So long as your glassware isn't chipped/cracked and of decent quality there are no vacuum limitations based on the size of the flask.
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Dr.Bob
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I mnight be biased, as I use a Rotovap a lot, which is not common to home chemists. But they are great for recycling solvent, and evaporating large
amounts. So I favor single necks for that. And I often use flasks of 2 and 3 L size on the rotovap, which again may be unusual for home work. But
I used to do lots of 500-1000 ml stuff in my home lab. I often use solvents for stripping paint and varnish, and other solvents for cleaning stuff,
so recycling it is very handy, as it is getting harder to find and even harder to dispose of, so I try to just keep using same stuff over and over.
I used to be able to recover 80-90% of solvent and just generate about 10% waste goo/residue, which means I lost about 10% to evaporatioon.
When I was stripping baseboard and wood trim in my last house, I used gallons of DCM, but was able to reuse much of it. This house I had to remove
some linoleum tile, which was glued down with asphalt or pitch, and that required a similar process, but on the floor. What a mess, but it was a wet
way to do it, and given the age of the tile, that was best. I was not able to recover as much of that, but did discover that adding some mineral
spirits to the DCM made it evaporate slower, which helped a lot. Also used organic gas mask and large fans to help breathe clean air. But I
digress.
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