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Tsjerk
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Erlenmeyer flasks can be heated just as well as round bottom flasks, I heated them to 300+ without problems. Just don't heat the tick walled ones
designed to pull a vacuum on.
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Eddie Current
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Quote: Originally posted by SWIM  |
You have, however, given us all a fine insight into your personality.
[Edited on 17-7-2020 by SWIM] |
So you're one of those internet heroes that likes to go out of their way to provoke others and then attempt to hide behind the mob? I tend to ignore
people who are under the delusion that they speak on behalf of everyone. Jog along knackers.
| Quote: Originally posted by Tsjerk | | Erlenmeyer flasks can be heated just as well as round bottom flasks, I heated them to 300+ without problems. Just don't heat the tick walled
ones designed to pull a vacuum on. |
Indeed. It's amazing how some folks seem to have a problem with identifying the difference.
[Edited on 17-7-2020 by Eddie Current]
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RogueRose
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| Quote: Originally posted by Heavy Walter | Years ago, I evacuated a large dessicant, not knowing that the cover has to match exactly to the lid (grounded surfaces), I used a cover from other
one.
I left the evacuated dessicant in a table, left the room and then the dessicant exploded filling the room with shattered glass.
Then I learned about stresses in glass... and recognized a lucky day!
[Edited on 4-6-2020 by Heavy Walter] |
I'm glad you said something about this. I just did the math on a 12" desiccator and there is almost 1700lbs of pressure across the top of the unit at
full vacuum! Even at -10psi its almost 1200lbs over the surface.
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Fyndium
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I used to heat erlenmeyer flasks directly on a hotplate at earlier era when I did chemistry. I had zero issues because I had zero cracks. Lucky me.
That was also the time I pulled vacuum on flat bottom flasks. I eventually broke them all, with my clumsiness. I now always use heating bath.
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Fyndium
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Are glass frit filters designed to handle ordinary vacuum pump pressure differential? Does size matter, as is 1000mL filter as suitable as 100mL?
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paulll
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Mood: It's fine. Really.
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"Are glass frit filters designed to handle ordinary vacuum pump pressure differential? Does size matter, as is 1000mL filter as suitable as 100mL?"
I'm not sure what counts as,"ordinary," but I pull 50-100mmHg through mine quite a bit and it doesn't seem to care. Shame it's liable to be destroyed
when I finally get around to making an inevitably somewhat clumsy attempt to get the graphite out of it.
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Fyndium
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Ordinary vacuum pumps easily reach from 32mbar to 0.1mbar vacuum of absolute pressure in my books. I have used hand primer pumps, but they are
somewhat PITA when dealing with larger amounts and especially when drying stuff on a vacuum, so just using a pump is much more convenient. I use to
also increase the vacuum volume for example adding a chromatography reservoir between the filter and receiving flask so I need to apply vacuum only
once and let the filter drain on it's own if it is a high resistance filtrate.
Piranha solution or nitric acid seem to be good carbon removers. I just cleaned one frit from carbonous residue with hot HNO3 after filtering nitrites
from thermal reduction. I'm not that afraid of clogging my filters, even CaSO4 is soluble in cold water up to 2g/L so pulling lots of water through a
filter quickly de-clogs it and pretty much all other stuff can be consumed by some chemical.
I always pre-heat my filters with warm water, then hot water and if necessary, finally with boiling water if I need to filter something hot. I've
broken one filter when drying it in oven and I'm afraid that it does it again if I dump hot liquid into one. The frit is not as tough as borosilicate
glass for thermal shock.
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paulll
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Mood: It's fine. Really.
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Makes sense.
Piranha's the plan but it's going to take me a while to get to the big distillation so I'm trying to be patient and not go down the oven route for the
reasons you mentioned.
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Fyndium
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For drying filters (and all other glassware if in hurry and no oven), I nowadays rinse them with little acetone and pull air through. It gets them dry
in a minute.
A good trick I learned just yesterday when cleaning a filter is to invert a hand primer pump and carefully squeeze air backwards through the filter.
It creates a very fine curtain of bubbles and agitates the liquid, and this should be good at dislodging some non-soluble crap and for leaching low
solubility stuff into the fluid.
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SuperOxide
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| Quote: Originally posted by beerwiz | | It happened to me in the early days during my first vacuum distillation. I taped the flat bottom flask as a precaution, then when I pulled the vacuum,
it imploded. Apparently, you should not pull a vacuum on flat bottom flasks. The tape kept the implosion at bay and no shrapnel was
generated. |
Wow... glad I ran across this. I was planning on doing [my first] vacuum distillation, and the receiving flask is a flat bottom boiling flask... lol.
Thanks!
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Fyndium
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I always wonder how NurdRage can hold on his erlenmeyers when doing vacuums. One would expect them to implode on a routine basis.
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Fyndium
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I came up with this video when googling about the subject.
https://www.youtube.com/watch?v=c4VdSuU8TC0&
It seems that glass is phenomenally resistant against implosion even when seriously scratched, if it's compressive structural integrity remains rigid.
Even when they actually made a crack in the glass, it held up.
The final outcome is very violent, though, so when it goes off, you'd better be prepared. To that, add 200C flammable, possibly toxic liquid splashing
everywhere.
I'm sort of more confident pulling vacuum on my chinese flasks, but still the potential for implosion haunts me.
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Fyndium
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What would be the burst pressure of rbf, for example immersed in sea? How many atm would it take to collapse the flask from static pressure? This also
correlates to safety factor for normal vacuum use.
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Grizli7
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The root cause is "resistance of materials" is an applied science that all students hate. The fact is that a ball as a geometric figure works best for
compression, therefore, stresses are evenly distributed in a round flask.
The quality of glass, its thickness, chemical composition are all secondary factors, first of all, the uniform distribution of stresses.
To understand better, do a simple experiment. Take a raw chicken egg, clasp your fingers, set the egg so that one end rests on one palm, and the other
on the other, and try to squeeze the egg. Be surprised.
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FluoroPunch
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| Quote: Originally posted by S.C. Wack | "Thick walled glassware, such as jars, bottles, cylinders and filter flasks, should never be heated on hot plates."
Care and Safe Handling of Laboratory Glassware |
I can attest to this personally from practice. I once used a very nice thick walled 1L vacuum flask for heating since at the time it was my largest
flask.
I was reacting HCl and some Fe nails to make FeCl2 and crystals started to precipitate before the metal all had been digested. The volume
was too large for my other flasks so I decided to use the vacuum flask. Big mistake. After maybe 15 or 20 min I believe I heard a crack and noticed a
fairly large crack formed on the side close to the bottom. I immediately removed from heating and fortunately had no leak.
So no, I do not believe it is a good idea. Vacuum flasks have thick walls to withstand physical stress, which unfortunately due to greater thickness
means greater thermal expansion & thus greater stress. Pulling vacuum on a flask stressed by heat adds even more stress due to the geometry of the
flask i.e. conical, vs round bottom flask, which has better distribution of stress. This is the unfortunate catch 22 of glassware: you make it thick,
its great for banging around and vacuum, you have it thin and its better for heating but physically weaker.
(I should add that the flask was old, donated to me from a lab I worked at and did have some surface imperfections.)
[Edited on 23-10-2021 by FluoroPunch]
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S.C. Wack
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I assume that this glass is all rounded surface, and isn't thick glass with a flat surface being heated or cooled?
This shows specific conditions needed for heating and cooling empty glassware:
SIMAX Glass Thermal Properties
High resistance of products made of SIMAX glass to sudden temperature variations – heat resistance – is given by a low coefficient of mean linear
thermal expansion, relatively low modulus of elasticity in tension (E) and relatively high heat conductivity.
In cooling down and heating a glass product, an undesirable internal stress formation occurs. Cracking may occur in quick cooling down, if the
undesirable internal stress exceeds the admissible limit. Simax glass products‘ values of resistance (D 0C) to a heat shock, depending on the wall
thickness, have been specified in the below table:
Wall thickness (in mm) Resistance to a heat shock (D °C)
1 303
3 175
6 124
10 96
SIMAX Glass Annealing
Annealing represents a thermal process, the purpose of which is preventing the formation of undesirable and unadmissible high thermic stress inside
glass, which could decrease the product resistance, and, eventually, removing the already formed stress.
The annealing cycle consists of three stages:
The temperature rise (product heating) at a heating speed from the inlet temperature to the upper limit annealing temperature.
Dwell on temperature (annealing, tempering, stabilization) of a product for a certain time at the upper limit annealing temperature, when temperature
variations inside the product must be equalized and the stress decreased to an admissible limit.
Temperature drop (annealing and after-cooling) of a product at an annealing speed from the upper limit annealing temperature to the lower limit
annealing temperature (this period is essential, as constant stress may be formed) and from the lower limit annealing temperature to the final
temperature or up to ambient temperature (which is important for the product subsequent practical handling).
Specific annealing cycle is mentioned in the below table.
Range of Temperature
Rise Dwell Temperature Drop
Maximum wall thickness 20 - 550 °C 560 °C 560 - 490 °C 490 - 440 °C 440 - 40 °C
3 mm 140 °C/min 5 °C/min 14 °C/min 28 °C/min 140 °C/min
6 mm 30 °C/min 10 °C/min 3 °C/min 6 °C/min 30 °C/min
9 mm 15 °C/min 18 °C/min 1,5 °C/min 3 °C/min 15 °C/min
12 mm 8 °C/min 30 °C/min 0,6 °C/min 1,6 °C/min 8 °C/min
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Fyndium
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Round bottom flasks are the only type I would subject to vacuum. The exception I have been using are erlenmeyers (2L and 5L so far) for vacuum
filtrations with small membrane vacuum pumps that generate only fraction of vacuum, in order of 10-30%, but these flasks are and will never be heated
or otherwise disturbed. If I need higher power vacuum, I switch the receiver flask to a round one, and use my rotary pump.
I'm extremely scared of heating any glass with direct hotplate or flame nowadays. I just don't do it without a heating bath. Heating baths are also
different, a metal shot bath would be close call to ordinary hotplate, but liquids that are oil or water based are good to go. Their downside is
limited temperature, 150-200C stable max, but other option is to up the vacuum to make stuff boil at lower temps. When the glass is placed in a bath
with thermal gradients far below 100C, thermal stress should remain within spec, unless a very thick walled or non-round glass is used.
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