I decided this belonged in 'beginnings' because I'm sure it's a fairly basic, and quite irrelivant question.
Oh, and it's not a homework question...
My problem is my beakers, when I put them on a hotplate and heat them to ~102C, the start to make unsettling 'BANG' noises, and jump up and
down on the hotplate. I've had problems with super heating before, and as a result, i put crushed glass AND Silicon Dioxide in my beakers before
boiling - and bubbles seem to form pretty easily.
The problem is that these explosions are quite violent (the problem is eliminated in my furnace and on my super-hot-bunsen). I think it may have
something to do with the small bubbles forming so fast that they make a shockwave which bounces off the hotplate and onto the beaker. It's a bit
of a wierd explanation, but it's the best i can think of.
Does anyone have any reasons or, even better, solutions?
Thank you in advance for any replies
Sincerely
-Ramiel
Fiberglass cloth (woven).....lovely stuff
Hermes_Trismegistus - 10-2-2004 at 09:09
Go down to any store that carries woven fiberglass (like a boatswain's suppy house) or any place that sells supplies to boatbuilders or whatnot.
You are looking to get yourself a piece of bi-axial woven fiberglass a little bigger than the surface of your hotplate. Place it between the hotplate
and the beaker. It will stop the beaker doing the jitter bug.
Bi-axial woven fiberglass cloth is great stuff. Soft and supple, it doesn't give off much glass shards unless you play with the frayed ends too
much. I use it in a lot of applications where the old books call for asbestos cloth.
*I am assuming you're not just overlooking the possibility of there being water droplets on the underside of the beaker...of course you're
not...right....*
also, it just occurred to me that when you say hotplate, you don't mean a hotplate/stirrer combo. You might mean a cheap walmart hotplate. In
which case the problem is probably related to uneven heating due to the spiral element. If this is the case either put a metal topper on (1/4 inch
steel plate) or use a pot filled with sand or zinc shot. these will certainly solve the problem but beware!...don't expect the lightning fast
heatup times you are used to from cooking on a stove top......
if you are using a proper scientific hotplate (and they are often underpowered) and you find you are getting too much parasitic loss from your heating
setup... and the final temperature of your beaker doesnt get high enough...you may want to add a heating shroud over your beaker. made from bi-axial
fiberglass cloth of course!BromicAcid - 10-2-2004 at 13:18
Quote:
My problem is my beakers, when I put them on a hotplate and heat them to ~102C, the start to make unsettling 'BANG' noises, and jump up and
down on the hotplate
Does this just occure during certain reactions? I remember way back in high school we did a lab called, the resurection of copper. One of
the steps called for heating of a thick precipitate till it dissolved. Out of the whole class only 2 of the 15 or so beakers made it. For some
reaon, on this one step everyone's beakers were jumping around and breaking from landing. Didn't happen to me because I held down my beaker
with a ring stand after I saw everyone else's going crazy.
My teacher had a term for it but I would say it was due to the thick precipitate, so what conditions do yours do this? All? Are they boro-silicate
glass? Also, if you go with the fiber glass or sand bath or whatever be sure to note that both of these insulate the top of the hotplate and if you
crank it up really high it can kill youre heating element.tom haggen - 10-2-2004 at 21:45
My beakers make unsettling noises when i'm boiling water. I think it has do to with the percussion made by the bubbles. It gets louder when I use
a bigger beaker. I just want to get a ring stand and start using oil baths.Tacho - 12-2-2004 at 10:17
Do you put broken glass and silicon dioxide to act as boiling stones and prevent bumping?tom haggen - 12-2-2004 at 15:57
I don't like using broken glass it takes to long to heat up.Tacho - 13-2-2004 at 03:16
Sorry tom, I meant to ask Ramiel if he was using broken glass as boiling stones because AFAIK neither broken glass nor silica would work as boiling
stones.
This problem sounds so much like common bumping during boiling.
I would hate to sound too basic if you actually have a more complex problem but boiling stones have to be pourous, they have to retain some gas on the
surface to promote smooth boiling.Ramiel - 13-2-2004 at 04:03
aah, okay. So diatomous earth is the go? I just didn't think it was superheating again because bubbles form quite easily in saturated solutions.
Thanks TachoTacho - 13-2-2004 at 09:55
Ramiel,
Just try putting one or two pieces of dry porous ceramic in your solution before you heat it.
Pieces of a broken coffee mug or plate, the size of a pea. Or chips of a brick.
I think you will see a miracle happen.axehandle - 13-2-2004 at 10:42
Or what about putting an aluminium net (the kind used as insect shield) between the hotplate and the beaker?Tacho - 13-2-2004 at 15:26
Believe me axehandle, there is NO substitute for proper boiling stones when heating a solvent to ebulition in glassware. When I say solvent I include
water.
People, do a quick google search on BOILING STONES or check on any practical organic chemistry book. I gess Polverone has put Vogel`s available to
download.axehandle - 13-2-2004 at 19:45
I believe you. Would clay/air pellets (as in the ones used in flower pots to highten the porosity) do the same trick?
(As a side issue, why is it that every chemical textbook worth reading was written by a German?)
(As another side issue, why were most of the elements discovered by Swedish chemists? =)
[Edited on 2004-2-14 by axehandle]
[Edited on 2004-2-14 by axehandle]
Boiling stones.
Hermes_Trismegistus - 13-2-2004 at 22:19
you certainly can use just about anything for boiling stones. The purpose of boiling stones is as a nucleating agent.
(I once asked my lab tech (masters in chem) what boiling stones are made of. So I did some research.
Turns out that until about fifty years ago, boiling stones were all just little peices of broken teacup from the cafeteria.
Now they are usually made of fused alumina. (broken alumina balls from making the balls used in ball mills)
but they really aren't high tech.
Still you may want to go for the porcelain instead of the clay. Being vitrified it would most likely be alot less soluble (the only real
consideration)
As for the German thing. You may have noticed ( as a student of history) that certain regions went through types of vogues where certain endeavors
were fashionable.
While the English and French were busy with the Naval Arms race the Germans were taking brewing and the art of heat-resistant glassware to a new art.
When the English began to rule the world because of thier industrial prowess, the Germans were the undisputed master chemists.
If you wanted to be a mechanical engineer of the day, you moved to England, learned to speak english, attended British schools and apprenticed to an
Englishman. It is because of this fascination with prime movers that we now have the science of thermokinetics (and thermochemistry in general).
If you wanted to be a chemist.........Germany.
This regional specialization is just now beginning to break down because of the transportations/communications revolutions.
But still some skills remain regionally specialized.
Like say you want to get a diamond cut?
Antwerp.
or jade carved.
Hong Kong.
Or silver worked ...Mexico
(in north america)
or gold?
or crystal cut?
or haute couture fashion?
or tulips?
.....etc......etc
even now there are places that are becoming new mecca's
Like, for instance, more than 70 percent of the worlds spammers operate out of Fort Lauderdale, Florida
70%!!!!
if you want to be the best of the future, you gotta go where the best are today.axehandle - 14-2-2004 at 00:45
Hermes_Trismegistus: Your discourse makes sense and explains much.
And I'm in luck when I might want some boiling stones then, I've got LOTS of broken ceramic cookware which I've saved (in case it might
be useful one day (I'm an astute saver =)).Tacho - 14-2-2004 at 01:50
Quote:
Originally posted by Hermes_Trismegistus
(snip)
Still you may want to go for the porcelain instead of the clay. Being vitrified it would most likely be alot less soluble (the only real
consideration)
(snip)
Vitrified? Well, maybe. Just make sure it has some porous surface. I think it has to have some air trapped in.
If you boil a solvent, cool it, and boil again, the "used" boiling stone won't work. Boiling stones also don't work in vacuum
distillations, I never understood why.
Vogel's "Textbook of practical organic chemistry" third edition, is available somewhere to download. It's a masterpiece, download
it. There you will find a whole section dedicated to bumping, boiling stones and such.
Edit1 - Hermes, what you said about germans and chemistry in the late 19th century and early 20th century is a remarkable note in history. Germans
were far ahead of the rest of the world.
I just read a book named "Magic in a bottle" published in 1941. Its about the early history of medicines. Althogh it was writen by an
american and published during the war, it's quite balanced and one gets stunned to see what germans could do so early in the history of organic
chemistry.
Edit2 Eventually, as it happens so often in history, germans found a way to screw it up big time.
[Edited on 14-2-2004 by Tacho]
[Edited on 14-2-2004 by Tacho]unionised - 22-2-2004 at 07:14
IIRC this is why things bump when you boil them.
According to the textbooks a liquid will boil if it is heated until its vapour pressure is equal to the pressure of its surroundings. OK, but in order
to boil it has to start of making bubbles. These bubbles start very small and the surface tension of the liquid raises the pressure in the bubbles.
This, in turn, means that the bubbles can only grow when the vapour pressure is large enough to overcome the atmospheric pressure and the surface
tension.
Once the bubbles start to grow the effect of the surface tension is reduced,
(for those who care, the equation is
p(bubble) = P bulk EXP(-2gamma (v(l))/rRT)
Where r is the bubble radius; v(l) is the molar volume of the liquid (IIRC) and gamma is the surface tension.
Anyway, once the bubles grow it is easier for the liquid to vapourise, so it does so violently. That explains the problem of bumping.
Solving the problem means providing a bubble for the boiling to start from. Any porous material would do this, Sintered glass is fairly common and I
have even seen teflon boling stones.
If you try to do a vaccum distilation the air gets lagrely sucked out of the holes in the porous material so it doesn't work so well.
(Bizarely, a matchstick works quite well. The air trapped in the fibres takes a while to get sucked out.)
This also explains why boiling stones don't work if the liquid is cooled then reheated. While the liquid is boiling, the vapour of the solvent
mixes with the trapped air and the air escapes with the bubbles. When they cool the stuff condenses adn there is no longer a bubble there to start the
boiling.
Final word, if you forget the boiling stones or anti bumping granules and heat a liquid without them please remeber to cool the liquid until it is
well below the boiling point before you add the anti bump. Otherwise you will get scalded when the liquid bumps suddenly as you add the nucleation
source.
(OK that's more than one word)tom haggen - 29-7-2005 at 19:20
If I used an unpainted broken coffee cup for boiling stones, how inert would this material be?Fleaker - 29-7-2005 at 19:37
If it's not glazed you should be ok, porcelain is typically inert to most chemicals the exceptions being hot alkalis and HF.
I use grog from broken up refractory bricks and they work fine and last really long.frogfot - 29-7-2005 at 22:30
I've tested to use a glassrod (pointed with one end into the bottom) when boiling aqueous solutions in a beaker.. Bumping is reduced alot and the
rod can be easily removed at any time.
I guess the bubbles are formed when the rod is rubbed against the beaker. The rod moves from the energy formed when bubbles are evolved at it's
end.. I guess this process is self sustaining...Eclectic - 30-7-2005 at 06:57
Chips of teflon carved from a solid chunk will work as boiling stones.bio2 - 30-7-2005 at 14:50
.........glassrod (pointed with one end into the bottom) when boiling aqueous solutions........
Those long bamboo skewers that look like a giant toothpick work even better!tom haggen - 30-7-2005 at 19:27
I believe that most coffee cups are made out of ceramic, and not porcelin. Anyway it seems like placing chunks of porcelin in a glass container would
be a bad idea. Have you ever seen what porcelin chunks from a spark plug does to safety glass?
Also solid chunks of teflon are hard to come by in my neighbor hood
[Edited on 31-7-2005 by tom haggen]Fleaker - 30-7-2005 at 19:39
I have porcelain and ceramic coffee cups come to think of it :-) I don't know how much damage pea sized chunks can cause in a thick-walled,
quality beaker. Having thus far used scores of different hotplates, I've never had that problem, at least with the severity you fellows talk of.
Ramiel, perhaps you're just heating it too strongly since you theorize that the bubbles are forming so quickly.Magpie - 30-7-2005 at 20:10
I have used pottery shards (as recommended by Organikum) with good success. I always place one or two split pea sized pieces in my boiling flasks.
This is especially important with liquids of unknown boiling characteristics. Place the shards in before heatup. If you forget and
place the shard in after it has been superheated you likely will end up with the liquid on the ceiling, or worse, in your face.
My understanding is that the porous pottery contains trapped air which is released as microbubbles as the heating proceeds. These bubbles provide the
necessary nucleation sites for boiling.
Even a splinter of wood will work and is recommended for some experiments.
When the shard cools in the liquid it will suck back liquid into its pores instead of air. So as Tacho says it won't work anymore.
[Edited on 31-7-2005 by Magpie]Cyrus - 30-7-2005 at 21:47
Just as a clarification, the ceramics you'll find in coffee cups and such are all probably inert. They are composed mostly of alumina and
silica. Some may have small percentages of other compounds, but just as glass has Na2O that just sits there, these other compounds usually won't
be a problem. Glazes can be poorly formulated or fired, or just not made to withstand boiling acids and such, so be careful with them.
The following is taken from answers.com.
"ceramics (sərăm'ĭks) , materials made of nonmetallic minerals that have been permanently hardened by firing at a high
temperature, or objects made of such materials. Most ceramics resist heat and chemicals and are poor conductors of heat and electricity. Traditional
ceramics are made of clay and other natural occurring materials, while modern high-tech ceramics use silicon carbide, alumina, and other specially
purified or synthetic raw materials. Ceramic materials are used in all forms of pottery, from crude earthenware to the finest porcelain, and in
industrial and engineering products."
Anyways, porcelain is a subcategory of ceramics. But it probably won't work as well as other more porous ceramics because it's pretty
dense. Also, ceramics are pretty dang hard, and I'm sure some will scratch glass (alumina, for example), sometimes, but I'm not sure that a
gentle boil will be enough to ruin a flask. I like the idea of teflon though- I've got plenty, it's inert, soft, and it's reusable.
12AX7 - 30-7-2005 at 23:18
Porcelain is like glass, actually it's mostly glass, that's why it has so little porosity. Average glazed ceramic is somewhat porous (like
how the unglazed bottom ring of your coffee cup is rough) and would work very well. As for scratching, borosilicate is harder than soda-lime IIRC,
which should then also be harder than most pottery. Alumina and such will definetly be able to scratch it though.
Huh, the main crystalline phase present in ceramics - mullite - is specified as mohs hardness 6-7 on one website, and 7-8 on another. Soda-lime glass is 6.5, IIRC.
TimEclectic - 31-7-2005 at 05:50
Teflon scrap is easy to find on Ebay. I think it works because of low wettability and low surface adhesion. Microbubbles form easily along sharp
edges.
boiling chips
demax - 7-11-2007 at 16:00
Max has some boiling chips and is looking to reflux some reagents.
max will reflux different quantities of liquid over the next week and is not sure what weight of boiling chips to add to say 500ml, 1000ml or 2000ml
of liquid.
If someone could help me please.
-maxvulture - 8-11-2007 at 01:31
Usually it's sufficient if the bottom of the recipient is covered in a layer of boiling stones, as that is where boiling likely starts.
Soda-lime glass is 6.5, IIRC.