bob800
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overheating hotplate with Al foil?
In this thead, there have been many suggestions of using Al foil to make a warm air bath around the flask to heat it better. However, there's also a
comment on how it's easy to overheat your hotplate that way.
Quote: Originally posted by User  | | One should be careful not to overheat the hot plate, you wouldn't be the first to mess one up. |
I don't quite understand what "overheats" a hotplate or what doesn't. Is there a certain way you have to use the foil to avoid overheating, or does it
mainly depend on the hotplate?
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crazyboy
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I have never heard of a hotplate overheating. I would suggest using a water or sand bath or a heating mantle. Heating a round bottom flask over a hot
plate even with insulation won't work very well. If you are using a sand bath or a mantle and it's losing too much heat try using fireproof insulation
rather than aluminum foil which is a pain in the ass and not very effective unless you really pile it on.
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bob800
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Oil baths seem dangerous to me and apparently take awhile to heat up.
The only problem with a heating mantle is that you have to invest in a $75 variac, and the only thing you can heat is a specific RBF.
Is there any other acceptable ways to heat an RBF? (other than an open flame)
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aonomus
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Here is some further reading for you regarding heating bath types:
http://www.ilpi.com/inorganic/glassware/heatsources.html
To answer the question 'are there any other ways...': for an amateur, not really, in the industry, yes.
There are aluminum heating blocks shaped to fit standard RBF sizes, which sit ontop of the popular Ika/Heidolph/Radleys round top hot plates and
transfer heat quite effectively into your reaction, with no mess. Unfortunately they are really really expensive.
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bob800
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@aonomus:
Thanks for the link.
I finally found some good hotplates on ebay, but now it seems if I want to do any distillations I'll have to pay another $150 for a heating mantle.
In that case do you recommend that I use flat-bottom flasks on a hotplate until I get a heating mantle?
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Magpie
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Oil baths don't have to be dangerous: use non-flammable DOT-5 brake fluid, a silicone oil. Available at your nearest Schucks/O'Reilly for $8/12 oz.
This oil heats up fast.
The single most important condition for a successful synthesis is good mixing - Nicodem
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bob800
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@magpie
If a few drops of water fell into the bath, would it spatter the hot oil everywhere?
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Magpie
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A few drops? I don't think so. I would expect to just see some steam coming off.
The single most important condition for a successful synthesis is good mixing - Nicodem
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ziqquratu
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Water can (will) result in splattering with most oils, I don't know about the DOT-5 stuff, never used it. The trick is to, firstly, not spill water in
it, and second, if you do, warm it in an oven at about 110 C for a few hours - usually gets rid of most of the water without splashing. If you're
still concerned after that treatment, just wrap the bath in foil when you use it next, until you're sure it's not going to spit at you.
If you want them to heat faster, throw a slightly bent paper clip (bend the inner loop so it's slightly raised compared to the outer loop, if that
makes sense) into the bath, which will stir the oil and distribute the heat better (and is much thinner than a normal stir bar, so you can still sit
your flask nice and close to the bottom).
As far as safety goes - yes, oil baths can be dangerous, if you're not careful. Don't, for example, turn your hotplate on high "just to get to the
right temperature" then walk away and forget about it... I've seen a couple of fires start that way. On the other hand, they're used every day in labs
everywhere with minimal incidents.
We use simple paraffin oil in most of our baths, it's good to about 150 C in my experience; for something a bit higher (up to 200 C with minimal
smoke), we use, I think, (partially) hydrogenated coconut oil... Don't ask me where to get that, I've not been able to find any. I've never personally
had any reason to go higher than that, although I'd probably move to sand if I did.
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entropy51
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My experience is that a
little water in an oil bath causes some spattering, but the result is not as disastrous as a RBF breaking over a hotplate.
It would be well to note that if distillations are conducted under vacuum, more often than not a hot water bath will be adequate to connect the hot
plate to the flask. Many solvents such as ether, CHCl3, methanol etc. can be refluxed on a water bath. I know it's old fashioned, but I prefer to
use a sand bath whenever possible. They require patience but they never smoke, spatter, or catch fire.
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aonomus
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My experience is a bit different with silicone oil. Typically an overworked chemist uses a external temperature probe in the oil bath to maintain the
temperature. The assumption is that if you crank the hotplate to the max temperature, and use the external controller to regulate, everything will
work fine. Unfortunately the heating ramp is too fast and it superheats the oil before it can start convecting, causing stratification. Any water at
the bottom gets superheated and bumps alot.
The general advice of heating slowly is fine, although putting an extra stirbar into the oil bath to help circulate it also helps. It breaks up a
droplet of water at the bottom of the bath, suspending it, and giving it a chance to slowly heat up to 100degC, and boil off closer to the surface in
a more controlled fashion (ie: smaller bubbles).
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bob800
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"It would be well to note that if distillations are conducted under vacuum, more often than not a hot water bath will be adequate to connect the hot
plate to the flask."
This is probably a dumb question, but are all RBFs, condensers, adapters, etc. designed to be under a vacuum, or is special vacuum-safe glassware
required?
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mnick12
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It depends on how strong of a vacuum your pulling. Generally speaking it is advised to use a rbf when doing vacuum work. Though if you have a well
made fbf, in my experience it works fine. So regular distillation equipment is good for most vacuum work.
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entropy51
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Not a dumb question at all! I have never had a problem with applying vacuum to glassware with standard taper ground joints. I think the modern
ground glassware is designed for vacuum use. This applies to round bottom flasks, I wouldn't try it with conical flasks unless they were of very
thick glass. I would also be careful with older glassware that wasn't ground joint.
However it's a good idea to inspect glassware for cracks and scratches before using it under vacuum. I test new glassware under vacuum with a towel
wrapped around it (empty, no heat) before I use it the first time.
Avoiding thermal stresses is probably also important in avoiding accidents. I'm pretty careful about applying an open flame to glassware under
vacuum.
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Magpie
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| Quote: Originally posted by entropy51 | | water bath. I know it's old fashioned, but I prefer to use a sand bath whenever possible. They require patience but they never smoke, spatter, or
catch fire. |
I've never used a sand bath but it might be a good solution to a small problem I'm now having. I'm using a silicone oil bath at 200C. At that
temperature there is some small amount of smoke (or vapor) and odor. If it wasn't for my fume hood I would have to stop.
Would a sand bath be a good alternative for that temperature? What type of sand is recommended?
The single most important condition for a successful synthesis is good mixing - Nicodem
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entropy51
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Magpie, I have run sand baths up to around 200 C. I think the sand I'm using was sold for use in children's sandboxes as "play sand". If it's dirty
it will smoke a little the first time it's heated, but that's as exciting as it gets.
I think my nice ceramic hotplate says not to use a sandbath on it because sand is not a good conductor. So for high tempeatures I use the sand in a
steel bowl on an old fashioned hot plate with a spiral nichrome heating wire, controlled by a variac.
[Edited on 26-9-2010 by entropy51]
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Rogeryermaw
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i was planning on using synthetic motor oil. i've seen it get pretty hot without any problems. any thoughts on this? any brand you all think would do
best? mobile 1 is big name and works well in engines...i have also used royal purple in my vtec honda engine with mild turbocharging and it seems the
hotter you get that oil the more it likes it (within reason).
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Magpie
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Thank you entropy. That sounds like a good plan, ie, using the calrod cheapo hotplate.
Calrod elements are made of a steel tube filled with MgO and the nichrome wire that obviously can take red heat. Also, these hotplates can be
cannibalized, removing the calrod element and bending it into a more desired shape. I recently bent one into a sine-wave of 1" amplitude and 4"
period. Ie, bends of 1" radius.
[Edited on 26-9-2010 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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unionised
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Barely.
If you have any meaningful vacuum then the force on the equipment is pretty much the same.
760mmHg - 20 mmHg is pretty close to 760 mmHg - 0.0001 mmHg.
The difference is less than that caused by the weather.
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Mr. Wizard
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Heating elements can be ruined by overheating. The more powerful and higher wattage they are, the more easily and quickly they can fail. Heating
elements either count on the heat being removed quickly, by some medium or method, or have a thermostatic device to cut the heat production. As an
example, the heating elements in a hot water heater draw 2 or 3 KW. If this heat is not efficiently moved from them, they will fail in much less than
a minute. They count on the heat being quickly removed by the water, and the water's heat activating a thermostat, which is not directly connected to
the heater.
I have seen water bed heaters catch a bed on fire when the flat heating element was separated from the plastic bag by a photo album. The heat
generated had nowhere to go.
If your hotplate has an interruption device, such as a bimetal thermostat, and a metal heating surface, I'd guess it would be safe no matter how you
insulated it. If it is a continuously variable device, controlled by a variac, or triac type regulator, it will not shut off if it gets too hot. It
may 'fail safe' due to a high temperature fuse included in the heating element, as hair dryers do. Hair dryers are another device that run full bore,
and if the heat is not removed quickly, they fail. The fail safe device can be replaced, if you can find one. They normally don't reset on cheap
equipment.
A small 7 watt decorator light bulb will catch fire if you wrap it with paper or cotton, keeping those few watts of heat in one spot. The
manufacturers count on free air flow to keep it functional.
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Magpie
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I accept what Mr Wizard is saying about burning out elements due to overheating. However, I have some recent experience in calrod heating that may
be interest:
I placed the sine-wave calrod element (see above post) against a large steel heat sink and secured it tightly with a radiator clamp. This was then
covered with a double layer of aluminum foil. Over that I placed about 2" of fiberglass insulaton. I then heated this 120V, 1000w element using a
variac. The variac was set at 30%. I measured the amp draw at 3.6a, so the power delivered was 120(3.6) = 432w. After 2 hours of heating I shutdown
the heating. There was no apparent damage to the element.
I present this information only as a single data point - not as a claim that calrods can be insulated and heated without consequence in all
circumstances.
The single most important condition for a successful synthesis is good mixing - Nicodem
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