I just got a hot plate / stirrer. I find the heating of flasks to be somewhat unsatisfactory (compared to that provided by a flame). This is expected,
but I'm wondering: are there any generally used methods for improving heat flow from hot plate to flask? For example, does anyone use thermal paste in
this application?BromicAcid - 2-1-2010 at 23:07
The first big question is round bottomed or flat bottomed flasks?User - 3-1-2010 at 05:29
Placing a piece of stove pipe on top of the plate effectively decreases the heat loss by surrounding air.
A 15/20 cm pipe which diameter is slightly smaller than the plate itself works well.
One should be careful not to overheat the hot plate, you wouldn't be the first to mess one up.
[Edited on 3-1-2010 by User]bbartlog - 3-1-2010 at 10:05
Flat-bottomed boiling flask. Which is still less contact area than I would get with an Erlenmeyer, especially since the bottom of the flat-bottomed
boiling flask I have is actually slightly concave. An actual round-bottomed flask seems like it wouldn't work at all.
The stove pipe idea sounds good. Should surround the flask with hot air. Could even neck it down on top to try and reduce the airflow.matei - 3-1-2010 at 10:18
Place the flask in an oil bath! It will take longer to reach the desired temperature but it will heat the flask uniformly.smuv - 3-1-2010 at 10:46
Make an aluminum foil house around the flask with the hot plate's heating surface as the floor. In this way you are essentially creating an
air bath. This does a satisfactory job allowing flasks to heat up faster and reach higher temperatures.
@Matei oil baths are great, but are also messy and often smell. As I have become a lazier chemist I have started to avoid them when possible.aonomus - 3-1-2010 at 11:22
I have recently been distilling about 25L of dilute solvent in a 3L RBF, which requires a ton of heating. Warmup time is 2 hours!
I initially started with a large glass crystallization dish as the container for the oil bath, but it was too shallow, so I tried out a stainless
steel mixing bowl with mineral oil instead. The steel container had better heat transfer, but a unforseen disadvantage. The center of the bowl heats
up and expands more than the surrounding steel, and turns convex, making the bowl somewhat unstable. With the hotplate/bath jacked all the way up and
in contact with the flask its fine, but if its even a little loose, for some reason the heat causes the metal to expand and contract in such a way as
to start the bowl rocking back and forth like crazy.
Also, with oil baths, you need to be careful and not heat the oil up too much, different oils have different vaporization temperatures, which also
determine how much they smell. They do however over time slowly oxidize or polymerize and become thick, brown, and nasty.
Another tip like mentioned by smuv is to use aluminum foil above the flask, this has 2 advantages. The first is that it insulates and creates a pocket
of warm air around the flask, preventing high heat loss to air. The other advantage is that it prevents any water droplets from leaks, or just random
occurrences (eg: drop from a wet glove) from entering the oil bath. Water will sink to the bottom of oil baths and then 'bump' due to general lack of
nucleation sites, and being superheated at oil bath temperatures.
Another way of doing this is taking 2 steel bowls of the same size, making sure the flask fits in entirely, cutting out a hole in one, and making a
'lid'. The whole assembly will probably be hard to assemble, but it would probably insulate better, since the two bowls in contact will transfer heat
from the hotplate into the upper bowl, making sure the air around the flask is hot.User - 3-1-2010 at 12:18
The downside of using metal is that it often messes with my magnetic stirring.
For this reason i switched to using ceramic containers , the ones used for oven application work quite nice.
The heat distribution is very smooth, thats an advantage.anotheronebitesthedust - 3-1-2010 at 22:17
Quote:
Make an aluminum foil house around the flask with the hot plate's heating surface as the floor.
This actually works very very well. During a vacuum distillation of glycerine, with my hotplate cranked to the max temp., it took like 8 hours to get
only half of the expected yield. I decided to wrap the flask in aluminum foil and almost instantly, the glycerine began flowing through my condenser,
and the other half came over in less than 30 minutes. The more aluminum I used the quicker it distilled.densest - 5-1-2010 at 22:04
The aluminum (aluminium) foil trick is very useful! Some glass wool inside the foil (think cheap unfaced fiberglass from the big box hardware store)
and you have a very good heat container.
I always cover Vigreux columns, etc. like this and get very sharp fractions.
A closed foil "house" containing the top of the hot plate and the flask or lower part of the flask will transfer a great deal of heat to the flask.
It's easy to overheat using this!
[Edited on 6-1-2010 by densest]unionised - 6-1-2010 at 01:31
There's a "cheat" for getting flat bottomed flasks to heat quicker on a hotplate if you are trying to get them up to some temperature below 100C.
Add a few drops of water to the hot plate. Even with the best of flasks, the base is only in contact with the hot plate at a few points because they
are not perfectly flat. With a little water in the gap, the heat is conducted by water rather than by air and water is a much better conductor.
Eventually the water boils and the steam then refluxes between the plate and the bottom of the flask- that's a lot better for transfering heat than
air is.
A layer of foil round the flask to keep the draft off helps a lot, particularly if you are working in a fume cupboard.User - 6-1-2010 at 01:59
Quote:
A layer of foil round the flask to keep the draft off helps a lot, particularly if you are working in a fume cupboard.
Indeed it is amazing what airflow can do.
When i do reactions involving steam I often use a ventilator to drive away the clouds and to keep moisture levels from skyrocketing in my small
working area.
When the fan is turned in direction of a boiling flask it actually drops below boiling point.
Used it a couple of times for quicker cool downs.
My experience with glass wool in the lab isn't all that good.
I had a couple of melts in contact with one of my plates that thing gets insanely hot , but still.
Iam not sure but rockwool can reach up to higher temps right??
[Edited on 6-1-2010 by User]12AX7 - 6-1-2010 at 10:46
Well, if you want to maximize heat transfer, first of all construct an aluminum pot with the desired dimensions. It might have a round hollow for
your RBF, a cylindrical shape for general use and holding liquids (e.g., oil bath), or just a flatter plate for better heat transfer (flat bottom
beakers/flasks). Whichever the purpose, clamp it to your hot plate with a suitable silicone based thermal compound.
As long as you've got your quart of silicone oil sitting around, pour some around your RBF in its aluminum mantle. This will suck it in place to some
extent, while maximizing heat transfer.
Or use a microwave magnetron, suitably shielded. Who needs conduction.
