daragh8008
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A simple choice oil or water
HI I was wowndering if anyone would be able to guide me on this choice. I have a reactor that I want to heat. It has a rounded bottom on it which
means I can't just sit it on to my hotplate so I was thinking of turning my plate into a water/oil bath. The temperature will need to be able to reach
at least 95C- max 100C and maintain it at that temperature of upto 12 hours. Would a water bath loose to much to evaporation? requiring constant
topping up. If an oil bath were best for this application is there any particular oil that would be easy and cheap to come by? My local shop doesn't
sell Diphenyl-Dimethylsiloxane. No customers for it apparently! Any suggestions.
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jarynth
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Water is perfect! Of course you'd have to refill every now and then, but it comes with the temperature control you need. Use distilled water, unless
as a side project you're planning on writing a paper about the solid residue of tap water.
If you decide for the oil, the evaporation will be minimal at 100°. Personally I'd use some kind of paraffin. But listen to someone more experienced
first, I never used or needed an oil bath so far.
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daragh8008
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In terms of keeping a constant temperature in the reactor for a prolonged period would topping up the water not effect that. What kind of paraffins
could one get?
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jarynth
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It all depends on how endothermic your reaction and how big your water bath is. You can keep it boiling with very little power. You could even reflux
the water, but that seems like overkill. Are you going to be sleeping the whole 12h, or can you drop by from time to time?
I was suggesting paraffin wax (cheap), around C25. Mineral oil (as low as C15) has a higher vapor pressure. The usual setup should work fine, but
you'll need to think about the temperature control, stirring the bath, etc.
[Edited on 15-10-2008 by jarynth]
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daragh8008
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I'm not really sure how endothermic the reaction is, I've never had to measure it. THe conc of the solution is quite dilute (0.05M-0.1M) and the
reaction is performed in water. The reactor will be refluxed and I suppose I don't want to have to invest in more relfux equipment. I think I'll give
the wax a go. See how I get on with that. Thanks :-)
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watson.fawkes
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As came up recently elsewhere, sand works as a heat transfer medium, and it has no evaporation problems. It's main issue is thermal mass, but you can
preheat it on the stove in a cast-iron frying pan.
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jarynth
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Sorry I didn't mention that alternative. Sand has convection problems. It's safe for working with higher temperatures because it's inert, and it would
absorb your mixture in case the flask breaks.
Seriously though, we should quit hyping sand. Soon we'll be building our houses with sand, and drink our water from objects made with sand 
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xwinorb
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I would say use an oil bath, cooking oil is OK. Use a pyrex container big enough to hold your round bottom BF with space enough left. Keep the oil
bath at 130 C or so, because there is a temperature gradient from the bath to the interior of your BF. The temperature delta will be roughly 30 C, but
will depend on how much you submerge your BF on the oil, among other things. It is best if you monitor both the inside temperature and the oil bath
also, but just the oil bath is fine. The bottom of the oil is hotter than the top.
Water won't be hot enough for that, also too much steam, hard to see things well and annoying.
Careful when disassembling, I would say let everything cool down first.
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DrP
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Water is better at lower temps, but at 100 deg I reckon that's a bit too high for water.
If you did use water - you can slow the evapouration down by reducing the surface area of the water bath by floating polystyrene or platic balls all
over the surface.
\"It\'s a man\'s obligation to stick his boneration in a women\'s separation; this sort of penetration will increase the population of the younger
generation\" - Eric Cartman
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watson.fawkes
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| Quote: | Originally posted by jarynth
Sorry I didn't mention that alternative. Sand has convection problems. It's safe for working with higher temperatures because it's inert, and it would
absorb your mixture in case the flask breaks. |
There's a possible chimney effect with cold sides and a hot
center (assuming a closed bottom). If it's a problem, put a lid on the sand. A simple piece of sheet metal with a hole for the reaction necks(s) would
be sufficient.
The convective properties of sand used to be used to use combustion heating. The exhaust gases would rise up through the sand in a slow updraft.
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