subsecret
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Thiele Tube vs Beaker
Are Thiele tubes really necessary for testing melting points? I read that the convection of the oil heats the sample evenly, but I fail to see why a
beaker full of heating oil with a stirbar could not be used instead.
Any pointers are appreciated.
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Crowfjord
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You make a fair point. I think that a well-stirred beaker of oil would make a fine substitute, provided your hot plate/stirrer can reach the necessary
temperature to melt the compound of interest. I think Thiel tubes are designed to be heated by flame, which is old-fashioned, but also can reach
higher temperatures (maybe) than an electric hot plate.
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Pyro
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and a thiele tube is smaller, meaning you need less oil and less heat to warm it.
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Crowfjord
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Ah yes, there is that, too. Not to mention less heating time, as well.
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AvBaeyer
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Actually, a beaker of stirred, heated oil is the core of the Thomas-Hoover melting point apparatus. So with modest stirring and a method of controlled
heating, you certainly can use a small beaker. The T-H uses a variac controlled nichrome coil for heating which can be closely controlled.
AvB
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Dr.Bob
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Any simple system will work for most melting points. It is not rocket science. The key to remember is that even the best system will not work well
if heated too fast. That is why some people make a first pass run quickly to get a rough idea of the MP, then go back and go up quickly to about 25
degrees below that and then slowly heat up the rest of the way fir the final run. But while a MP is a useful piece of data, if you are just
comparing two samples or an unknown with a known, if you put them side by side, the main question is do they melt at the same point and in the same
manner. If so, then they are likely the same material with the same rough purity. But if a sample is not pure, even just slightly impure, then
often the MP will be quite a bit lower than for pure material, so it is only a good test for reasonably pure compounds, not crude material.
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aga
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SMelty ?
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Burner
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It all really depends on the "quality" of the data you wish to obtain. To get really precise melting poits requires that you are raising the
temperature of the sample slowly (<1 degree/min) as you enter the MP range. This requires very good heat control and environmental isolation to
limit the effects of transient heat sources and sinks which can alter the desired slow monotonic heating of the sample. Larger thermal masses (such
as a larger beaker of oil) can reduce some of these environmental effects - adding some insulation around the large thermal mass will help greatly
also.
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aga
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SMelty with rockwool insulation ?
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Burner
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Should improve the performance IMO.
“There is a cult of ignorance in the United States, and there has always been. The strain of anti-intellectualism has been a constant thread winding
its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your
knowledge.”
- Isaac Asimov
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