Crucible
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thermometer placement in distillation
I bought a 76mm immersion thermometer at the same time as my distillation glassware. But now that I have set it up, I can see that with the
thermometer adapter on top of the distilling head, the opening for the condenser is more than 76mm from the top.
I can plug the thermometer into a stopper directly into the distilling head and it is almost exactly at 76mm, but the extra height of the thermometer
adapter means that more vapor will be refluxed and less collected which should result in a cleaner end product. So I'm torn as to whether I should
use the thermometer adapter and try to compensate for the temp inaccuracy it will introduce or not. Any advice on whether I should try to compensate
or not? Since the fractions are fairly close in temp it may be better to go with higher accuracy.
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Magpie
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The top of the thermometer bulb should be level with the inside bottom of the neck to the condenser. You will find this information in most organic
lab manuals.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Crucible
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Right, I understand that. However, lab thermometers are calibrated to be immersed to a certain depth. If I use the thermometer adapter and push the
thermometer to the opening of the condenser it will be in further than the depth it was calibrated for. If I don't use the thermometer adapter it
will be at the proper depth when positioned in front of the opening to the condenser, but I will have less reflux and a higher rate of flow into the
condenser reducing the purity of my product. Do you see?
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entropy51
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Crucible, the correction for immersion depth can be calculated and applied to the reading. Kimble gives the equation here:
http://208.72.236.210/pdfs/thermometer_corrections.pdf
I think that Magpie's advice is sound. The error due to placing the thermometer bulb too high or too low in the still head is likely to exceed the
stem correction error.
In any case, the stem correction is usually not applied unless you are distilling a very high boiler.
If you really need extreme accuracy then calibrate the thermometer for the depth to which it will be immersed using materials of known boiling point.
[Edited on 4-7-2009 by entropy51]
[Edited on 4-7-2009 by entropy51]
[Edited on 4-7-2009 by entropy51]
[Edited on 4-7-2009 by entropy51]
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Crucible
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Ok, thanks for that advice and the link looks quite helpful. Since you don't mention reflux, I'll assume the ratio of reflux to carry over is less
important than accuracy and proper placement.
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entropy51
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In a fractionation, the ratio of reflux to takeoff is very important, but I can't see how either one is significantly affected by the thermometer
placement. Those first two variables are primarily a function of heat input to the boiler, heat loss in the column and cooling to the reflux
condenser if one is used on top of the fractionating column. But it sounds like you are using a plain still head without a column? If you're strying
to perform more than a gross separation you will likely need a column.
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Crucible
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Oh, I have a column. The reason the reflux ratio is affected by the thermometer is because of the thermometer adapter I bought for this setup. It
adds an extra 30mm to the top of the setup. Everything is oriented vertically, so from bottom to top: RB flask, column, distill head, therm adapter.
My understanding of this is that the extra space above the distilling head (for the therm adapter) means there will be more reflux and less take off.
The problem is that the extra 30mm for the therm adapter puts my 76mm therm at 106mm if positioned at the opening to the condenser where it should be.
Which is why I asked if I should ditch the adapter to put the therm in the right position at 76mm or calculate correction for the wrong position but
get a higher reflux to take off ratio with the therm adapter.
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entropy51
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If you insulate the thermometer adapter well it should not greatly increase the reflux ratio. Fiberglass pipe insulation is good, but other materials
will work for insulation.
Actually a high reflux ratio is conducive to separation. You want to conduct a fractionation slowly.
I think this may not be as great a problem as you are thinking, no matter which arrangement you use.
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