1) What can be said of doing a melting point determination of a sample if it isn't always an indicator of the compounds identity / purity?
Can someone please help me utilize this method. If it doesnt always work, isn't it equivalent to guessing?!?
[Edited on 10/7/2018 by Yttrium2]j_sum1 - 6-10-2018 at 17:42
Usually by the time you get to the point of doing a melting point test you have already narrowed down the possibilities -- typically you are analysing
the product of a synthesis and you are aware of the possible side reactions.
Also you have typically done some recrystallisation and so you have some confidence of purity.
In this situation, a MP test is done as a confirmation. Useful if you are then going to use that material in subsequent syntheses. If the test is
good you can proceed with the next step. If the test comes out negative you go back and try again.
In this situation it is not so much a qualitative analytical technique as it is a means of monitoring your process.
Of course it can be used as an analytical test but generally for determining what the substance is not rather than what it
is. If I buy some vanillin from eBay and I get a MP of 95°C, I know I have been sent the wrong stuff.CharlieA - 6-10-2018 at 17:45
The idea is to measure at least 2 parameters of a substance to identify it. if you are pretty sure of the identity of a substance then the melting
point, measured by any method, is a good indication of the purity of the substance. The trick to measuring an accurate mp by the Thiele method, is to
increase the temperature slowly when near the mp of the substance.Yttrium2 - 6-10-2018 at 19:40
why does it need a triangular shape for convection current?
wouldn't a thermometer/capillary tube in a test tube work?
I read recently somewhere % purity could be determined with a m.p. tube. Information here?
[Edited on 10/7/2018 by Yttrium2]j_sum1 - 6-10-2018 at 20:03
If you happen to have a well-defined relationship between purity and MP, and if you can guarantee what components might be present, then yes you could
in theory determine purity this way. (In much the same manner as sulfuric acid concentration can be determined by density for example.) But it would
require some calibration data to be collected first.Ubya - 7-10-2018 at 00:33
why does it need a triangular shape for convection current?
wouldn't a thermometer/capillary tube in a test tube work?
[Edited on 10/7/2018 by Yttrium2]
by heating the bent part of the thiele tube, you create a flux of hot liquid always hitting the sample, then it would cool a bit, sink, and rise in
the side arm to be heated again a bit more raising the temperature costantly. a test tube would have convection currents in the same regions creating
turbulences, so temperatures are not homogeneous around the sample, and this can give bad mp readings or bad reproducibility.
