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Date: | Wed, 17 Nov 2021 16:03:26 +0000 |
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The melting point of anhydrous oxalic acid is 189 to 191 deg C.
https://www.google.com/search?client=firefox-b-1-d&q=melting+point+of+oxalic+acid+
For those who have never run a melting point I will describe a couple of methods. The most common is to put a small sample to be tested in a glass melting point capillary tube that is roughly 0.5 mm ID. The bottom is flame sealed. You put this capillary in an oil bath or in a heated copper block and raise the temperature fairly rapidly up to about 20 deg C below the melting point. Then you slow the temperature rise to about two deg C/min. You can watch the solid naked eye or thru a magnifying glass or low powered scope. You record the temp at the very first sign of liquid and again when the last of the solid just disappears. Another kind of apparatus is a glass surface on top of a heated block with a glass cover slip and watch thru a low powered scope. Same sort of temp profile. A reported range of two degrees C by these methods is quite normal for a decently pure substance. As a sample becomes more impure the melting range will both drop and get wider. So it is a decent quick test of purity. There are also more accurate methods such as scanning calorimetry that can determine melting points, but that equipment is fairly expensive.
At any rate, I see oxalic fog coming off both the pan sublimers and the Provap at temperatures below the melting point so we are indeed sublimating oxalic acid and not boiling it. And I have seen without question fog coming off a solid in a pan sublimer.
This is supported by:
https://pubchem.ncbi.nlm.nih.gov/compound/Oxalic-acid#section=Boiling-Point
In section 3.2.4 titled Boiling Point they state anhydrous oxalic acid does not boil, rather it sublimes.
Also this ref:
https://www.chemicalbook.com/ChemicalProductProperty_US_CB0323998.aspx
They list an estimated boiling point of 365.1 deg C. No vaporizer gets any place close to that hot. So, once more this ref says we are indeed not boiling it. We must be either subliming it or evaporating it. Perhaps a bit of both depending on heating rate? I would assume they estimate the boiling point by taking vapor pressures at much lower temperatures which is easy enough to do but tedious as it can take weeks per data point. From those determine things like the shape of the heat of vaporization curve and entropy of vaporization curve from which an extrapolation to higher temperatures and pressures can be done mathematically. Due to rapid decomposition at such a high temperature you have no choice other than do the extrapolated estimate as it is impossible to do a direct measurement. They report an estimated boiling point to a tenth of a degree and I doubt very much if their data is that good. More likely it is only good enough for +/- two or three degrees.
Dick
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