Sciencemadness Discussion Board

Ethylenediamine query

HydrogenSulphate - 7-8-2021 at 03:37

Hi all,

I have what I believe to be ethylenediamine, 99%. While it certainly has a characteristic odour I'd expect (ammonia-like), I am baffled as to why it has not solidified at the subzero temperatures I'm storing it at. Its MP (literature value) is quoted to be in the region of 8 to 11 degrees C.

I have no particular reason to desire it in its solid form; it is just that I decided to store it in a repurposed chest freezer, considering that I could detect a very faint odor (suggesting a slight leak) around the cap of the original glass bottle it was in (despite being stored in a cool place, well out of direct sunlight). Leakage, no matter how minor, means loss of precious reagent over time, which I wish to minimize. So, I transferred it to a Simax borosilicate reagent bottle with a PTFE-lined cap (the red cap), then I coated the bottle in several layers of clingfilm/Saran wrap to protect the fragile glass against accidental knocks.

Its liquid volume has reduced a little since being stored below zero degrees C (some contraction, which is to be expected), but it has not solidified, as I thought it would have, considering the literature MP. Maybe factors such as nucleation sites (a lack of, in this instance) are the explanation? Also, I've noticed that it does not fume at all (well, not to any noticeable degree) in humid air (as high-conc ethylenediamine is 'supposed' to., according to literature).

Maybe its concentration is less than the stated 99%? I was planning on making some en-ligand organometallic complexes with it, but I think I'll perform some chemical identification tests beforehand.



[Edited on 7-8-2021 by HydrogenSulphate]

Bedlasky - 7-8-2021 at 05:05

Melting point isn't the same as freezing point. For example, glycerol melts at 17,8 °C. But it doesn't freeze at this temperature. I tried it myself - put 99,5% farmaceutical grade glycerol in to the fridge and didn't freeze. I asked about this behaviour my friend which teach physical chemistry at high school and he told me, that melting and freezing point actually doesn't have to be at the same temperature. Look at this:

https://en.wikipedia.org/wiki/Melting_point

We tried to freeze glycerol using liquid nitrogen and it work. Solid glycerol is glassy like material.

I never worked with pure ethylenediamine, I have just its hydrochloride salt. What I saw on photos, it should fume. But I sometimes work with diethylenetriamine and this compound doesn't fume. On the other hand it have 100 times lower vapor pressure than ethylenediamine (according to wikipedia).

[Edited on 7-8-2021 by Bedlasky]

AvBaeyer - 7-8-2021 at 18:20

Ethylenediamine is rarely found anhydrous. The mp/fp quoted is for the anhydrous material. Usually it is sold as the equivalent of a monohydrate although it may contain even more water. It is very difficult to make and keep anhydrous. That has been my experience. I have the "monohydrate" material and it only fumes in very humid air.

AvB

woelen - 8-8-2021 at 01:38

I do not know anything else than anhydrous ethylene diamine. I have purchased this chemical 3 times already (I used it quite a lot for coordination chemistry experiments) and it always is anhydrous. It fumes heavily, even in quite dry air. The fumes, however, are remarkably mild. They are not nearly as pungent as e.g. ammonia gas or HCl-fumes. Ethylene diamine has an ammonia-like odour, but much less pungent and a little more 'soil-like', 'organic'.

unionised - 8-8-2021 at 04:05

Quote: Originally posted by Bedlasky  
Melting point isn't the same as freezing point. For example, glycerol melts at 17,8 °C. But it doesn't freeze at this temperature. I tried it myself - put 99,5% farmaceutical grade glycerol in to the fridge and didn't freeze. I asked about this behaviour my friend which teach physical chemistry at high school and he told me, that melting and freezing point actually doesn't have to be at the same temperature. Look at this:

https://en.wikipedia.org/wiki/Melting_point

We tried to freeze glycerol using liquid nitrogen and it work. Solid glycerol is glassy like material.

I never worked with pure ethylenediamine, I have just its hydrochloride salt. What I saw on photos, it should fume. But I sometimes work with diethylenetriamine and this compound doesn't fume. On the other hand it have 100 times lower vapor pressure than ethylenediamine (according to wikipedia).

[Edited on 7-8-2021 by Bedlasky]

The melting temperature and the freezing temperature are the same.
What you have measured is the temperature at which it doesn't freeze.
That isn't the same as the temperature at which it does freeze.

You have prepared supercooled glycerine which is no surprise glycerine supercools very easily (in part because it is very viscous).

If you took your cold glycerine and put a tiny seed crystal of glycerin into it the liquid would freeze and the temperature would rise, possibly as far as 17.8 C (as long as it doesn't start off too cold.)

The melting and freezing points are the temperature of the solid and liquid in equilibrium.

That mixture does not have a memory. It can not know if you are adding or removing heat.
So it can not know if it it melting or freezing.
So how could it know which temperature to choose?

Bezaleel - 9-8-2021 at 07:15

It's a phenomenon similar to supersaturation. For solids to start to form, a nucleation point will help. This could be a rough surface, a piece of dust or an impurity, for example.
You can cool pure water to -30C and as long as you continue to move the vessel, it will not solidify.