soma
Hazard to Others
Posts: 297
Registered: 31-7-2010
Member Is Offline
Mood: No Mood
|
|
thermal dissociation of LAH
I have read that LAH thermally dissociates when dissolved in ethyl ether. I'm wondering what this means in terms of actual behavior.
Help appreciated.
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
As with dissolution in THF, there is a large enthalpy of solvation to take into account.
As a related aside, I'm kinda surprised that purification can be effected by soxhlet extraction with ether...
[Edited on 22-4-2015 by DJF90]
|
|
|
soma
Hazard to Others
Posts: 297
Registered: 31-7-2010
Member Is Offline
Mood: No Mood
|
|
Thanks.
Yes - it seems the impurities aren't soluble in ether and can be filtered out. Actually, I was thinking of dissolving it and then pouring off the
supernatant, taking a small aliquot, evaporating off the ether in a water bath (at 70C), and weighing, to get an idea of the activity of the LAH
without having to try to measure the amount of hydrogen generated.
I'm not sure how long to give it to dissolve though.
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
| Quote: | | As a related aside, I'm kinda surprised that purification can be effected by soxhlet extraction with ether... |
What I meant was, given the large enthalpy of solvation, I'm suprised that it is a safe method of purification. Normally when preparing LAH solutions,
you charge LAH to the vessel and charge the required volume of solvent down the condenser as fast as practicable. Even then it may end up refluxing
due to the exotherm (THF as solvent) so I'm cautious about having a small stream of ether dripping onto solid LAH in a soxhlet extractor. I'd be
interested to hear from members here who have experience of this procedure.
Quote: Originally posted by soma  |
...taking a small aliquot, evaporating off the ether in a water bath (at 70C), and weighing, to get an idea of the activity of the LAH without having
to try to measure the amount of hydrogen generated.
|
That seems like a fairly long winded approach when you can just titrate an aliquot. Supposedly iodine can be used, but personally I'd opt for
salicylaldehyde phenylhydrazone (see: http://www.sas.upenn.edu/~marisa/documents/gtitrate.pdf)
|
|
|
soma
Hazard to Others
Posts: 297
Registered: 31-7-2010
Member Is Offline
Mood: No Mood
|
|
I'm going to be dissolving and pouring it off anyway to purify it so it won't be that much extra to evap off a small amount and weigh it.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
