wild_rabbit
Harmless
Posts: 4
Registered: 4-3-2019
Member Is Offline
|
|
Distillation: not reaching hydrobromic acid azeotrope temperature
Hello everyone,
I was running a simple distillation of hydrobromic acid to purify it. Initial concentration was approx. 30%. I was expecting at some point to reach
azeotrope vapor temperature of 124 degrees C but the distillation ran almost all the way through at fixed 112-113 degrees C and didn't get any higher.
Distillation rate was approx. 15 drops/min. My budget hotplate has a pretty low maximum temperature of 280 degrees C and I used it on highest setting.
Does it mean that it is maxing out and there isn't enough heat or is there another explanation?
[Edited on 4-3-2019 by wild_rabbit]
|
|
|
Assured Fish
Hazard to Others
Posts: 319
Registered: 31-8-2015
Location: Noo Z Land
Member Is Offline
Mood: Misanthropic
|
|
Your problem is likely that you havent reached the azeotrope and a large percentage of your distillate is still water.
Meaning you have distilled some comparatively dilute hydrobrimic acid.
To get to the azeotrope you would ideally want to opt for fractional distillation, also you shouldnt be pumping that much heat into it.
You could possibly acheive something close to azeotropic with a standard distillation by controlling your heat alot more, you want enough heat
generate about 20 drops of acid per minute, the faster you try to distil it the more water will come over with it.
Now chances are you have more water in your solution that 50% so you want your first fraction to be mostly water, this will concentrate the acid down
in the boiling flask prior to distilling over your acid.
Sufficiently advanced science is indistinguishable from madness.
|
|
|
BromicAcid
International Hazard
Posts: 3253
Registered: 13-7-2003
Location: Wisconsin
Member Is Offline
Mood: Rock n' Roll
|
|
Can you guarantee that you had your thermometer in the right position and that it is accurate at these temperatures? Believe me, both of these issues
crop up with great frequency.
|
|
|
wild_rabbit
Harmless
Posts: 4
Registered: 4-3-2019
Member Is Offline
|
|
Yes, I will do fractional distillation, I just found it strange that initial simple distillation ran the way it did. Maybe I heated it too much or
stopped too quickly when temperature started to decrease. Thermometer position was correct. Accuracy at higher temperatires might be a factor as well.
|
|
|
woelen
Super Administrator
Posts: 8027
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
You will have to distill nearly all liquid, otherwise you will lose a lot of your HBr. The highes tconcentration is obtained at the end of the
distillation.
|
|
|
Sulaiman
International Hazard
Posts: 3721
Registered: 8-2-2015
Location: 3rd rock from the sun
Member Is Offline
|
|
Assuming that your dilute HBr is pure,
because HBr has a maximum boiling point azeotrope with water,
you can just slowly boil away the water in an open vessel until you reach Az. HBr solution.
If you very slowly 'boil away' via something like a Vigreux column then I'd expect more water and less HBr to leave the mixture = higher yield.
With the thermometer in the boiling liquid you should get a reading of about 124 °C (47.6% HBr by mass, 8.89 mol/L)
If you can do fine fractionating then you should get an even better yield
- if you consider it worth the extra time.
CAUTION : Hobby Chemist, not Professional or even Amateur
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
