Pumukli - 28-2-2018 at 13:19
It always intrigued me why on Earth does a distilling solvent bump when there is a fair ammount of salt present in the flask?
Isn't it counterintuitive?
I mean we use boiling stones, boiling glass rods, glasswool, etc. to introduce small cavities (capillaries?) into the solvent where bubbling can start
in sort of a controlled way.
Then why those cavities (capillaries) produced by the salt (inorganic, NaCl, sulfates, etc.) are not good for this? Are they?
Maybe if we reduced the heating rate a bit would make a difference?
I just finished such a distillation with severe bumping (basically ethanol was refluxed in the presence of lots of NaCl).
When I finally turned off the heat there were minutes where the bumping has stopped but the ethanol was still refluxing approximately at the same
rate as before. There were lots of small bubbles in the liquid but not those "flask and equipment shaking" big bumps we all know.
Opinions?
Vomaturge - 28-2-2018 at 13:30
Maybe the salt isn't porous enough? The main capillaries in a mass of salts are just the gaps between the grains. The grains are usually solid
crystals, which retain smooth impermeable faces as they dissolve. I guess that this geometry doesn't encourage nucleation and steady boiling?
Diachrynic - 28-2-2018 at 13:39
Assuming dissolved salt, maybe the mixture is more viscous, making it harder for bubbles of steam to rise and causing more bumping?
Sulaiman - 28-2-2018 at 15:35
I think that the liquid between grains gets superheated as there is minimal liquid circulation to bring bubbles or the surface in cose proximity.
j_sum1 - 28-2-2018 at 16:48
All three? Plus another one.
Concavity is best for nucleation. You don't get that on the surface of a growing crystal.