Cloner - 17-8-2006 at 04:05
To make some 48% HBr I distilled sulfuric acid mixed with water and LiBr. At a temperature of 121-123 there was a fraction coming over which I assume
is hydrobromic acid, but oddly, the temperature LOWERED after this fraction was over. It went gradually down to 105 degrees, then I stopped.
How is it possible that the temperature goes down in a distillation? There were good amounts coming over, nothing was changed, etc...
Magpie - 17-8-2006 at 14:48
Cloner, I cannot answer your questions directly, but will relate my own experience:
I made HBr by adding sulfuric acid to NaBr. I then removed the NaHSO4 by filtration. HBr forms a high-boiling azeotrope with water at 48% HBr.
Since my filtrate was rich in water the distillation temperature first came up to 100C. It eventually rose up to the bp of the azeotrope (126.9C). I
switched out my receiver at 125C and collected the azeotrope until the temperature started to drop. I assumed it was dropping because there was no
more vapor/condensate at the bulb of the thermometer. I just stopped at this point.
Nick F - 17-8-2006 at 17:13
That seems like a good explanation - if nothing is coming over then the temp at the thermometer will drop.
Just thought I'd mention that sulphuric acid can oxidise bromides to bromine if you're not careful...
Cloner - 18-8-2006 at 00:41
Let me be more clear then: The mixture contained sulfuric acid (about 50% strength) and LiBr. I got about 80 ml of distillate slightly over 120
degrees, but this thermometer has seen better days so it can just as well be 125 degrees. After that, the temperature gradually went down from 120 to
105 degrees WHILE COLLECTING ABOUT 100 ML of additional distillate. In my book, low boiling fractions come out first, then the high boiling ones.
I am aware of the fact that if you distill your solution to dryness, the top thermometer will reach room temperature even if you melt the glass with
heat 
edit: ok, after redistilling the combined distillate, there was a good quality 125 degrees azeotrope after an interval slightly over 100 degrees. It
is the sulfuric acid / LiBr / HBr / water system that is responsible for the strange boiling point.
[Edited on 18-8-2006 by Cloner]