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S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
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Mood: Enhanced
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You said lagging a column is nonsense. I said it isn't. You say I'm so full of shit because I'm so stupid that I can't see how this is a problem. I do
nothing but post quotes from *real chemists* saying that columns must be lagged. You say my response to you is childish, underhanded, and lacking in
substance.
I gave you the links, read the books for yourself. Why do I need to argue the author's points for them? Especially to you?
There is something that I'm missing here all right.
[Edited on 3-7-2008 by S.C. Wack]
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Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
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Mood: Chemistry: the subtle science.
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I have performed many fractional distillations using a packed column without insulation. But if the more volatile component (or azeotrope) that you
want to recover as distillate has a high boiling point, then I can see where insulating the column would be desireable or even necessary. You have to
get this component to the top of the column in a vapor state. Just giving it more heat in the pot may not be enough, or may cause flooding. It seems
like insulating the column is the obvious answer.
Slow distillation, high reflux ratio, and use of many theoretical plates are factors that promote effective separations.
I don't have a Vigreaux column and don't see the need for one. If I want a short column with fewer plates (to reduce holdup & losses) I just use
less packing in the column.
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DJF90
International Hazard
Posts: 2266
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Nobody has yet answered my question. What are the advantage(s) of isolating hydrobromic acid rather than preparing it in situ?
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not_important
International Hazard
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| Quote: | Originally posted by DJF90
Nobody has yet answered my question. What are the advantage(s) of isolating hydrobromic acid rather than preparing it in situ? |
Depends on the use. Will the reaction you want to performs be affected by the materials used to make the HBr? Related - will the reagents used and
byproducts may workup of your reaction more difficult? Will the concentration of the HBr be great enough to give good yields in your reaction?
Isolating constant boiling HBr means you only have to worry about HBr and water, and you have a pretty good idea of the concentration of the HBr.
In many cases, though, there is no real advantage A number of procedures for making alkyl bromides either prepare the aqueous HBr and use the
solution without isolation, or prepare the HBr in situ.
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woelen
Super Administrator
Posts: 8031
Registered: 20-8-2005
Location: Netherlands
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Mood: interested
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I also tried to make hydrobromic acid, 48%, but my attempt only is a partial success.
I took 50 grams of NaBr and dissolved this in water. The total volume of the solution is appr. 100 ml.
I took approximately 50 ml of water and added appr. 25 ml of concentrated sulphuric acid to this. When mixing this 50 ml of water with the acid, quite
some heat was produced.
I finally mixed the liquids. The resulting liquid is yellow. A very tiny pinch of Na2SO3 was added, but the yellow color did not disappear. So,
apparently it is not bromine, but some organic stuff, due to impurity in the sulphuric acid (this was very light brown, it was drain cleaner). No
crystals were formed after some time, everything remained in solution.
All of the liquid was tranferred to a 500 ml RBF, to which a 40 cm vigreux condenser was attached and to that, a setup, similar to Jor's was attached.
When distilling, the temperature remained constant within 0.2 C around 100 C for more than 45 minutes. In that time approximately 70 ml of liquid was
distilled. At a certain point in time, temperature was rising quickly, and in 2 minutes, the temperature went from 100 C to 120 C. At that point I
stopped the distillation, removed the vigreux condenser, changed the receiving flask and then continued. Approximately 15 ml of colorless clear liquid
was distilled, but the liquid started bumping more and more. Lots of solid particles were formed. I decided to stop the distillation, first the solid
needed to be removed.
The liquid was allowed to cool down, and quite some solid matter settled at the bottom. I then decanted the liquid and continued distilling. Quickly,
the temperature rose to 125 C (without the vigreux condenser) and a colorless liquid comes over again. When just over 50 ml of colorless liquid has
been collected, the liquid in the heated flask starts to foam considerably, and at that point I stopped distilling.
Total result: 70 ml of colorless liquid, obtained at 100 C and 55 ml of colorless liquid, obtained at 125 C.
The first batch of liquid is almost pure water. When a pinch of NaHCO3 is added to 1 ml of this liquid, then no bubbles of CO2 can be observed. This
is a clear indication that this liquid hardly contains any acid.
The second batch of liquid has a density of 1.37..1.38 gram per ml. This is less than I expected, the 48% azeotrope has a density of 1.49 gram per ml.
At a density of 1.37 ... 1.38 gram per liter the concentratoin is almost 40%. The second batch does not give a precipitate with a solution of
Ba(NO3)2, not even a slight opalescence can be observed, so it is totally free of sulfate. After addition of a few drops of H2O2 to the solution with
the barium nitrate it remains clear, so it is also free of sulphur dioxide (otherwise it would become turbid). After a while, the liquid becomes
orange/brown, due to oxidation of bromide by the H2O2.
The foaming liquid, which was left in the heated flask solidified to a light yellow solid, which dissolved in water moderately well. Probably this is
mostly NaHSO4/Na2SO4.
Total yield is 55 ml of a solution of 1.37 gram/ml, with just below 40% of HBr. This means that total weight of HBr is appr. 30 grams. From 50 grams
of NaBr, according to theory, one can get almost 40 grams of HBr, so my yield is 75%.
I am only partially satisfied with this result.
Yield is acceptable to me. The solid which caused the bumping in the distillation most likely also contained some bromide. A second type of loss is in
the decanting. Quite some liquid remained stuck inside the solid mass, just as Jor observed. A third loss is in the final distillation, when the
liquid started foaming excessively and I decided to stop. On opening of the equipment, however, quite some fumes of HBr evolved from the foaming
liquid.
The concentration, however, is not as high as I hoped it to be. Even with the vigreux condenser, and the rather sharp transition from 100 to 125 C I
still have a liquid which is quite below the azeotrope.
Purity of the resulting liquid is very good though. It is completely colorless and does not contain sulfate, not sulphur dioxide. I also have a
commercial sample of HBr (48%) and this is a light yellow liquid, so my product is more pure. My product has a pungent smell, but it is not fuming in
air. The 48% azeotrope is fuming slightly and has the same pungent smell.
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Jor
National Hazard
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I tested my HBr for sulphate and sulphur dioxide as well, excactly the same way, except for that I replaced barium nitrate for strontium nitrate. Is
this OK as well? There was no observed precitipate.
Your HBr seems to be about as concentrated as mine. How is that possible? Anyways, I concentrated it, by distilling, and discarding the almost pure
water that comes over.
My HBr has a density of about 1,41 now. I'm happy with this. It completely colorless and seems to be pure, unless SrSO4 is not as insoluble in HBr as
in water.
Maybe you didn't get crystals because NaHSO4 is too soluble. Why not try KBr? AFAIK, we don't have any OTC NaBr, so NaBr is even more expensive than
NaBr! Or am I wrong on this one?
[Edited on 5-7-2008 by Jor]
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woelen
Super Administrator
Posts: 8031
Registered: 20-8-2005
Location: Netherlands
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I obtained my NaBr from eBay a few years ago. Only EUR 5 for 1 kilo! That's why I use NaBr.
The test with Sr is not nearly as sensitive as the test with Ba. It confirms that there is not much sulfate in it, but the test with Ba is more
sensitive.
I am also surprised by my result. The transition from 100 C to 120 C was really sharp, and I changed receiving flasks, when the temperature was 120 C.
It might be that there still was some water left inside the Liebig condenser, but that cannot account for such a low density.
I have a table with densities at 15 C:
5% --> 1.038 g/ml
10% --> 1.077 g/ml
15% --> 1.117 g/ml
20% --> 1.159 g/ml
25% --> 1.204 g/ml
30% --> 1.252 g/ml
35% --> 1.305 g/ml
40% --> 1.365 g/ml
45% --> 1.445 g/ml
50% --> 1.515 g/ml
82% --> 1.78 g/ml (this is the highest possible concentration at a pressure of 1 atmosphre)
The azeotrope of almost 48% has a density of 1.486 gram/ml.
[Edited on 5-7-08 by woelen]
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