bmays
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Preperation of HBr in acetic acid.
I need to prepare a 15% solution. I think i will use NaBr and H2SO4, this should make HBr and Br2 which will be bubbled into the acetic acid. Then H2
gas will be bubbled into the acetic acid to convert this unwanted free bromine into HBr. This should be a good way to avoid having to use hot
phosphoric acid and NaBr?
thanks
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weiming1998
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Quote: Originally posted by bmays  | I need to prepare a 15% solution. I think i will use NaBr and H2SO4, this should make HBr and Br2 which will be bubbled into the acetic acid. Then H2
gas will be bubbled into the acetic acid to convert this unwanted free bromine into HBr. This should be a good way to avoid having to use hot
phosphoric acid and NaBr?
thanks |
No, this is not a good way of making a 15% HBr solution. Firstly, bromine and hydrogen doesn't react spontaneously at room temperature and pressure
(bromine as vapour and hydrogen as gas can be mixed together at room temperature without HBr being formed) as hydrogen is quite inert at room
temperature and pressure. Secondly, the reaction between H2SO4 and NaBr also produces SO2, which will react with Br2 when it is diluted/cooled to
produce H2SO4 and HBr. It depends on the purpose of your 15% HBr solution, but the presence of H2SO4 as an impurity is likely to be problematic.
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bmays
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Oh, i see. Then i will make an 50% aqueous HBr solution first using my NaBr, the diluted H2SO4 will not be concentrated enough to react. Then use
MgSO4 to dry and liberate HBr gas.
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AJKOER
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For preparing a variety of dilute acids, I would suggest reacting the appropriate acid salt with Oxalic acid, H2C2O4 (or the dihydrate, H2C2O4.2H2O).
For example, with an aqueous solution of NaBr:
2 NaBr + H2C2O4 --> Na2C2O4 (s) + 2 HBr
I started a thread on this general approach awhile back. Please note, however, that safety issues can arise when attempting to produce concentrated
acids via this approach. As such, I do not generally recommend this procedure to form concentrated acids as many acids can be inherently dangerous
and/or unstable (including explosive, always research and check MSDS before experimenting) by themselves especially at high concentrations.
As an example of potential safety issue, with NaClO3, the action with H2C2O4 forms Chloric acid (HClO3). However, with excess H2C2O4 in this instance,
the Oxalic acid takes on a new role as a reducing agent, and from the newly created HClO3, explosive ClO2 gas will be evolved. This will occur even
for a supposedly safe concentration level of Chloric acid.
[Edited on 15-5-2013 by AJKOER]
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bmays
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NaBr does not seam to react with an oxalic acid and sodium bisulfite. Does it require heating for HBr gas evolution?
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Metacelsus
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Sodium oxalate is water-soluble, although not very much so. Unless you use a dehydrating agent to liberate HBr as gas, your solution will be
contaminated with sodium oxalate. If you had, say, calcium bromide, then that would be different.
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Organikum
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H3PO4 and NaBr or KBr produce anhydrous HBr upon heating, with slow production and strong cooling plus good dispersion by a frit or at least good
stirring 15% in GAA is doable. A slim tall vessel, a tube basically, inlet to the bottom, filled with crushed glass 5cm high for dispersion.... Take
care the system must be sealed there is some pressure to overcome. The receiver needs only a tiny openeing favorably equipped with a acid trap,
something from PP lightly stuffed with cotton and a carbonate, a few drops of water worked all the time. A suckback arrestor/washbottle/onewayvalve
between gas generator and receiver may be helpful. Better not all in once but in steps, say not all needed bromide to acid but a part is good.
Most of all take your time, the HBr will dissolve in GAA but not always immediately. Time is the key.
Best way of making anhydrous HBr (and HI) is Bromine + Tetraline. Smooth reaction, quantitative.
/ORG
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