condennnsa
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boric acid synthesis
I want to make boric acid from borax, but I don't have HCl. I do have H2SO4 though, and i read that it also works.
But what stoichiometry should i use? Will the reaction produce sodium sulfate or a bisulfate?
And what would be the best way to extract the boric acid, by cooling the solution, or by boiling it till the boric acid crystallizes? thanks
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blogfast25
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Never tried it with H2SO4.
Use a concentrated solution of Borax, really saturated. Add the stoichoimetric amount of H2SO4, preferably concentrated, very carefully (e.g. precool
both the borax and the acid, and add in small amounts at a time).
Then cool the solution of H3BO3 and Na2SO4 to about 0 DC: fatty looking boric acid will drop out but this may take a while. The highly soluble Glauber
salt should stay in solution (to be certain, consult the solubility of Na2SO4 at 0 DC). Filter off and wash with small amounts of iced water.
Probably works with half the amount of H2SO4 too: you should obtain NaHSO4 as a byproduct instead of Na2SO4...
I'm guessing that if you have access to H2SO4 you can get hold of HCl too: look for 'patio cleaner' for instance.
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condennnsa
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Ive been looking for hcl for a long time, i think i'll just buy from a chemical supplier some. I'm not from the US.
Thanks for the answerr, but you only say add the stoichiometric amount of h2so4, but how much would that be?
I mean how would the reaction look like ?
Na2B4O7 * 10H2O + H2SO4 -> H3BO3 + Na2SO4 ....?
I can't figure out how to balance this
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entropy51
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Na2B4O7 + H2SO4 + 5H2O -> Na2SO4 + 4H3BO3
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bbartlog
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If you cool a solution of Na2SO4 to near freezing you may find that crystals of the decahydrate form (this can happen quite suddenly), making it hard
to separate the boric acid. Sodium sulfate is not all that soluble in water at low temperatures. I think HCl is really a better choice than H2SO4
here. An alternate sulfuric acid approach would be to slowly add an ethanolic solution of H2SO4 to an aqueous solution of borax, which should
ultimately result in a solution of boric acid in ethanol as a separate phase from water plus sodium sulfate.
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blogfast25
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Borax is a decahydrate:
so the stoichiometry is:
Na2B4O7.10H2O + H2SO4 -> Na2SO4 + 4H3BO3 + 5H2O
or with HCl:
Na2B4O7.10H2O + 2HCl -> 2 NaCl + 4H3BO3 + 5H2O
I get 32 w% HCl (500 mL) bottles from my local hardware store (in the UK).
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entropy51
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I knew one of you would say that! The crystals are a decahydrate, not the aqueous solution in which the
reaction occurs. I wrote the reaction as it occurs in solution, but the decahydrate is amount you would need to weigh out for the reaction.
[Edited on 3-7-2010 by entropy51]
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condennnsa
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So the reaction will proceed all the way to sulfate, instead of stopping at bisulfate?
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entropy51
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I don't know. But
when one titrates H2SO4 with a strong base like NaOH, it doesn't stop at NaHSO4. The salt of a weak acid is a strong base, and boric acid is a weak
acid, so sodium borate is a strong base (pH about 9.5) so I would expect it to go to Na2SO4.
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not_important
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You just about have to titrate this, as borax loses and gains water depending on relative humidity and temperature. Ether that, or dehydrate it
completely and use weight.
The solubility curve for sodium sulfate is a bit funny, although not too atypical for sulfates. Above 32.3 C it crystalises as anhydrous Na2SO4 and
the solubility decreases a bit as you increase the temperature - going from 49,7 g/100 g H2O down to about 42,7 at 100 C
So you can make a hot concentrated solution of borax, adding the needed amount of H2SO4, separating the hot solution from the precipitate of Na2SO4,
cooling to 33 C, and filtering off the boric acid crystals as boric acid's solubility at ~30 C drops to around 1/4 of that at 100 C while sodium
sulfate's actually increases over the same reduction in temperature. You might have to first concentrate the hot solution to bring it almost to
saturation of boric acid, filtering out any further Na2SO4 crystals - you'll need to run the numbers to calculate the amount of water you'll have.
Another possibility is to use moderately dilute H2SO4, say around 25%, and distill a mix of water and B(OH)3 out of the borax-H2SO4 solution, chilling
the condensate to force as much boric acid out of solution as possible. Boric acid is somewhat volatile with steam, I don't remember the exact ratios
but the volatility is a problem when performing analysis where you want to determine borate.
J. Am. Chem. Soc., 1939, 61 (4), pp 917–920
Attachment: phpTWokBT (254kB)
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[Edited on 4-7-2010 by not_important]
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