I have another question here which refers to me asking how to separate boric acid from other compounds, and this is partially a follow-up.

Suppose I was to go down the route of making boric acid by the reaction of: $$\ce{Na2B4O7·10H2O + H2SO4 → Na2SO4 + 4 H3BO3 + 5 H2O}$$

Which in actual fact, I just did 10 minutes ago and failed miserably.
The issue I'm having is that no matter what happens in a home-experiment, you're going to have contaminants, and in this case, they are the sodium sulfate and left-over sulfuric acid.

I added sodium bicarbonate in to remove the sulfuric acid which was stupid because I required so much but then realised that it will also neutralise the boric acid. So next I am going to attempt to neutralise the acid with sodium hydroxide.

But sodium hydroxide will also target the boric acid and just change it back into borax. Is there a way I can neutralise the sulfuric acid without touching the boric acid?


2 Answers 2


You've got to know that sulfuric acid is a strong acid and boric acid is a very weak acid. So, sulfuric acid can neutralize most of the bases where boric acid cannot. So, you have to take a base that can be neutralized by only sulfuric acid and not boric acid. So, you should probably take group 2 hydroxides and try to react with the unreacted sulfuric acid because sulfuric acid reacts with group 2 hydroxides whereas boric acid can't.

$$\ce{M(OH)2 + H2SO4 -> M2SO4 + 2H2O}$$

$$\ce{M(OH)2 + H3BO3 ->nothing}$$

where $\ce{M=Be,Mg,Ca,Sr,Ba}$.

So, which group 2 hydroxide to use? Group 2 hydroxide react with the sulfuric acid to form group 2 sulfates. Now, solubility of group 2 sulfates decreases down the group. So, you should use calcium hydroxide so the calcium sulfate that forms will not dissolve in the solution due to its low solubility and form a precipitate. You can separate that precipitate by filtration.

Formation of calcium borate is highly unlikely and so only calcium sulfate will only form.

Also, barium hydroxide is the best chemical for this process because barium sulfate is very insoluble. But barium hydroxide is costly if you buy it or making it is difficult due to the cost of other reagents. Instead, calcium hydroxide is your best bet.

  • $\begingroup$ In actual fact, its a good thing it forms a precipitate, because the water left over in solution will take in the boric acid, and therefore the salt can be filtered off. Should I use maybe calcium hydroxide? $\endgroup$
    – user36217
    Jan 29, 2017 at 4:44
  • $\begingroup$ Oh no, I'm just asking weather or not calcium hydroxide will cause something to precipitate out. I don't have any on hand at the moment and since I'm going to be doing a fair few experiments along these lines, I might need to buy or synthesize some. $\endgroup$
    – user36217
    Jan 29, 2017 at 5:01
  • $\begingroup$ Solubility of group 2 sulfate decrease down the group. So, if you use calcium hydroxide, calcium sulfate will be formed which is not soluble in water and so, you can separate that precipitate. I will edit the answer. $\endgroup$ Jan 29, 2017 at 5:52
  • $\begingroup$ Great! Thanks alot. Now to go get some calcium hydroxide... :P $\endgroup$
    – user36217
    Jan 29, 2017 at 5:57
  • 1
    $\begingroup$ Something's funky here. Calcium and heavier G2 hydroxides are generally considered strong bases and I would argue that magnesium hydroxide is close to one. I think they would neutralize the boric acid too. Beryllium hydroxide might work in principle but beryllium compounds are horrendous from a safety/health point of view. $\endgroup$ Apr 1, 2017 at 11:56

Sodium bicarbonate is actually a good choice. Bicarbonate ion is a stronger base than both sulfate and bisulfate ions, but a weaker base than borate ion (corresponding to boric acid being only a very weak acid). So the sodium bicarbonate will form sodium sulfate with the sulfuric acid leaving the boric acid unreacted.

Alkaline earth hydroxides, mentioned elsewhere, are not recommended here. Beryllium compounds are toxic and magnesium hydroxide, if dissolved by acid, may be too strong to avoid reaction with the boric acid. Heavier alkaline earth hydroxides would be stronger still.


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