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I want to create some boric acid with borax in the following reaction:

$$\ce{Na2B4O7·10H2O + 2 HCl → 4 BH3O3 + 2 NaCl + 5 H2O}$$

However as you can see, the leftover solution will contain a mixture of boric acid, sodium chloride and water. Both chemicals are soluble in cold water so neither will want to immediately crash out of solution.

My question is, how can I remove the excess water and sodium chloride to leave myself with some powdered/dry boric acid crystals?

I know I can probably just use some liquid ammonia, but then the boric acid is only slightly soluble in it leaving much of it behind. I could perhaps use glycerin also, however I am limited to how much I currently have.

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  • $\begingroup$ I guess ordinary ethanol dissolves boric acid pretty well. $\endgroup$
    – Ivan Neretin
    Jan 27, 2017 at 14:50

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Here is a potential solution that shouldn't even require heating. Note that I have not tried this myself.

Boric acid is soluble in lower alcohols like ethyl alcohol and isopropyl alcohol.
The solubility of sodium chloride in ethyl alcohol at room temperature is about 0.05%$^1$.

Given these properties, I suggest dissolving the dried boric acid / sodium chloride in a minimal amount of cold 95% denatured ethyl alcohol or 99% isopropyl alcohol (available at most pharmacies).

The boric acid should dissolve readily and the sodium chloride should not, so you can just filter it and let it dry to a fairly sodium chloride free boric acid solid.

To make the sodium chloride even less soluble you could cool the solution in a freezer prior to filtering. If you notice any significant increase in solids that means boric acid is coming out of solution and you'll want to abort this step.

1) Journal of Chemical and Engineering Data, Vol. 50, No. 1, 2005

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  • $\begingroup$ Is there any steps I can add to this solution by heating/distilling or rather anything to get a pure solution of boric acid? I'm going for a sodium chloride-free environment here. :) $\endgroup$
    – user36217
    Jan 27, 2017 at 22:44
  • $\begingroup$ If you don't mind sacrificing your recovery of boric acid a bit, and any purification procedure will likely result in decreased recovery, here is what I would do: When all but a small amount of alcohol has evaporated, pour off the remaining alcohol then give the solid a quick rinse in a small amount of ice-water, pour off all the water that you can and dry. As the boric acid crystalizes out of solution, the crystal formation should exclude most of the NaCl, so that most of the NaCl will be in the alcohol and at the surface of the solid. NaCl is very soluble in water, so most should rinse off. $\endgroup$
    – airhuff
    Jan 27, 2017 at 23:03
  • $\begingroup$ Hmmm... I'm sorry but I'm no good at these crystallization attempts... I'm giving it a go, but is there any other way I can do this? :( $\endgroup$
    – user36217
    Jan 29, 2017 at 4:49
  • $\begingroup$ Could you please share the link for the specific research paper from the "Journal of Chemical and Engineering Data, Vol. 50, No. 1, 2005"? I tried but am not able to find it on Google Scholar or Google search. $\endgroup$
    – Gaurang Tandon
    Jul 11, 2018 at 11:14
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You could consider using ion exchange resin, there are resins intended for use in the nuclear industry for the removal of boron (borate / boric acid) from water. A common method in light water reactors (PWRs) is to control the reactivity by adjusting the boron content of the coolant.

While you can imagine it is easy to add boric acid / lithium borate to the water, a method of removing it is also needed. One method which is common is to use an ion exchange resin. It is interesting that the acid strength of boric acid increases when it is combined with a polyalcohol which can chelate to the boron.

What I would imagine going for is a weak acid resin such as IRA 67 which has been modified with surgar groups which are able to increase the binding of boric acid / borate. There is a resin which is marketed for this purpose (IRA743) which could be used to remove boron from sodium chloride.

You would then need to elute the resin, I would suggest using a mineral acid solution to release the boric acid followed by treatment with sodium hydroxide to restore the resin back to being the free base form.

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