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I have found recommendations for making a soldering flux using boric acid mixed in with ethyl alcohol in 50/50 ration or 60/40 depending on the consistency desired.

Some have recommended that methyl alcohol is better to use as more of the boric acid can be dissolved in it? Is this correct?

If so, how do I calculate how much boric acid I can dissolve in isopropyl alcohol? What would be the best solvent to create a paste of boric acid to use as a soldering flux?

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  • $\begingroup$ For safety reasons, I would stick with ethanol, as it is less flammable than methanol (your using hot soldering equipment) and much less toxic. $\endgroup$ – DrMoishe Pippik Jun 7 '16 at 2:55
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For safety reasons, I would stick with ethanol or isopropanol, as they are less flammable than methanol (your using hot soldering equipment) and much less toxic, even though methanol is a better solvent for $\ce{H3BO3}$. If it weren't for the expense, due to heavy taxation of ethanol, it would be preferable to isopropanol. From the table below, you'll need almost twice as much isopropanol (though 1-propanol is the solvent listed, it is an isomer of the more common 2-propanol, AKA isopropanol).

Solubility of $\ce{H3BO3}$ in various alcohols at 25 C, percent by weight:

Methanol: 22.66%

Ethanol: 11.96%

1-Propanol: 7.34%

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    $\begingroup$ And methanol flames are hard to see, so it might be on fire and you might not notice until things get exciting... $\endgroup$ – Jon Custer Jun 7 '16 at 3:30
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    $\begingroup$ In general, boric acid dissolves well in alcohols. BUT the usual recommended solvent is ethylene glycol, which dissolves boric acid very well and has a low vapor pressure (i.e. low flammability). I'm guessing it would also dissolve well in propylene glycol. $\endgroup$ – AlaskaRon Sep 21 '16 at 2:13

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