I am looking to replicate instructions provided in the last paragrph of this article that requires dissolving borax (Sodium Tetraborate decahydrate ($\ce{Na2B4O7·10H2O}$)) and boric acid in ethylene glycol, for reasons of practicality and cost efficiency supplied in the impure form of concentrated automotive anti-freeze.

For reasons out of scope of this question I've modified the ratio provided as follows:

  • 5 kg anti-freeze
  • 2.5 kg Sodium Tetraborate decahydrate
  • 1 kg Boric acid

In order to dissolve the borax completely it is necessary to boil off the water of crystallisation plus any water impurities in the anti-freeze.

While I have water in the system, the temperature of the mix is expected to be stable at around 100 °C, but once the water is removed the temperature will rise to potentially dangerous levels if the heat source is not removed in time.

I would like to understand the risks of this operation. The flash point of (pure) ethylene glycol is, according to Wikipedia, at 111 °C. The autoignition temperature is listed as 410 °C and seems to be safely above the boiling point of ethylene glycol (it would boil off before it auto ignites). However this hazard sheet for ethylene glycol states it 'causes ignition' at 100 °C in combination with (amongst others) Sodium Chlorite (no more details given).

Can I assume that Borax and Boric acid are 'safe' as they are not listed on the hazard sheet?

Is there an (online) reference where I can find out how Borax and Boric acid modulate the flash point and auto ignition temperature of Ethylene Glycol?


1 Answer 1


Chlorates, perchlorates, permanganates and other strong oxidants conflagrate even at room temperature with ethylene glycol.

Boric acid and borax, however, are not considered strong oxidants and should probably be safe to dissolve below the boiling point of water. That said, I would test a small batch in a safe place, to be sure there is not a large gain in temperature on dissolving.

BTW, you might be interested in reading of the preservation of the Swedish warship Vasa in polyethylene glycol, and long-term issues with acidification. N.B. Polyethylene glycol is generally considered innocuous, but ethylene glycol is toxic if ingested.

  • $\begingroup$ I think the ultimate answer here is 'test a smal batch in a safe place'. Probably with planned 'catastrophic failure' to have an idea of a worst case scenario. Then I'll repeat this with sodium chloride added, as this is always present in marine environments. Although I think they might have a typo on that safety sheet and meant to say sodium chlorite. $\endgroup$
    – Flint
    Sep 1, 2017 at 11:08

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