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TLDR: I need a liquid with a high mass density of boron that has several physical properties constraints: Needs to be similar in density and viscosity to water (replacing water in the system). Needs to have a low vapour pressure as it is exposed breafly to high vacuum (10^-5~-6 torr). Can be cooled down or even supercooled if nesesery. Needs to be not too toxic (we're physicists, we don't have experience handling dangerous chemicals).

Currently in considering: Boric acid dissolved in water. Nanoparticles of boron suspended in water. Some organic liquid with boron, boron trifluoride tetrahydrofuran.

Context: I'm a Physics grad student, and for my research I need to find a liquid with a high density of boron. Currently we have a system that pumps distilled water, that is exposed breafly to high vacuum (10^-5~-6 torr). To lower the vapour pressure we supercool the water, and the flow is kept high. This still degrades the vacuum, but to a manageable level. The flow is kept laminar and in a special configuration to achieve a specific shape.

I would like to replace the water with a liquid that has boron to research interactions with the boron. The constraints are thus: Can't be to toxic. We are physicists and we don't have the experience to handle dangerous chemicals. The density and viscosity needs to be similar to water to achieve the special laminar flow configuration. The vapour pressure needs to be minimal, to lower the impact on the vacuum. We manage with the supercooled water, but we can't handle much more then that.

My chemistry knowledge in quite rudimentary, so I don't even know how to best approach the search. I've first considered dissolving a boron rich compound in water, like boric acid, But it seems that the solubility drops down a lot (too much?) as the temp is lowered for the vapour pressure. Second, I've consider some kind of organic liquid containing boron, based on a search in chemical supply vendors. Finally, I've considered nanoparticles of boron suspended in water. I'm not sure about sourcesing such a suspension, but there's budget for it.

Thank you in advance for your time and expertise in this matter.

P.s. Apologies in advance in any error in conduct (tags, explanations etc), I'm obviously new here. And the same for my English, I'm dyslexic and it is not my first language.

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  • $\begingroup$ If I understand correctly, you are looking for the heaviest Boron containing liquid or solution. Your expression "a high boron density" is not clear. What is a Boron density ? Has your substance to be soluble in water ? Do you consider borotungstic acid, density 3.0, but melting at 45°C ? Or Boron trifluoride, density 2.99, melting point -127°C (decomposed in water) $\endgroup$
    – Maurice
    Commented Jul 22 at 10:10
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    $\begingroup$ Seems to me you want a large number of moles of boron atoms per volume of liquid? Trimethyl borate seems about as good as you can get with simple chemicals, though it needs to be kept away from moisture in the air, and it will catch fire if exposed to a flame and oxygen. There are certain boranes which could do better by an order of magnitude, but most are extremely pyrophoric and dangerous to handle. $\endgroup$
    – Nicolau Saker Neto
    Commented Jul 22 at 10:24
  • $\begingroup$ By boron density I refer to the molar amount of boron in a given volume. So the density of the liquid times the proportion of boron in it. Other components in the liquid are not important for the reaction. $\endgroup$
    – Noam Popper
    Commented Jul 22 at 10:24

2 Answers 2

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Easiest to try might be sodium tetraborate, $\ce{Na2B4O7·10H2O}$ (borax), which is more soluble in water at room temperature (31.7 g/L) than boric acid, if the sodium will not interfere with your work.

However, solubility does decrease with temperature. On the other hand, it tends to form supersaturated solutions, and it might be possible to use such a concentrated solution in you apparatus -- if it doesn't start to precipitate.

Perhaps potassium tetraborate, $\ce{K2B4O7·4H2O}$, might be even a better choice. At 0°C, it still has a solubility in water of 8.2%, ~80g/L. With so much dissolved, there would be a freezing-point depression, allowing you to drop below 0°C, reducing evaporation.

Consult the chemical safety data sheets for sodium tetraborate and for potassium tetraborate. Both show no major warnings, with the only noted concern being prenatal toxicity.

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  • $\begingroup$ Thank you and everyone else who commented, I appreciate the help. I believe the potassium tetraborate is a good candidate, and the freezing point depression is an added boon I haven't even considered (freezing is a major issue in this system). $\endgroup$
    – Noam Popper
    Commented Jul 23 at 11:11
  • $\begingroup$ Sorry for the stupid question, but is the 80 g/L consitration includes in the mass of the 80g the 4(H2O)? $\endgroup$
    – Noam Popper
    Commented Jul 23 at 11:53
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    $\begingroup$ @NoamPopper Yes. $\endgroup$
    – Mithoron
    Commented Jul 23 at 12:21
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Boric acid itself works well if you are willing to use ethanol as a solvent. Up to 11% boric acid by weight can be dissolved into the ethanol [1].

enter image description here (From Ref. 1)

Boric acid is only a weak acid, and in its predominant molecular form it is quadrupolar rather than dipolar as well as non-ionic. These features render boric acid much more soluble in lower alcohols than in water.

  1. Demirel, Hilal & İnce, Taylan & Uysal, Duygu & Uysal, Bekir. (2015). Boric Acid Production From Sodium Metaborate With Sulfuric Acid. Celal Bayar Üniversitesi Fen Bilimleri Dergisi. 11. 10.18466/cbujos.22763.
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