According to Wikipedia all of the alkali metals, as well as $\ce{Ca, Sr, Ba, Eu,}$ and $\ce{Yb}$ (also $\ce{Mg}$ using an electrolytic process[4]), dissolve in ammonia to give the characteristic blue solutions.

Could I just dissolve $\ce{Mg}$ in $\ce{NH3}$ and then filter it to remove traces of any elements other than those substances (supposing my aim is to get any other element to get $<\pu{0.1ppm}$)?

  • 1
    $\begingroup$ Blue solutions are temporary, until solvated electrons ( metals are already in form of ions ) reduce ammonia to amide and hydrogen. When electrolysis is involved, I doubt about selectivity. $\endgroup$ – Poutnik Jun 22 '19 at 15:27
  • $\begingroup$ What other elements do you mean, alkalis and earth-alkalis? Why and in what form would they precipitate? And for what purpose? If you condense NH3 gas into a clean flask, it's already pretty ion-free. Sorry, totally unclear what you ask. $\endgroup$ – Karl Jun 22 '19 at 17:40
  • $\begingroup$ @Karl All other elements. They wouldn't precipitate, but would simply not dissolve to start with. My goal is to purify Mg, not NH3. $\endgroup$ – Veritas Jun 23 '19 at 8:39
  • $\begingroup$ You are not likely to find much francium in your metal. There is only an ounce or so in all of Earth's crust. $\endgroup$ – Oscar Lanzi Jun 23 '19 at 22:06
  • $\begingroup$ @OscarLanzi: You are right. I just listed all the elements that theoretically dissolve in NH3. $\endgroup$ – Veritas Jun 24 '19 at 12:30

I can only give you a qualitative answer based on my experience. But I'm doing a lot of Europium based chemistry where I use Europium metal as reagent, often together with sodium as a fluxing agent. The sodium is then removed using liquid ammonia by constantly dissolving it, decanting it off and warming it up.

Just to give you an idea, I'm using about 90 - 100 mg of Sodium metal usually. My container is filled with, I'd say about 10-15 ml ammonia per run. It will take me about 20-25 repetitions until I have most of the Sodium removed, which is about 1-2h of work. And that is just for the Sodium. If we use Lithium it will take much longer. And I usually find metals like Sr, Ba, Sm or Yb still in my washed sample.

If you have traces of other metals present, say Eu or Sr the removed sodium will often adopt a golden color. So it's usually a mixture and not well separated, just as you assumed it. But I imagine you could perhaps dissolve the Magnesium and separate it from other metals that way. But using standard pressure would most likely take forever. I never tried Magnesium but I can tell about Sodium, Lithium, Strontium, and Europium. And the latter ones are extremely slow. This could perhaps be increased if you left the metal for a while in the ammonia or if the pressure or temperature is increased.

Our setup works by cooling down the vessel to - 80°C so the ammonia will liquify. And then immediately pour it off to save time. So there isn't much time for the metals to dissolve and the pressure is then often at around 350 - 400 mbars. But as I said this is just from my experience, a specially designed setup may work in your case.

  • $\begingroup$ Thanks a lot for your answer! Would you perhaps have an idea of another way to purify magnesium? I'm most concerned with Al & As which I'd like to bring down under 0.1ppm or so. For As I guess I could oxidate my Mg sample (by calcination?) and then dissolve & filter it "rinsing out soluble As oxides", and repeat a few times if necessary. When it comes to Aluminum, I don't have the slightest clue how to bring it down that much in a convenient way... Maybe I should ask a new question for that specifically? $\endgroup$ – Veritas Jun 22 '19 at 21:12
  • 1
    $\begingroup$ I would guess magnesium takes so long to dissolve that it decomposes before a large concentration is built up. That's why they use an electrolytic process. $\endgroup$ – Oscar Lanzi Jun 24 '19 at 13:50

Not the answer you're looking for? Browse other questions tagged or ask your own question.