Aluminum/Nitrate Reactions

[TranslucentDragon]

I was wondering if anyone can walk me through the reaction that happens when you mix Aluminum and a nitrate that causes it to spontaneously ignite, as well as why Boric Acid will keep this reaction from happening. How does the changing of the pH value cause the mixture to ignite, or keep it from igniting?

[Mumbles]

8 Al + 5 OH- + 3 NO3- + 18 H2O ---> 8 Al(OH)4- + 3 NH3

 

By using boric acid, you neutralize the required hydroxide for the reaction to go forward. The reaction doesn't exactly spontaneously ignite. What happens is that this reaction happens, and it's exothermic. You will also notice that ammonia is formed, which is additionally basic. Essentially it causes the reaction to run away. When the reaction gets out of control, it heats up to much, and can ignite some compositions. This is really only a problem if you're horribly negligent. In the presence of other things, like sulfur, you can end up reducing that instead of the nitrate, producing H2S.

 

Boric acid is sort of a neat acid. It doesn't actually produce any protons, like one might think of with a conventional arrhenius acids. Instead it absorbs hydroxide. This effect inhibits the reaction from going forward. More base around, just makes the reaction more likely to happen and get out of control. Aluminums with a high surface area accordingly also react faster, and are easier to get out of control.

[dangerousamateur]

What will happen if we swap Mg for the Al, will this produce ammonia too?

[Mumbles]

It might. They do react. I'm not sure if it also produces ammonia, but seems reasonable. Boric acid is incompatible with magnesium though, so you have to use dichromate or other surface protections.

[TranslucentDragon]

One more question for this topic:

Is it the heat from the exothermic reaction that causes the runaway

or

Is the accumulation of bases as the reaction continues that causes the runaway.

 

So more heat or more basic causes the runaway?

[Mumbles]

It's both. Heat causes the reaction to go faster. When the reaction goes faster, more base is generated which self perpetuates and causes a runaway. It's only with very large batches that there is really any risk of ignition. Usually it would just heat up, ruin the batch, and generate a lot of ammonia which is unpleasant.

[shouldnoteatindat]

i was wondering the same thing the other day

[dan999ification]

I've had magnalium glitter react on me, ammonia smell and some warmth coming from the stars.

After 10hrs drying they felt cold and the smell was faint, the glitter still worked ok.

This batch had 20% sulfur and was over wet while rolling, I still can't decide if it was acidic sulfur that caused it or un coated mgal, a high chance it was the water that started it

 

Dan.

[Zumber]

or un coated mgal(

Dan.

Magnelium can not be coated ever and it is stable than magnesium.

[dan999ification]

I'm sure I read that mgal can be and sometimes is coated in some situations.

 

Dan.

[Mumbles]

I honestly have no clue what swapinsutar is talking about either. It's not really coated like magnesium is, but if you wet the composition with potassium dichromate solution, it will impart some stability and resistance to attack.

[dan999ification]

Cheers mum, and linseed oil ? strobe comps need coated metal iirc.

I always thought that the oxide layer provided by the al would be enough to passivate it, but I got a reaction. Maybe the metal was fine since it still glittered..who knows.

 

The smell was sulfur and ammonia, strangely this has never happened when using al but I'm sure the problem is overwetting. The rolled stars always look dry to me.

 

Dan.

Edited December 19, 2013 by dan999ification

[LambentPyro]

For Bleser White Strobe, does the Magnalium need a coating? I do not want to use Boric Acid because of the Mg content.

 

I have seen a picture in the gallery here of Magnalium in 3% Acetic Acid and it had no reaction however in Mg and Al alone, it reacted somewhat violently.

[Mumbles]

3% acetic acid shouldn't really react with straight aluminum, though it might with actual MgAl. You might be thinking of some of the tests done by either Peret or Swede (I think) on using Parlon to coat the particles to impart resistance to them.

 

There's no reason you can't coat MgAl with linseed oil, it's just probably unnecessary. Just remember that AP will eat right through linseed oil. You need dichromate for that. If you're talking about nitrate based strobes, I suppose linseed oil would work. I personally wouldn't bother, and would just add dichromate to the solution or the composition. It has the benefit of speeding things up a little bit too.