Hang-Man
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Mg + Mg(NO3)2 --> What the hell
Clearly I’m an idiot and am just forgetting some fundamental aspect of chemistry… Anyway, here is my problem:
I have these big blocks of Mg that I drill to get Mg turnings. But once a block has been drilled to a certain extent I can’t drill it anymore
because it’s so full of holes. So I think; “Hey, I could electroplate these mangled blocks onto a small square and get a drillable block again.”
So with my limited knowledge of electrochemistry, I make a solution of Mg(NO3)2 and put in the Mg blocks. Before I even turn on the power, the Mg
blocks start reacting with the solution. I figure “oh, must just be the Mg oxidizing in the water” and plug it in anyway. Both blocks start
disappearing so I pull one out and it smells really basic so I think “stupid me, this is just making Mg Hydroxide” but I checked the Ph; 7.
After this I made a new Mg(NO3)2 solution and submerged a chunk of Mg in it. It bubbled allot and on its removal smelt basic again. Why in gods name
is Mg(NO3)2 reacting with Mg? And why the basic smell but Ph of 7? Arrgh my brain!
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Saerynide
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How does something smell basic? Ammonia? Maybe there's excess acid in the magnesium nitrate solution?
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BromicAcid
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| Quote: |
After this I made a new Mg(NO3)2 solution and submerged a chunk of Mg in it. It bubbled allot and on its removal smelt basic again. Why in gods name
is Mg(NO3)2 reacting with Mg? And why the basic smell but Ph of 7? Arrgh my brain!
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Did you make the solution by dissolving Mg in HNO3? If so there may be residual acid, although you state the pH of the solution is only 7. Mg(OH)2
does not necessarily make a solution basic to any noticeable extent because Mg(OH)2 is VERY insoluble in neutral or basic solutions so there could be
an appreciable quantity, if there is a precipitate it is most likely Mg(OH)2. Elemental magnesium will react with water but not vigorously, just a
bubble here or there on a clean surface but you make it sound like there is significant activity, Mg may be able to reduce the nitrate anion in
solution, it is normally not considered oxidizing in solution but it may be with Mg. In this case you could be getting oxide, hydroxide, nitrogen,
nitride, oxygen, maybe nitrogen oxides too.
Also, magnesium cannot be the product of electrolysis of aqueous solutions, it is too reactive, just as aluminum is, and zinc is almost impossible too
from what I understand, you will just get the oxide and hydroxide and maybe the subnitrate.
And I also ask the same question as Saerynide, "How does something smell basic?"
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Hang-Man
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I got the Mg(NO3)2 from a chem supplier and have several pounds, Its so god damn hydroscopic I can hardly use it anywhere though. I will restate that
the pH is 7, and thus it isn't excess acid eating my Mg.
I suppose I should have said it smelt caustic instead of basic, In retrospect I have no idea why I assoiated the smell with a base. It smelt like a
strong mix of Ammonia and bleech.
It did react rather vigorusly, like Mg about 25-20% HNO3. It made my reaction vessel white with bubbles.
Edit: I might also add, that I have found Mg(NO3)2 to be a damn good oxidizer for metals when not in soluion. For example, coarse Mg powder with a
little Mg(NO3)2 makes a really good flash, much better than KNO3. However it seems to be worse than KNO3 when mixed with sucrose.
I have to look into this stuff a bit more. ...
[Edited on 17-7-2004 by Hang-Man]
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BromicAcid
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Reduction of nitrate to ammonia
It's probably a process that feeds on itself. The Mg(OH)2 keeping the solution neutral by precipitating out but forcing the remaining magnesium
to reduce the nitrate to nitrite, then nitrogen, or ammonia, that is where your basic smell comes from.
Instead of trying to electroplate more nitrogen on your block, put it in a cast iron vessel, heat strongly enough to melt it with a lid on, then allow
to cool, you will be left with a new, smaller brick with no holes.
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vulture
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Magnesium nitrate is very useful for concentrating HNO3, as it is easier to recycle and handle than sulfuric acid.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Hang-Man
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It can be used to concentrate sulfuric acid too. And we all know how much fun heating H2SO4 is.
I will melt the block now that my fancy meathod failed. And this time it's getting mixed with Al.  .
One last thing, the Mg block will NOT react in a KNO3 solution, only the Mg(NO3)2.
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vulture
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Magnesiumnitrate + H2SO4 ===> HNO3 + MgSO4 no?
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Hang-Man
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Sure.
But I mean that Mg(NO3)2 is so hydroscopic it will suck the H2O out of the H2SO4 solution, thus concentrating it.
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The_Davster
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Not if the sulfuric acid is in contact with the magnesium nitrate, then the reaction Vulture posted occurs.
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Hang-Man
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yes obviously.
|============|
|****************|
|**|HHHHHH|**|
|**|HHHHHH|**|
|**|HHHHHH|**|
|**|HHHHHH|**|
|****-----------***|
|XxxxxxxxxxxxxxX|
|============|
*= space
H= H2SO4
X = Mg(NO3)2
There. No more confusion. I suppose I should have said it can be used to dry sulfuric acid, not concentrate.
[Edited on 18-7-2004 by Hang-Man]
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Magpie
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speculation
Mg(NO3)2 is the salt of a strong acid and a weak base and therefore should be acidic in water. The Mg metal would react with the H+ to produce Mg++
and H2 gas. This continues until H+ has been reduced to 10^-7molar, or pH 7.
What is the pH of your Mg(NO3)2 solution before inserting the Mg?
The single most important condition for a successful synthesis is good mixing - Nicodem
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Hang-Man
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Saturated solution pH = 5 to 4 (I only have pH strips for testing). I should have though of that 
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