woelen
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Making Mg3P2 from Mg and red P?
I would like to have a means of making PH3 gas easily. I know of the method of heating white P in a solution of NaOH, but that method is quite
dangerous and it does not work cleanly and is rather slow.
So, my idea is to make Mg3P2, which by adding to dilute acid simply makes PH3, as easily as making CO2 from baking soda and dilute acid.
I tried making Mg3P2 by heating a stoichiometric mix of red P and Mg, but this reaction is VERY violent and is not suitable for synthetic purposes.
The reaction is as if black powder is ignited or something close to that. Way too violent. Fortunately I only had 100 mg or so of this mix, but it was
quite scary already.
I have read textbook preparations for Mg3P2 or Ca3P2 and these involve using a quartz tube in which the metal is heated strongly and in which vapor of
phosphorus is passed over the hot metal. I don't have the equipment for that and messing around with phosphorus vapor also is not something I feel
easy about.
As I am interested in making PH3 only and not in the purity of the phosphide I have the idea of making the reaction slower by mixing in an inert
filler. I have been thinking of very fine white sand (SiO2), but this leads to formation of Mg2Si as well.
I also have a lot of MgO lying around (off-white prills, which can easily be ground to a powder, cheap stuff, fertilizer which slowly releases Mg in
soluble form due to soil acidity).
My idea is to mix in powdered MgO. What do you think of this? I can imagine that it reduces the reaction speed as it is an inert filler, and I expect
that it does not react. Or do I overlook some side reaction and could adding this stuff lead to undesirable side-products?
I just can try this reaction, but any ideas from other people before doing so are welcomed. I don't want to spoil precious red P on a failed reaction.
I intend to carry out this reaction on a scale of a few grams (e.g. a test tube filled with the mix for 1/4 of its length).
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ScienceSquirrel
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Mood: Dogs are pets but cats are little furry humans with four feet and self determination! 
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Have you thought about making aluminium phosphide instead?
It is a standard material used to get rid of moles, rats, rabbits, etc as it reacts with moisture to produce phosphine gas.
There is a link in Wikipedia to Inorganic Syntheses and I always found their procedures quite good when I was a synthetic chemist.
http://en.wikipedia.org/wiki/Aluminium_phosphide
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blogfast25
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Your idea is to mix in a so-called heat sink. I would suggest fluoride (CaF2) for this purpose, it's guranteed to be inert in these reaction
conditions and has a high heat capacity (and high MP). It's even fully recoverable. I've used it to 'tame' some very violent thermite reactions...
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woelen
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@ScienceSquirrel: That's a good idea. Next weekend I'll try your suggestion first at a small scale, just to be sure that this is not a violent
exothermic reaction like the one with magnesium. But I can imagine that this reaction is somewhat more tame. I have 400 mesh Al-powder and will use
that.
@blogfast25: I do have potteries grade CaF2 (off-white powder), but this is said to contain quite some silica-based material as well. With the Mg this
can lead to formation of Mg2Si and this leads to strong contamination of the phosphine when acid is added. Do you know anything about this? I
unfortunately have no pure reagent grade CaF2. On the other hand, the MgO I have is free of silica-based material. When it is added to acid, then all
of it dissolves, leaving a clear solution.
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Neil
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Wouldn't MgO or CaF2 create water insoluble barriers, preventing the water from reacting with all of the phosphide?
What about washing the silicate out of table salt with phosphoric and then using the chloride salt?
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woelen
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The presence of MgO is not a real issue, because I intend to add the resulting material to dilute acid. The MgO quickly dissolves in that. CaF2 might
indeed be an issue and might prevent quick dissolving of the resulting material.
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kmno4
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Cheap Ca3P2 by aluminothermy -->> Brauer.
[Edited on 22-4-2011 by kmno4]
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blogfast25
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Woelen:
Hmmm… I think the silica contamination in most CaF2 must be quite low. Mine doesn’t contain any.
Alumina is another example of a possible heat sink.
I think kmno4 is referring to Ca3(PO4)2 + 16/3 Al --- > Ca3P2 + 8/3 Al2O3, that would be an interesting possibility indeed.
It’s similar to CaSO4 + 8/3 Al --- > CaS + 4/3 Al2O3
These are extremely exothermic but very safe to do (stand well back  )
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unionised
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If you use MgO make sure it's dry.
Generating unexpected high temperature steam is a learning process that's worth avoiding.
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Neil
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I'm pretty sure most/all natural fluorite contains some silica. Any flux grade fluorite is going to have large amounts of impurities such as CaO and
SiO2.
"The term fluorspar refers to crude or beneficiated (extracted) material that is mined and/or milled for the mineral fluorite (calcium fluoride). It
is the common commercial term used to describe concentrates of fluorite sold as metallurgical grade (97 percent or less calcium fluoride) or acid
grade (more than 97 percent calcium fluoride)."
Fluospar grades
If you washed some of your MgO with phosphoric and added that to the mix, would not the phosphate react to produce silicate by binding up the silica
while still producing useful phosphide?
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blogfast25
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Neil:
Not all fluorite on the market is mined from fluorspar though.
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Neil
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I agree
"...most/all natural fluorite..."
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thaqrhys
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At this moment I'm conducting experiment on Aluminium Phosphide Production. Based on the literature I read : US Patent 4,412,979 describes few
examples which I summarize : in a reaction vessel exists 130 kg Aluminium Phosphide from earlier batch production, then the reactor is heated until
480C, afterwards mixture (250 kg of finely divided Aluminium powder + 1kg of Iodine) and 275 kg of Yellow Phosphorus is simultenously added with
addition rate is maintained in order the temperature of the vessel is not over 570C. At the end, it is produced 501 kg product with ALP (Aluminium
Phosphide) content of 95%.
And another US Patent 3,532,464 describes few examples of Aluminium or Magnesium Production that one of the example I used is : 2 kg of Aluminium
powder, 1.6 kg of Red Phosphorus, 400 grams of finely pulverized CaCl2 is rapidly mixed in a covered reaction vessel and then the mixture is ignited.
The result is 3.6 kg product with ALP content 80%.
I did the experiment based on the second US Patent because I ever read some literature about Self Propagating Exothermic Reaction. The reaction I did
was not that succesful as there was leakage so the molten phosphorus came out of the reactor and then I stopped the reaction. I let the reactor
naturally cooled until the next morning I opened the reactor and there was a fire because the heated raw material is burnt by the fresh air came into
the reactor. Although there was no ignition source and the reactor inside temperature is 26C, but the fire kept took place. However, there was a tiny
amount of ALP produced because I can smell it.
I'd like to ask if there is someone has experience in Solid State reaction like this? It is possible theoretically but it is not that simple on the
practice.
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AJKOER
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Here is a path to zinc phosphide that uses inexpensive starting materials by the carbon reduction of pure zinc orthophosphate.
Link:
http://www.springerlink.com/content/w021g4340575251r/
zinc phosphide is also an interesting compound with new applications in material science.
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