Sciencemadness Discussion Board

H3PO4 + Al = phosphine ?

papaya - 16-7-2014 at 06:44

Recently I obtained phosphoric acid, that is advertised to be of 85% concentration. When I put a ml of it into test tube, added some aluminum foil and started to heat a vigorous gas evolution started, which I thought must be hydrogen.. BUT it smells strongly of phosphine, how could this happen, is Al capable of reducing the acid ( P +5) to phosphine (P +3) ?

Metacelsus - 16-7-2014 at 09:36

Phosphine has phosphorous in a -3 oxidation state, not +3.

If it was making significant amounts of phosphine, it would probably have been on fire. Maybe phosphine is just present in trace amounts.

Amos - 16-7-2014 at 09:42

I have no idea how this could happen, either. The fact that I just read "it smells like phosphine" in a matter-of-fact way is kind of disturbing, considering its tendency to KILL people in small doses. Aluminum Phosphide, AlP, does produce phosphine on contact with water or acids, but it is beyond me how the phosphide could have formed.

Amos - 16-7-2014 at 09:45

Also, the smell commonly associated with phosphine may or may not come from impurities of P2H4, disphosphane. You should look into whether or not this could have formed.

arkoma - 16-7-2014 at 09:52

I used to get sent into a grain bin with a can of Aluminum Phosphide pellets to sprinkle on the floor before we'd fill it with wheat/corn/ rice whatever. Would hyperventilate, hold my breath and RUN in a circle dumping them out. From what I understand, would "time-release" phosphine gas and keep the vermin out/dead.
These bins, in fact:



(Granpa's farm, in Garland City, Arkansas)

aga - 16-7-2014 at 12:20

What does Phosphine smell like ?

Sounds pretty useless as a poison gas if you can smell it more than once.

arkoma - 16-7-2014 at 12:32

aluminum phosphide:
Quote:
It generates phosphine gas according to the following hydrolysis equation.[6]

2 AlP + 6 H2O → Al2O3∙3 H2O + 2 PH3

It is used as a fumigant when other pesticide applications are impractical and when structures and installations are being treated, such as in ships, aircraft, and grain silos.


I remember that stuff "smoking" on contact with the humid air in Ark-In-Saw

[Edited on 7-16-2014 by arkoma]

papaya - 16-7-2014 at 12:56

Really, typo - oxidation state is -3 in phosphine. The smell is very similar to one of technical calcium carbide when you want to make acetylene, don't know whether it's PH3 or P2H4, but I don't think it's going to kill in that amounts (otherwise I wouldn't write here anymore). When diluted acid is reacted with Al there's no smell anymore, so I think the concentration is important here - must be conc. H3PO4 can act as oxidizer like conc. H2SO4 does. It's pity it's only 85% and not more - might be interesting to test that.
Btw., I thought most phosphates are insoluble, but when in other experiment I add some CuSO4 to conc. acid and heat it dissolves giving blue solution, where's the expected precipitate ?

[Edited on 16-7-2014 by papaya]

aga - 16-7-2014 at 13:22

Glad you are still here to comment.

I gotta go pronto, as i feel an AJKOER comment will be coming up any second now.

Polverone - 16-7-2014 at 15:10

Quote: Originally posted by papaya  
Recently I obtained phosphoric acid, that is advertised to be of 85% concentration. When I put a ml of it into test tube, added some aluminum foil and started to heat a vigorous gas evolution started, which I thought must be hydrogen.. BUT it smells strongly of phosphine, how could this happen, is Al capable of reducing the acid ( P +5) to phosphine (P +3) ?


Concentrated phosphoric acid is dehydrated to a complex polyphosphoric acid mixture at elevated temperatures and can be reduced to phosphorus or possibly phosphine by many metals. It only takes traces of phosphine to produce that characteristic odor. If you do this in the dark you will see a greenish glow where the vapor reacts with air. When illuminated you see the smoke from the oxidation instead. But you'll see significant smoking/glowing only after the acid has become more concentrated and heated; I used borosilicate test tubes and the acid grew hot enough that the glass was glowing red.

I've tried the reaction with zinc, lead, tin, and aluminum. The extremely long phosphorus thread contains some discussion if you have the patience to read it.

papaya - 16-7-2014 at 15:18

Thanks Polverone, I'll read, one more question - is it possible to concentrate 85% acid even more some way (heating, maybe electrolysis?), and also I don't understand why polyphosphoric acids should form when there's still 15% of water present, are they stable in water?

Ascaridole - 16-7-2014 at 16:03

For the most part no, 85% is the limit for ortho phosphoric acid. If pyrophosphoric acid is not a problem yes you can heat it with awesome heating and drive it all the way to tripolyphosphoric acid and maybe even to the meta form. P2O5 can be used to generate the tetrapolyphosphoric acid.

As for why polyphosphoric acid can exist in 85% its due to protination of an ortho phosphoric acid molecule from another ortho phosphoric molecule and then subsequent lost of water from the protonated ortho phosphoric acid to form the pyrophosphoric acid.

Just as 98% sulfuric acid can be used for sulfonation without the need for oleum, SO3 is formed in situ due to auto protonation.

The pyrophosphoric acid does react with the 15% water but just as acetic anhydride doesn't just hydrolyze away into acetic acid when added to water neither does pyrophosphoric acid. What is converted back into the ortho phosphoric acid once again goes through the protonation process and as you can see an equilibrium exists.

woelen - 16-7-2014 at 22:45

The effect you observe with phosphoric acid is quite common with oxo acids and oxo anions. E.g. when you add zinc powder to moderately concentrated H2SO4 you get small amounts of H2S (rotten egg smell). When you add zinc powder to moderately concentrated HNO3 you get lower oxides of nitrogen, but you also may get some ammonium ions in the solution. In alkaline solution (e.g. a solution of KNO3+NaOH in water) you get ammonia if you add aluminium and heat a little.

There is only one simple oxo anion which cannot be broken down in this way, and that is the perchlorate ion and moderately concentrated HClO4. Breaking down perchlorate ion either requires high temperature (as in pyrotechnic mixtures) or very high concentrations (well over 70% by weight). Adding metals like zinc or aluminium to moderately concentrated HClO4 just gives H2 and no chloride ions or HCl. Amazingly, the same so inert anion forms an insanely reactive acid when it becomes (nearly) anhydrous. Anhydrous HClO4 even at room temperature ignites most organic stuff like paper and reacts explosively with metals like zinc, aluminium and magnesium.

papaya - 17-7-2014 at 04:35

Yes woelen, that's what I also thought - a reduction of phosphate anion or more probably of free acid (since it takes place at high conc. only). Now I'm interested what will happen if I try to electrolyze conc. acid - it may be reduced at cathode the same way giving off interesting products , else the water will be electrolyzed giving more concentrated (maybe poly-) phosphoric acid. Either is good to see.

woelen - 17-7-2014 at 06:33

Electrolysis of the concentrated acid will be hard. The acid is quite viscous and only weakly ionized. I expect that the current, flowing through the liquid, will be very small and hence the electrolysis will be very slow. So, concentrating the acid in this way is not practical.

You may get small amounts of phosphine, but I do not expect more than trace amounts. You mostly will get H2 at the cathode.