Sciencemadness Discussion Board

Pyrophoric bulk elements

chornedsnorkack - 28-12-2013 at 06:20

Elements which in bulk rather than in fine powder are said to burst in fire in room temperature air are said to be three.

Phosphorus, rubidium and caesium.

Phosphorus certainly does not always burst in fire, because it also glows in dark without fire.

How fast do chunks of rubidium and caesium, respectively, burst in fire when exposed to dry, cool air?
How does the behaviour of rubidium compare to potassium under the same conditions (dry air, no mechanical disturbance to, say, force the metal in contact wit its hyperoxide)?

blogfast25 - 28-12-2013 at 07:52

I don't think these questions are very interesting.

But here's a bit of 'caesium porn':

http://www.youtube.com/watch?v=PxUV4cB9Ehw

macckone - 28-12-2013 at 08:52

The allotrope of phosphorus is important for 'spontaneous' ignition. Ie. white phosphorus. Red and Black phosphorus are not 'easy' to ignite. Lithium, Sodium, and Potassium may also 'spontaneously' ignite but require some moisture in the air.

And add cerium and plutonium to the list.
Plutonium again requires some moisture while
Cerium may require a disturbance of some kind
to damage the outer oxide coating.

blogfast25 - 28-12-2013 at 09:23

Plutonium, pyrophoric? Explain and/or provide some references. Remember, he's talking about BULK pyrophoric.

Brain&Force - 28-12-2013 at 10:14

Plutonium is pyrophoric, as shown in this photo:



The radioactivity of plutonium may contribute to the effect.

Cerium ignites when struck. It's used in lighter flints for this purpose, and cutting a piece of it can cause a spectacular shower of sparks, as demonstrated in this video. But in air it just oxidizes away.

blogfast25 - 28-12-2013 at 11:04

Quote: Originally posted by Brain&Force  
Plutonium is pyrophoric, as shown in this photo:

The radioactivity of plutonium may contribute to the effect.

Cerium ignites when struck. It's used in lighter flints for this purpose, and cutting a piece of it can cause a spectacular shower of sparks, as demonstrated in this video. But in air it just oxidizes away.


Hmmm... Pu in very specific conditions.

Re, Ce, by that reasoning so is Ti (a common test is to hold an object to a grinding stone, bright sparks are an indication of Ti). It's stretching the definition of bulk pyrophoricity a bit, IMHO.

macckone - 28-12-2013 at 11:53

A mechanism for plutonium pyrophoricity
http://www.sciencedirect.com/science/article/pii/00223115949...

Plutonium doesn't just spark, it burns. There are plenty of photos and videos of this around the internet.

Cerium is pyrophoric in a major way if the outer oxide coating is disturbed. Ie. It ignites when the coating is disturbed.

This is not at all the same as applying a grinding wheel.


blogfast25 - 28-12-2013 at 13:21

http://en.wikipedia.org/wiki/Pyrophoricity:

A pyrophoric substance (from Greek πυροφόρος, pyrophoros, "fire-bearing") is a substance that ignites spontaneously in air at or below 54.55 °C (130.19 °F)".[1] Examples are iron sulfide and many reactive metals including uranium, when powdered or thinly sliced.

http://en.wikipedia.org/wiki/Cerium#Precautions:

Cerium, like all rare-earth metals, is of low to moderate toxicity. Cerium is a strong reducing agent and ignites spontaneously in air at 65 to 80 °C.

http://www.sciencedirect.com/science/article/pii/00223115949...

A proposed mechanism for plutonium pyrophoricity quantitatively predicts the ignition temperature of plutonium as a function of surface : mass ratio and particle size. Plutonium must exceed 475°C before self-ignition occurs. External heating of massive samples is necessary to achieve this condition, while finely divided materials can reach the ignition point by an alternative, two-step mechanism. First, the thin layer of surface PuO2 on the metal undergoes kinetically controlled reduction to Pu2O3 near 150°C. Second, the trivalent Pu2O3 reacts with gas-phase oxygen to reform PuO2. Heat generated from the second reaction is sufficient to raise the temperature of small particles or thin foils above the 475°C ignition point. Details of this mechanism are given, including a discussion of plutonium oxidation and a calculation of adiabatic temperature increase due to oxidation of the Pu2O3 surface layer. Plutonium pyrophoricity data are summarized and compared to model results.

Macckone:

You are willing these materials to be more pyrophoric than they actually are.

”There are plenty of photos and videos of this around the internet.”

Would love to see them!

macckone - 28-12-2013 at 22:53

Quote: Originally posted by blogfast25  


Macckone:

You are willing these materials to be more pyrophoric than they actually are.

”There are plenty of photos and videos of this around the internet.”

Would love to see them!


Interesting the the first link of your supporting material includes cerium and plutonium as pyrophoric.

And as I stated cerium is only pyrophoric if the protective oxide coating is disturbed. Plutonium I haven't played with so I have to
go with the wiki article you posted which states it is. One note on plutonium is that it is also self heating (which is not chemical in nature).

From a safety standpoint I would consider both of these pyrophoric while from a strictly definitional standpoint of
less than 130F they may not make it. Cerium as previously
stated requires the oxide coating to be disturbed. While
plutonium is much more dangerous than just flammable.
I think there are probably other definitions of pyrophoric as well.

blogfast25 - 29-12-2013 at 06:15

Quote: Originally posted by macckone  
Interesting the the first link of your supporting material includes cerium and plutonium as pyrophoric.


I could put up a ton of links that 'prove' (NOT) that the moon landings were a hoax too. Just because someone says so doesn't make it true.
Quote: Originally posted by macckone  
I think there are probably other definitions of pyrophoric as well.


And this is precisely the problem: poor definitions, in particular with regards to the temperature limit imposed. I could come up with a definition that makes bulk paper pyrophoric too!

It's far more meaningful to state the full set of conditions in which a given material will 'auto-combust' than to rely on sloppy definitions of pyrophoricity.

[Edited on 29-12-2013 by blogfast25]

Random - 29-12-2013 at 08:24

Phosphorus will glow in the dark for some time and then it will catch fire once it heats up enough. That glow in the dark is actually reaction of it to form phosphorus oxides.

Some radioactive elements as I have read can heat up to 600 degrees celsius regardless of air surrounding them because of strong radioactivity. I think I read that on polonium wikia page.

[Edited on 29-12-2013 by Random]

macckone - 29-12-2013 at 19:39

Quote: Originally posted by blogfast25  
Quote: Originally posted by macckone  
Interesting the the first link of your supporting material includes cerium and plutonium as pyrophoric.


I could put up a ton of links that 'prove' (NOT) that the moon landings were a hoax too. Just because someone says so doesn't make it true.
Quote: Originally posted by macckone  
I think there are probably other definitions of pyrophoric as well.


And this is precisely the problem: poor definitions, in particular with regards to the temperature limit imposed. I could come up with a definition that makes bulk paper pyrophoric too!

It's far more meaningful to state the full set of conditions in which a given material will 'auto-combust' than to rely on sloppy definitions of pyrophoricity.

[Edited on 29-12-2013 by blogfast25]


I would hope you wouldn't argue that the moon landings are a hoax (not that you would) and then send them to the nasa page
on the moon landings as 'evidence'.

I tried to be complete in my qualification of those two substances.
Ie. one requires the oxide coating to be disturbed.
And the second is self heating in bulk amounts.

So I don't think my definition was exactly sloppy, it may
not meet your exacting standards but none of us are likely
to happen upon a source for plutonium. And if we do
I hope the person reports it to the authorities because
there are way to many ways that can go badly.

blogfast25 - 30-12-2013 at 09:53

Quote: Originally posted by macckone  

So I don't think my definition was exactly sloppy, it may
not meet your exacting standards but none of us are likely
to happen upon a source for plutonium.


Your definition.

At least the latter part of that sentence goes a long way to explain why:
Quote: Originally posted by macckone  
There are plenty of photos and videos of this around the internet.


… isn’t true either. Unless you have your own definition of "plenty", of course...

Brain&Force - 30-12-2013 at 10:43

macckone, cerium doesn't have a protective oxide layer, its oxide just flakes off until the metal is destroyed. Otherwise the cerium in the lanthanide corrosion test would have ignited during preparation (all oxides were removed beforehand).

Pyrophoricity is really badly defined: iron sparks when ground with a grinding wheel; magnesium does not. Yet bulk Mg burns in air very easily while Fe just rusts. Which of them is pyrophoric?

I think a better definition of pyrophoric is being able to ignite without activation energy in the form of heat (but with mechanical or light energy as activation energy). At the same time, that would exclude several compounds, such as triethylborane, because the ambient heat is already enough activation energy. (Triethylborane ignites in air spontaneously at temperatures higher than -20ºC.)

blogfast25 - 30-12-2013 at 10:59

Quote: Originally posted by Brain&Force  
Pyrophoricity is really badly defined:[...]


That is the central point.

Re activation energy, that is itself a concept open to interpretation and often misinterpreted.

Personally I would ditch the whole pyrophoricity idea, bar in a number of indisputable cases.
Quote: Originally posted by Brain&Force  
Otherwise the cerium in the lanthanide corrosion test would have ignited during preparation (all oxides were removed beforehand).


What ‘lanthanide corrosion test’ are you referring too?


[Edited on 30-12-2013 by blogfast25]

Brain&Force - 30-12-2013 at 11:58

Sorry, this one. (I forgot to post the link)

blogfast25 - 30-12-2013 at 12:37

Quote: Originally posted by Brain&Force  
Sorry, this one. (I forgot to post the link)


Very interesting study. They behave markedly differently, for a group that is so similar.