Bystander
Science Advisor
Homework Helper
Gold Member
- 5,616
- 1,777
The moisture is irrelevant to the oxidation. The "ready" oxidation is the property that makes phosphorus useful as an incendiary.Art said:(snip). ... we live on a planet with oxygen in abundance and plenty of moisture available in IMH layman's O it would seem to me the oxidisation step would take place quite readily
The kinetics of the reaction are a function of surface area. Run down to the hardware store and buy yourself a small package of "four ought" steel wool (used for hand rubbing steps in furniture finishing); take it outdoors on a windless day, prop it, sans wrapper, off the ground by sticking 3 toothpicks in it as a tripod, and touch a match to it (you might want to use pliers or an extension to hold the match). Please don't do the experiment near (say 2 - 3 meters) anything you don't wish to set on fire --- guess that means wear old clothes.and somehow I doubt WP takes as long to 'rust' as oh, say iron.
White phosphorus when expelled from shells by the bursting charge may be in either the liquid or solid phase (function of storage temperature, cold gun, hot gun firing conditions, time of flight (heating from atmospheric drag)). If liquid, it will be in a spray of droplets having some size distribution that's no doubt been studied. Anything over a few mm is going to be broken up by aerodynamic braking forces as it moves toward the limit of the burst radius. If solid, ignition might be delayed by ms while aerodynamic drag and autooxidation eats the fragments to the ignition temperature, at which time the heat evolved melts the interiors of the solid fragments rapidly; and, we're back to liquid droplets. The droplets ignite quickly, and the combustion proceeds at a rate determined by diffusion of oxygen to the droplet, and diffusion of phosphorus pentoxide vapor away from the droplet --- in terms of droplet radius, 10s, maybe 100s of μm/s. A mm droplet is going to burn for 10 s to a minute, so long as elemental oxygen is available. The reaction with atmospheric moisture takes place within several cm of the drop as the vapor cools below 200 C, or thereabouts. This is the white smoke cloud, an aerosol of micron to submicron metaphosphoric acid particles scattering light.
The difference between the combustion of the steel wool and phosphorus is that the products from the steel are solid, even at the combustion temperature, allowing freer diffusion of oxygen to the fuel, whereas for the phosphorus, the phosphorus pentoxide product is in the vapor phase at the combustion temperature and in expanding away from the droplet hindering diffusion transport of oxygen.
It's the oxidation step --- call it the oxidation step.In fact to the person affected the temporal element of the transitional phase
This is the most observable effect to the victim, getting a hole burned into your hide with not a red hot, but a white hot poker. Does get one's attention. The ten seconds to minute burning time for the mm droplet is increased by the fact that oxygen transport is blocked by the victim's flesh except for one side. Gives a sputtering effect --- as things cool, more oxygen moves in, heats up, blowing phosphorus pentoxide out, cools, ...would probably not even be observable
Elemental phosphorus is lipid soluble. The metaphosphoric acid is not. You have the mm droplet of elemental phosphorus sputtering at the bottom of a dime size, dime diameter deep hole in your arm, back, leg, whatever --- it's hot, and percolating through char, destroyed tissue, and damaged tissue to tissue that is still live. If the live tissue adjacent to the wound includes fatty tissue, elemental phosphorus, will dissolve in it. If you wish to call the dissolution process a reaction, yes, there is a reaction. If you are asking if there is a chemical reaction, no, there is not. The metaphosphoric acid being produced by combustion of the droplet in the wound may be coagulating (denaturing) proteins in the wound, but they were already cooked.assuming somebody who's skin is melting is objective enough to care about such details. (snip)
BTW could the effect of WP (or it's oxide) reacting with and dissolving in fatty tissue be likened to melting by a non-chemist observer?
"Melting?" No. Not in any sense that a chemist uses the word.
Fatty tissues being fatty tissues, fat soluble materials remain in the body for long time spans; in the case of elemental phosphorus, there are a number of slow chemical reactions possible, none of which are desirable. Fatty tissues are not well aerated; there may be slow production of diphosphorus tetroxide, P2O4, which hydrolyzes to phosphorous acid, which is toxic. It's conceivable that phosphine is produced, PH3, also toxic.
Incomplete debridement of affected tissues from phosphorus burns can leave phosphorus in tissues; delay in treatment of wounds can allow phosphorus in fatty tissues to diffuse over a larger area surrounding the wound. Either event leaves the victim chronically poisoned. Over time this leads to fossy jaw, and whatever other occupational diseases were observed in matchmakers --- no, not Tevi, them little sticks pyromaniacs love. Does the medical profession actually know how to treat phosphorus burns? In principle, yes. In practice, I haven't seen studies of "diffusion rates of elemental phosphorus in fatty tissues," in the literature --- haven't looked, but that's one that would have caught my eye in the chemical literature.
At any rate, this is the "humanitarian" reason behind international discussions on use of phosphorus in an antipersonnel role, not "melting" people.
Experiment(s) were done in the concentration camps during WW II for "disposal" purposes with the intent of finding conditions under which "self-supporting combustion" could be achieved. Various stacking arrangements were tried, and the bottom line is that there is insufficient heat transported downward from the flame and exhaust gases of such a fire to vaporize the combustibles at the rate necessary to support sustained combustion.Just another thought; seeing as how per your mail above burning WP generates a temp of 2000 C and bearing in mind steel melts at ~1300 C isn't it likely that this would create self supporting combustion of the fatty body tissues should burning WP come into contact with skin?
Getting back to the current discussion, the tissues in the immediate wound area do vaporize and combust, but the heat necessary for the vaporization is coming from the phosphorus combustion.
