Experiments‎ > ‎

Burning of white phosphorus

The element phosphorus comes 2 common allotropes. The red and the white allotrope.
-Red phosphorus is a red flammable, nontoxic solid, wich is made by exposing white phosphorus to heat/light (in the absence of oxygen). It is used in matches and in the production of certain phosphorus compounds (e.g. reacting it with chlorine produces phosphorus pentachloride, a versatile chlorinating agent).
-White phosphorus is a yellow or darkyellow/orange, waxy, very toxic solid. It will ignite spontaniously in air if given enough time, as the self igintion temperature is 32C and it will react readily with oxygen at room temperature in an exothermic matter.
In very pure form white phosphorus is actually white and in extremely pure form it forms colorless crystals, however these are very unstable, and quickly turn to the yellow form.
White phosphorus is used by the military as a smokescreening agent, because it will emit high volumes of smoke when burned.
There are other allotropes of phosphorus, like violet phosphorus. These however are very rare and inert.
NOTE: I discourage people in the USA to perform this experiment, as elemental phosphorus is forbidden to posses by law,as it is used in the manufacture of methamphetamine. It is also banned in many other countries. I discourage anyone who uses large amounts of phosphorus (more than a few 100's of milligrams) to perform the experiment outside a fumehood, as it can be very dangerous.
Needed equipment:
-Round bottomed flask (or erlenmeyer)
-Pair of tweezers
-A sharp knife
-Sand or other inert material to phosphorus and it's oxides, like sodium chloride. I used sodium chloride as I do not have sand at hand :-(.
-Glas rod
Needed chemicals:
-'White' phosphorus (this is very hard to buy, so it's best to make it from red phosphorus. A good detailed synthesis is found here: )
Please beware that it is a very hazardous synthesis! Myself, I have not performed it, but got the phosphorus from someone else. I only have 2-3 grams total of white P, as it is too dangerous to have in large amounts.
-For cleanup dilute hydrochloric acid and bleach.
-In case anything goes wrong (wich should not happen if done responsibly) a dilute copper sulfate solution to wash a white P burn. It is used by military as a 0,5-2% solution to wash wound, to prevent white phosphorus poisoning. Beware that copper sulfate is a harmful substance by itself.

-White phoshorus is extremely toxic! It smells like garlic.
It is extremely flammable and will self ignite in air if given enough time. Cut phosphorus under water with a knife! Be very careful to not get phosphorus on the skin! The temperature of the skin is most of the time around 30-37C and the autoiginition temperature is 32C, so it will probably spontaniously iginite when contacted with skin. This will cause deep severe burns and possibly serious poisoning. .
-The fumes produced in the combustion of phosphorus consist of phosphorus oxides. These are toxic and irritating to the lungs when inhaled. Avoid inhaling this smoke. Is contact with water the phosphorus oxides will form phosphoric acids.
-The cleaning solution slowly gives of chlorine wich is toxic by inhalation. However the amount of chlorine is very small, so there is no real risk involved here.
-Only use a small amount of phosphorus. Using too much in a closed vessel will not result in complete combustion (lack of oxygen) and possibly white phosphorus will remain, wich will evaporize in the heat. This can be an inhalation hazard when opeing the flask. Also in the absence of sufficient oxygen, the lower oxide of phosphorus, P4O6, wich smells like garlic, and is very toxic.
Take a round-bottomed flask and add about 10 grams of inert material (like sand, or sodium chloride). This is too absorb the heat formation when burning the phosphorus, to prevent local overheating and possible cracking of the glass. Next take a SMALL piece of white phosphorus (I used not more than 100mg). If needed, cut it under water. Always quickly transfer phosphorus in open air, but not too quickly as you really don't want to drop the phosphorus on the bench or so.
Add the phosphorus to the flask. Have a heated glass rod at hand and touch the phosphorus. It will ignites, with a bright yellow flame, forming a LOT of smoke. I recommend to keep the flask open, and to not cover it. This ensures that most to all phosphorus is converted to the relatively harmless (compared to diphosphorus trioxide and white phosphorus) phosphorus pentoxide.

YouTube Video

There was a lot of smoke, and nothing got out of the fume hood, although it seems like in the video.

When the fire went out, the residue emitted light blue, very bright light. This is called chemiluminescence. This stopped after 10 seconds or so. If you look carefully, you can observe it at the end of the video.

Next the cleaning solution was added to the flask, prepared by a few mLs of 10% hydrochloric acid and a few mLs of 5% bleach.
White phosphorus consists of 4 P atoms bounded together like a pyramid-like structure. This structure is not very stable and very reactive.
When burned the following reaction takes place:
P4 + x O2 --> P4O2x
One would expect there would be one product: phosphorus pentoxide. This is not the case. Other oxides are formed as well, like diphosphorus trioxide. This can be proved by adding very hot water to the reaction vessel. Phosphorus trioxide will react with hot water to form phosphine (very toxic!). I did not do this, as phosphine is very toxic, and is a gas.
Remaining white phoshorus reacts with chlorine to form phosphorus trichloride, and this reacts with excess chlorine to phosphorus pentachloride. Both chlorides hydrolyse immediatly to phosphoric acid phosporous acids.