Burning Phosphorus in Air

White phosphorus is burned in air while monitoring the flask pressure.  The decrease in final pressure shows the partial pressure of oxygen in air.

Equipment

  • 1 L round bottom flask fitted with a threaded cap and syringe port
  • USB pressure sensor
  • tubing
  • laptop/software to monitor signal from pressure sensor
  • propane torch

Reagents

  • white phosphorus, P4

Presentation

  1. White phosphorus is placed in the round bottom flask, the threaded cap with syringe port is tightened onto the top.
  2. The pressure is displayed from the laptop showing that the initial pressure is approximately 1 atm.
  3. The propane torch is used to gently heat the P4. Stop heating when the phosphorus just begins to melt; it will spontaneouslt proceed to combust in the oxygen in the air contained in the vessel.
  4. The audience it directed to monitor the vessel pressure during the combustion reaction. 

Details

White phosphorus ignites spontaneously in the presence of oxygen at 50 °C.  The reaction that proceeds is:

P4 (s) + 5 O2 (g)   →  P4O10 (s)

The P4O10 is a white solid that will be expelled as smoke during the reaction and deposits onto the walls of the flask.  Initially, the heating of the phosphorus and air with the torch, as well as the exothermic reaction that follows will lead to an increase in pressure due to gas expansion.  As heat is dissipated, the pressure in the flask will drop to about 80% of its initial value.  In this case, the mass of phosphorus is chosen so that oxygen in the flask will be completely consumed, so the drop in pressure is the decrease in the partial pressure of oxygen. This can be used to demonstrate that air is about 20% oxygen.

Safety

fire

hot

smoke

Special Safety Notes

  • white phosphorus is very reactive and pyrophoric and should be stored under water until use
  • the P4O10 particles generated in the smoke of the reaction can be an irritant, adequate ventilation is needed
  • the burning phosphorus is hot and the flask will remain hot for some time afterwards
P4
Flask fitted with pressure sensor.
P42
White phosphorus burning in air.
p4 setup
The complete pressure sensing apparatus.