US EPA

8614010 

Journal Article 

The vapour pressure of red phosphorus 

Melville, HW; Gray, SC 

1936 

1 

Transactions of the Faraday Society
ISSN: 0014-7672 

32 

1026-1030 

English 

10.1039/TF9363201026 

A redetermination of the vapour pressure of red phosphorus has been made by evaporation in a vacuum. Application of the Herz-Knudsen effusion equation to the results gives values for the vapour pressure about 107 times lower than the normal static method. The new values are given by the equation log10 p (mm.)=-4110/T+1.1.The reason for the discrepancy is due to the fact that although P2 molecules condense on and evaporate from red phosphorus, the equilibrium P4 ⇌ 2P2 is set up in a closed system. At temperatures normally employed the equilibrium is almost wholly on the P4 side and hence in the static method the pressure is due almost wholly to P4 molecules. In the molecular distillation method no opportunity is given to the molecules to combine to P4 and therefore the apparent vapour pressure is much smaller.