# Phosphorus standard state

Why is white phosphorus considered to be the standard state of phosphorus although it is only metastable? Red phosphorus is thermodynamically more stable than white phosphorus, yet it is not the standard state.

[The] reference phase is normally the phase stable at 10$^{5}$ Pa and 298.15 K. The exception to this rule is phosphorus for which, by convention, the white form is chosen as the reference phase because the more stable red form is difficult to characterise.