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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.

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It is correct that white phosphorus is usually used as the standard state for tabulating thermodynamic data, in particular by the Committee on Data for Science and Technology (CODATA).

A comparison of thermodynamic data for phosphorus and some of its compounds can be found in Rard, J. A.; Wolery; T. J. The Standard Chemical-Thermodynamic Properties of Phosphorus and Some of its Key Compounds and Aqueous Species: An Evaluation of Differences between the Previous Recommendations of NBS/NIST and CODATA. J. Solution Chem. 2007, 36 (11), 1585–1599.. This publication also includes an explanation for the use of white phosphorus as the standard state:

Although white phosphorus is not the thermodynamically stable allotrope, the red and black forms are difficult to prepare in pure form, which makes them less suitable for quantitative thermodynamic measurements.

Similarly, from the other side of the Atlantic, A.T. Dinsdales' SGTE Data for Pure Elements, originally published as CALPHAD 15(4) 317-425 (1991), says:

[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.

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