What colour is tetraphosphorus, white phosphorus (WP)? This might seem to be an obvious question, but the answer is not so clear. The Wikipedia entry for phosphorus1 states the commonly-accepted view that:
… samples of white phosphorus almost always contain some red phosphorus and accordingly appear yellow. For this reason, white phosphorus that is aged or otherwise impure (e.g., weapons-grade, not lab-grade WP) is also called yellow phosphorus.
The term “yellow” phosphorus seems to be a recent occurrence. Traditionally, only the terms white and red were applied to the common allotropes of phosphorus.
The Wikipedia photo of “white phosphorus” shows a block with a white opaque coating and, from the sliced-off corner, a pale-yellow translucent centre (Fig.). This looks much like “yellow phosphorus” with a thin coating of phosphorus oxides, P4O6 and P4O10.
The yellow appearance is explained in several different ways in the literature:
Partington3 stated: “White phosphorus is commonly a white translucent soft waxy solid … On exposure to light it becomes yellow.”
Toy4 contended that the yellow colour is due to impurities, stating that: “Purified phosphorus is white or colourless … and has a waxy appearance not unlike that of paraffin. …Impurities such as arsenic and hydrocarbons are usually present.”
More recently, Patnaik5 contended that the impurity is due to traces of red phosphorus stating: “White phosphorus is a white, soft, wax-like transparent mass which often acquires a yellow appearance due to impurities, especially traces of red phosphorus.”
Intuitively, one would think that traces of red phosphorus would result in a pinkish colour, not yellow. Arsenic impurity seems possible, as one allotrope of arsenic (As4) is yellow. Interestingly, yellow arsenic is described as soft and waxy, somewhat similar to tetraphosphorus (P4).6
Now comes the interesting part — looking for images of tetraphosphorus in Google Images. As you look, be warned, there are endless images of the horrors of phosphorus used in military context listed under “white phosphorus”. The few images of “white phosphorus” show a cylinder or block, having a white, opaque and a powder-like appearance — obviously resulting from the white oxide coating over tetraphosphorus. By contrast, under “yellow phosphorus” one finds many images from chemical companies displaying their products. Irrespective of the company, and the purity, the tetraphosphorus displayed appears to be a consistent yellow colour. One company noted at least 99.9% purity with a maximum of 150 ppm arsenic and a maximum of 40 ppm sulfur. Another company advertises 99.9999% pure electronic grade “yellow phosphorus”, and that too, is accompanied by a photograph of a very yellow translucent solid.
The question arises, therefore, is there “white phosphorus”? If so, why are there no images of this truly colourless translucent material? Surely the images from all these chemical suppliers are of fresh, not “aged” phosphorus, yet they are consistently yellow. Are the parts per million of arsenic enough to always provide a yellow hue? Or is “white phosphorus” a myth? Is pure tetraphosphorus actually pale yellow?
References
- https://en.wikipedia.org/wiki/Phosphorus#White_phosphorus_and_related_molecular_forms
- https://en.wikipedia.org/wiki/Allotropes_of_phosphorus#mediaviewer/File: Wei%C3%9Fer_Phosphor.JPG
- J.R. Partington, General and Inorganic Chemistry for University Students, Macmillan, London, 1958, 3rd edition, page 592.
- A.D.F. Toy, The Chemistry of Phosphorus (Comprehensive Inorganic Chemistry, Chapter 20), Pergamon Press, New York, 1973, page 394.
- P. Patnaik, Handbook of Inorganic Chemicals, McGraw-Hill, New York, 2003, page 702.
- https://en.wikipedia.org/wiki/Arsenic
