Scarlet Phosphorus, and the Transitions to Violet Phosphorus

It has already been shown that the transition through the various grades of red phosphorus to violet phosphorus proceeds continuously as higher temperatures and longer times of heating are progressively applied. Liquid phosphorus, when in the early stage of transformation, shows a fine scarlet colour and probably then contains the form known as scarlet phosphorus or Schenck's phosphorus, from its discoverer.

Scarlet phosphorus is prepared by exposing to light a solution of phosphorus in carbon disulphide or carbon tetrachloride, or by boiling a 10 per cent, solution of phosphorus in phosphorus tribromide. In the latter case the product contains considerable quantities of the solvent, in which it is slightly soluble. The solvent may, however, be removed by reducing the tribromide with mercury at a temperature over 100° C. The remaining tribromide and also the mercuric bromide may then be extracted with ether. The lighter coloured preparations are more reactive than the darker, on account no doubt of their finer state of division.

It may also be noted that liquid phosphorus prepared by melting in sealed tubes under high pressures may deposit scarlet crystals.

Scarlet phosphorus has a density of 2.0, i.e. slightly less than that of red phosphorus. It is isotropic, and in this respect resembles red phosphorus which has been prepared at comparatively low temperatures. Red phosphorus which has been prepared at higher temperatures shows distinct evidence of crystalline structure.

Scarlet phosphorus is thus a transitional form and can be converted into the red variety by heating for some time at 300° C. in an atmosphere of carbon dioxide.