The addition compound P2S5 ? 7NH3, formedat —20°C by treating P2S5 (see p. 567) with gaseous, dry NH3, is gradually heated in a stream of NH3 to 230°C, followed by careful heating in a stream of H2 or N2 to a higher temperature. First, (NH4)2S is liberated, then P and S are evolved, and after heating to bright red heat only P3N5 remains. To remove the last traces of S, the heat must be increased steadily almost to the temperature of decomposition of the P3N5. It is best to use a Pyrex tube, open at both ends, which fits tightly inside an outer Pyrex tube. This is preferred to a porcelain boat since the substance is more exposed to the gas than in the boat. Hydrogen is preferred to nitrogen, since it reduces the extensive decomposition of the product. If the temperature is too high, an odor of NH3 will be noticeable, an indication that the temperature must be immediately lowered.
