Mike - 13-6-2004 at 01:12
Hi,
I found in a lab a very old ampoule with phosphorus trichloride. What’s bothering me is that it contains some residue. Can anyone know what the
residue could be?
BromicAcid - 13-6-2004 at 05:15
Possibly red phosphorus from light decomposition remember that PBr3 is the starting point for one of the variations of red phosphorus and although
PCl3 is not mentioned as a starting point for this to my knowledge it may well work. If it is red phosphorus then the remaining Cl2 produced would be
readily absorbed by the PCl3 solution forming PCl5 keeping the reaction clean and preventing excessive gas formation. No color of your residue is
mentioned but under these conditions the red phosphorus could be anywhere from light red to black depending on a number of forming factors (note I
don't think this would produce black phosphorus, it would just look black).
Mike - 16-6-2004 at 16:08
That's what I thought. But I wasn’t sure what’s happening with the chlorine gas. I was afraid that the pressure could rise in the ampoule.
Is the information about the absorption of chlorine by PCl3 100% sure?
The residue is bright but because of the dark glass I can't say if it's reddish or yellowish.
BromicAcid - 16-6-2004 at 18:40
Phosphrous trichloride will absorb chlorine at ambient temperature and pressure to form phosphorus pentachloride, therefore in the ampule where
chlorine evolution would lead to an increase in Cl2 pressure I would say very nearly 100% that the PCl3 would absorb the Cl2 to form PCl5
5PCl3 ----> 2P + 3PCl5