304 ELEMENTS OF GROUP Vof bromine. In both cases the reaction is very vigorous, and con-
siderable caution must be exercised in the following preparation.
Phosphorus tribromide is a colorless liquid which freezes at —40°
and boils at 173°. The pentabromide is a yellow crystalline solid
which, when heated, decomposes into the tribromide and bromine.
Since both bromides hydrolyze completely, perfectly dry materials
and apparatus must be employed.
Materials: bromine 12.5 cc. = 40 grams = 0.25 F.W. Br 2.
red phosphorus, 7 grams.
Apparatus: special Pyrex test tube 1 inch in diameter and 24
inches long.
2.5-inch funnel.
125 cc. distilling flask,
small beaker.
2-ounce glass-stoppered bottle.
250° thermometer.
Bunsen burner.
Procedure: Clamp the long test tube in a vertical position,
covering the clamp jaws with folded paper instead of rubber.
Under the hood pour the bromine into the test tube. Take
about 0.05 gram of the red phosphorus on a spatula and drop it
into the bromine. When the heat from the reaction has dissi-
pated add another portion of phosphorus. Continue the addition
of the phosphorus until all is used, being careful to avoid driving
red bromine vapors out of the top of the tube. About 20 minutes
is required. The tube now has a yellow deposit of PBr 5 along the
sides, red bromine vapor and liquid and excess phosphorus in the
bottom. Apply a small flame held by the hand to the bottom of
the tube, and boil the liquid. The level of the top of the vapor
column can be clearly seen. Boil the liquid until this level has
risen to within half an inch of the top of the tube. The hot vapor
decomposes the PBr 5. The condensed PBr 3 dissolves the bromine
and carries it back to the bottom of the tube to react with the
excess phosphorus. Continue this " refluxing " until the red color
of free bromine has disappeared from the tube. After partial
cooling, pour the liquid PBr 3 with the aid of a dry funnel into the
distilling flask. Close the neck of the flask with a cork stopper
carrying a thermometer. Insert the side arm 1 inch into the small
bottle, and distil the PBr 3 slowly into the bottle. The side arm