PHOSPHORUS TRIBROMIDE 303Procedure: Put the bone ash in a porcelain evaporating dish,
add 70 cc. of water, and stir the mixture until a thick paste is
obtained. Stir and add 47 cc. of concentrated sulphuric acid as
rapidly as possible without excessive heating. Continue to stir
vigorously until the mixture begins to stiffen. This usually
requires from 10 to 15 minutes. Add 500 cc. of cold water and
stir until a thin paste, free from lumps, is obtained. Filter the
cold mixture, using suction. Wash the solid in the funnel with
50 cc. of cold water. Combine the nitrates. Add anhydrous
sodium carbonate in small portions with stirring until effervescence
no longer takes place and a drop of the solution turns phenolphthal-
ein pink. This usually takes 55-60 grams. Filter and evaporate
the solution to 250 cc. Transfer the clear liquid to an 8-inch
crystallizing dish and allow it to stand until a satisfactory crop of
crystals is obtained. Decant the liquid from the crystals and
rinse them with a little distilled water. Dry them thoroughly
on paper towels (Note 9 (6), page 15). Disodium phosphate is
efflorescent.QUESTIONS- Test solutions of monosodium phosphate, NaH 2 PO4, disodium
phosphate, Na 2 HPO4, and trisodium phosphate, Na 3 PO4, with
litmus paper. See pages 116-117. Explain how a compound
can be an acid salt and react alkaline to litmus. - List the solubilities of the three sodium phosphates. How
would you prepare each from phosphoric acid? - To 5 cc. of magnesium chloride solution add 6N NH 4 0H
until it is alkaline. Add solid NH 4 C1 in small portions until the
precipitate of magnesium hydroxide is dissolved. This solution
is known as "magnesia mixture" and is used in testing for phos-
phates. Add a little Na2HPC>4 solution and allow the tube to
stand until a white, crystalline precipitate of MgNHaPO* is formed.
Write equations for all reactions.
PREPARATION 54
PHOSPHORUS TRIBROMIDE, PBr3
Phosphorus reacts directly with bromine to form two com-
pounds. With an excess of phosphorus the tribromide, PBr 3 , is
formed; the pentabromide, PBr 5 , results from the use of an excess