ORTHO-PHOSPHORIC ACID 299upon phosphorus contains a considerable quantity of phosphorous
acid, H 3 PO 3 ; but upon concentrating this solution, a point is
reached at which a rather vigorous reaction takes place, which
consists of an oxidation of the phosphorous to phosphoric acid by
means of the nitric acid still present.
Commercial phosphorus often contains a small quantity of
arsenic. This on the treatment with nitric acid is oxidized to
arsenic acid, which would contaminate the preparation of phos-
phoric acid unless removed by hydrogen sulphide.
Phosphorous acid may always be present in the product if
the oxidation with nitric acid has not been complete, and its
presence may be detected by its ability to reduce silver nitrate
and give a black precipitate of metallic silver,
H3PO3 + 2AgNO 3 + H 2 O = 2HNO3 + 2Ag 1 + H3PO4Materials: red phosphorus, P, 31 grams = 1 F.W.
6N HNO3, 295 cc.
16ATHNO3, 20 cc.
seed crystal of H3PO4.
Apparatus: 2-liter round-bottom flask.
condenser to hang in neck of flask (Fig. 25.)
8-inch porcelain dish.
250° thermometer.
125-cc. casserole.
2-ounce glass-stoppered bottle.
tripod.
burette clamp.
wood ring.
Bunsen burner.Procedure: Place 295 cc. of QN nitric acid in a 2-liter round-
bottom flask; connect the condenser with the cold water tap, and
hang it in the neck of the flask to condense acid vapors and allow
them to drip back into the reacting mixture. Add about one-fifth
of the phosphorus and warm the flask very cautiously until red
vapors begin to appear; then stand the flask in a wooden ring and
allow the reaction to proceed, keeping a pan of cold water at hand,
in which to immerse the flask if the reaction gets too violent.
After the foaming has abated somewhat add a little more of the
phosphorus, and again wait until action has abated, and so on,