CRYSTALLIZED ARSENIC ACID 305
of the flask and the sides of the bottle offer sufficient cooling sur-
face to condense all the vapor if the distillation is not too rapid.
Cover the mouth of the bottle with an inverted beaker until it
has cooled; then insert the glass stopper.
QUESTIONS
- Experiment. Caution at the hood. Place 0.5 cc. of PBr 3 in
each of two dry test tubes. To one tube add 0.5 cc. of water and
note that it forms a layer above the heavier PBr3. Agitate gently
to mix the layers and increase the surface of contact. The mixture
warms up, and rather suddenly the two liquids become homo-
geneous and a gas is evolved. Blow the breath across the mouth
of the tube while this gas is issuing and note the effect. What is
the gas? What is left in the residual solution? To the other
tube add suddenly 15 cc. of cold water and let the tube stand until
the two layers become homogeneous. Explain the difference in
the observed effect in the two tubes.
- Write the equations for the hydrolysis of PBr 3 and PBr 6
respectively.
- What are the specific gravity, melting point, and boiling
point of phosphorus trichloride — of phosphorus pentachloride?
How would you arrange an apparatus to prepare each of these
compounds?
PREPARATION 55
CRYSTALLIZED ARSENIC ACID, (HsAaO^-H^O
Arsenic acid in its properties shows a striking similarity to
phosphoric acid; and even the method of its preparation is similar,
in that use is made of the oxidizing action of nitric acid. Instead
of starting with uncombined arsenic, however, use is made of
arsenious oxide, As 2 O 3 , a product which condenses in the flues
wherever ores which contain arsenic are roasted. This is oxidized
by the nitric acid to the higher oxide, AS2O5, which, with water,
yields arsenic acid, H3ASO4. By evaporating its solution for a
long time on the water bath, crystals of ortho-arsenic acid having
the composition H3ASO4 can be obtained. By prolonged evapo-
ration at higher temperatures crystals of the composition H4AS2O7
and HAsO 3 , respectively, can be obtained. When a solution of
arsenic acid is concentrated according to the following directions,
a liquid is obtained of almost exactly the composition given by the
