306 ELEMENTS OF GROUP Vformula (H 3 AsO4) 2 -H 2 O. This liquid when cooled to 35.5°, or
below, can be crystallized to a solid product of the same com-
position, and this is the most satisfactory form in which to crystal-
lize arsenic acid. It is interesting to note that this liquid can be
much supercooled below 35.5°, but when once crystallization is
induced the temperature immediately rises to this point and
remains there until solidification is complete. Likewise when the
solid is being melted the temperature will not rise above the melt-
ing point, 35.5°, until the whole mass is liquefied.
Materials: arsenious oxide, As 2 O 3 , 50 grams = 0.25 F.W.
162V HN0 3 , 75 cc.
seed crystal of (HaAsO^-H^O.
Apparatus: 750-cc. casserole.
150-cc. casserole.
250° thermometer.
2-ounce glass-stoppered bottle,
iron ring and ring stand.
Bunsen burner.
Procedure: Place 50 grams of arsenious oxide in a 750-cc.
casserole; add 20 cc. of water, and then at the hood add 75 cc. of
16 2V HNO3, warm occasionally, just enough to keep up an action,
but do not allow the reaction to become violent, because the heat
would drive off nitric acid. When red vapors cease to be given off,
all the original white powder should have dissolved, and a clear
colorless or very pale yellow solution should be obtained. It will
sometimes happen, however, for no very apparent reason that the
reaction stops with a considerable amount of white powder still
undissolved although a plentiful excess of nitric acid may be
present. When this happens the addition of 5 cc. of 62V HCl
will make the reaction start up vigorously again and run to com-
pletion. Arsenious chloride is volatile and very poisonous; if
HCl is added keep the dish under the hood during the reaction and
the subsequent evaporation. Since HCl is not an oxidizing agent
its action must be essentially that of a catalyzer. Finally evapo-
rate the solution, holding the casserole with the hand and rotating
it to spread the liquid up on the sides, until the residue is just dry.
This residue should be arsenic pentoxide, and it should dissolve
completely, although somewhat slowly, when treated with 60 cc.
of water (see Note 1). Evaporate the solution by boiling it gently