CRYSTALLIZED ARSENIC ACID 307in a small casserole until the temperature has risen to 115°. Then
transfer the liquid to a very narrow beaker or a test tube, and boil
it carefully with a small flame until the temperature shown by a
thermometer inserted in the liquid has just risen to 160°. Cool the
product to below 35.5°, place it in a weighed sample bottle, and
seed it with a small crystal of (H 3 AsO4) 2 -H 2 O, whereupon the whole
will slowly crystallize to a solid mass. Stopper the bottle tightly,
since arsenic acid takes moisture rapidly from the atmosphere.
Note 1. If the residue on evaporation does not redissolve after
warming it 10 minutes with 60 cc. of water, it contains arsenious
oxide either from incomplete oxidation by nitric acid, or from a
decomposition of arsenic pentoxide by overheating. Test 1 cc.
of the suspension containing the undissolved substance by adding
10 cc. of water, then solid sodium bicarbonate until no more
effervescence occurs, and then a considerable quantity in excess.
Add to this a solution of iodine, drop by drop. The amount of
the latter which is decolorized (if any) corresponds to the amount
of arsenious acid (AS2O3), which was in the sample.
If arsenious acid is present it must be oxidized by further treat-
ment with 162V HNO3 and a little HC1.
QUESTIONS- Compare the strength of arsenic and arsenious acids. Of
what general rule is this comparison an example? - To a solution of arsenic acid (0.1 gram in 10 cc. of water)
add magnesium chloride and then NH4OH until strongly alkaline.
Compare with Question 5 under Phosphoric Acid. - Add a little potassium iodide solution to some arsenic acid
solution, and warm gently. Is iodine set free? Write equation.
Prepare a faintly alkaline solution of arsenious acid as follows:
Dissolve a minute quantity of arsenious oxide in not more than 2
or 3 drops of hydrochloric acid; dilute to 10 cc. and add, without
heating, a considerable amount of sodium bicarbonate in excess
of what is necessary to neutralize the acid. To this solution add,
drop by drop, a solution of iodine, and determine whether any
free iodine disappears. Write the equation. So far as the state of
oxidation of the arsenic is concerned, the reaction is exactly the
reverse of the one preceding. Recall a previous instance in which
the direction of a reaction of oxidation and reduction is changed on
passing from an acid to an alkaline solution.