SELENIOUS ACID 335
QUESTIONS
- Heat 1 gram of the product in a dry test tube. Smell cau-
tiously. Observations? Equations?
- Heat a few crystals with 5 cc. of hydrochloric acid. How do
you explain the results?
PREPARATION 69
SELENIOUS ACID, H 2 Se0 3
Elementary selenium constitutes a large part of the " anode
mud " obtained during the electrolytic refining of copper. Like
many non-metals, selenium is converted to an acid (H 2 Se0 3 )
when dissolved in nitric acid. Selenious acid, H 2 Se03, so formed,
is a solid, soluble in water, whose solubility increases rapidly with
rise in temperature. Selenious acid may be oxidized to selenic
acid, H 2 Se04, by KMnC>4. Selenic acid is very similar to sulphuric
acid. Selenious acid is easily reduced to selenium by reducing
agents, for example, by sulphurous acid.
The element selenium exists in several allotropic forms.
Materials: selenium, 39 grams = 0.5 F.W.
I6.ZVHNO3, 113 cc.
Apparatus: 500-cc. flask.
250-cc. beaker,
suction filter,
spatula, iron or glass,
iron ring and ring stand.
Bunsen burner.
Procedure: Warm the nitric acid in the flask gently until the
acid is almost boiling. Using a spatula, add a small amount of
selenium; when this has been dissolved, add a little more. Con-
tinue this intermittent addition until all the selenium has been
added. Regulate the addition and the heating so that the con-
tents of the flask will not boil over. After all the selenium has
been added, boil for 5 minutes after red fumes cease to be evolved.
Cool, filter if necessary, and let the filtrate stand in a previously
weighed beaker until it has evaporated to dryness. (Do not
heat, as SeO 2 is volatilized by heat!) Dissolve in one-fifth its
weight of boiling water, cool to room temperature, add a small seed
