270 ELEMENTS OF GROUP IVrepeating this process once or twice more, almost the entire mother
liquor should be used up and nearly the calculated yield of stannous
chloride should be obtained.
QUESTIONS- Explain why during this preparation no red oxides of nitrogen
are seen to escape in consequence of the reduction of nitric acid
by the metal. If nitric acid is reduced to NH3, show how many
more equivalents of oxygen it will yield for the oxidation of the
tin than if it were reduced only to NO.
To test for the presence of ammonium salt in the product, take
about 1 gram of the crystals; dissolve in 10 cc. of water in a small
beaker. Add sodium hydroxide solution until the precipitate
first formed redissolves. Place over the beaker a watch glass, on
the under side of which is stuck a piece of moistened red litmus
paper. Place some cold water in the hollow of the watch glass,
and warm the solution in the beaker very gently. What observa-
tion will indicate the presence of ammonium salt, and why? - Dissolve 1 gram of stannous chloride crystals in 1 to 2 cc. of
cold water. Then add a considerable amount of water. What
is the precipitate? What can be added to prevent its forma-
tion? - To a cold solution of stannous chloride add sodium hydroxide
until it has redissolved the precipitate first formed. Write the
equation. Save the solution. - Pour the solution saved from Experiment 3 over a little
bismuth hydroxide on a filter paper. (The bismuth hydroxide can
be precipitated for the occasion.) Compare the action with that
of stannous chloride on mercuric chloride. - Prepare a very concentrated cold solution of sodium stan-
nite: Dissolve 1 gram of stannous chloride in 1 cc. of water. Dis-
solve a small lump of sodium hydroxide in its own weight of water,
and add this solution, a drop at a time, to the first solution —
cooling all the while under the water tap — until the precipitate
at first formed redissolves. Then heat the solution. Compare
the action with that in Experiment 4. - In preparing a solution of stannous chloride for a laboratory
reagent, what is the necessity of adding hydrochloric acid and of
placing a piece of metallic tin in the bottle?