(5) REMOVING EXCESS NITRIC ACID
You were told to use no more aqua regia than was required. Be-
ginners always use too much. Experienced workers often do the
same thing, especially when in a hurry, because fresh acids work
more rapidly than those that are partly exhausted.
Sooner or later you must get rid of the unused nitric acid. Most
workers think it pays to do so right away, as follows:
Put the aqua regia solution into a big evaporating dish. It will
contain some sediment, mostly silver chloride; this does no harm at
this point, so do not filter it yet. Now add a little sulphuric acid,
stirring all the time, and working slowly, as it might spatter. Use
about an ounce or less of the sulphuric acid to each quart of liquid.
Now boil the solution down gently, to a syrup, being careful not to
spatter. The purpose of the sulphuric acid will be explained later.
This evaporation is a tiresome process. It goes rapidly at first, but
soon the liquid begins to spatter and you must lower the flame. Do
not cover the dish, as that merely impedes evaporation. A sand bath
or a steam bath is useful; these have already been described.
When using a sand bath, start work by pushing the sand to the
sides of the pan, so that the heat can quickly reach the evaporating
dish; later, as evaporation has progressed, scrape the sand down
underneath the dish, to lessen the tendency to spatter.
The advantage of the steam bath is that it does not cause spatter-
ing, and a dish can be left on it without supervision, without danger
of overheating.
Let the evaporation continue until the liquid becomes syrupy;
then slowly add a little full-strength hydrochloric acid. Brown
fumes will bubble off; this is the unused nitric acid being expelled.
Usually this evaporation must be repeated at least once, maybe twice
—evaporating to a syrup and adding a little hydrochloric acid—in
order to get rid of all the unused nitric acid.
Do not carry the evaporation far enough to form a hard crust.
One reason for the sulphuric acid is that it hastens the expulsion of
the nitric acid, and when it is present there is less tendency for a
hard crust to be formed. The sulphuric acid also serves another pur-
pose—to get rid of lead—which is described fully in a later chapter.
