413. Lead is the one which is most frequently suspected
from its use in pipes, and occasionally it is desirable to test for
copper or zinc. With any considerable quantity of the first
two, hydrogen sulphid gives a brown or black color. It will
usually be advisable before adding the sulphid to evaporate to
about one-tenth of its bulk, as much as a liter of water after
acidifying with hydrochloric acid, and the use of hydrogen
sulphid gas is here preferable to its solution, to avoid dilution.
If a black precipitate is obtained, it can be removed by filtra-
tion and identified by the method of Table IV. The zinc can
be separated from the filtrate or, in absence of a precipitate
with hydrogen sulphid, from the concentrated water by first
making the liquid slightly alkaline with ammonium hydroxid,
then passing in hydrogen sulphid and warming. It forms
white, voluminous zinc sulphid which may be identified by its
reactions.
/Ammonia.
414. Ammonia does^ot occur in natural waters in suflicient
abundance to render/possible its detection by its odor or by

-H"' litmus paper. Very minute quantities, however, yield a yellow

•4



-iM.
*
I

J



THE SANITARY EXAMINATION OF WATER 1 89

to brown color with Nessler's reagent. If the amount is con-
siderable there is a brown precipitate, but in ordinary waters
the liquid remains clear. The result is somewhat complicated
by the presence of calcium and magnesium compounds as they
are in part precipitated at the same time. The diflSculty can
be completely overcome by distilling a part of the water from
a retort and condensing the steam. The pure ammonia is
found in the distillate.
