50

QUANTITATIVE AGRICULTURAL AXALYMX

this serves also to insure against the objectionable action of
bases, noted above. Silver nitrate being used EH reagent, the
following reaction occurs:

AgNO, + MCI -> AgOl + MNOs.

The following experiments must be conducted in a room which
is not brightly lighted and they should not be unduly prolonged.

Gravimetric Determination.—Prepare two "Gooch" (Oaldwell) filters by
the following procedure, first marking them I and II an directed on page 3!:
Place the crucible in the holder as shown in Fig. 5, page 25. Apply the*
suction and pour in prepared asbestos suspended in water until a felt of
sufficient thickness is obtained on the perforated bottom. The* required
thickness will vary according to the condition of the oslwstos, a compara-
tively fine material making a compact pad which need not IKJ an thick an
one of coarser material. These points must be determined by experiment,
guided by the advice of the instructor. Place a small perforated porcelain
plate on the pad, to prevent injury when solution is poured in.
Finally give the crucible a single rinsing with redistilled alcohol to promote
rapid drying, drawing out as much liquid as possible. Ho move the* cruci-
ble, carefully wipe the outside and place in an oven which in maintained at a
temperature between 105 and 110° and dry for at least 30 xninut.es. Place
in the desiccator and weigh after 30 minutes cooling.
If an alundum crucible is to be used instead of the Gooeh it is placed in
the holder so that the top is even with the rubber. This in to provide for
thorough washing of the entire body of the crucible, which is porous. No
asbestos is used but a new crucible should be given a preliminary washing
with hot water, followed by alcohol. It should then be dried at 105 to
110° before weighing.
While the filters are drying proceed with the weighing arid precipitation
processes. Fill a clean, dry weighing bottle with the powdered and well
mixed chloride sample. Provide two clean, 250-cc beakers of Pyrex or
other resistance glass and mark them I and II. If the substance is known
to be unaffected by contact with air it may be poured directly into oni» of the
counterpoised glasses on the balance, until about 0.2 gm is obtained, (The
glasses should have been brought to balance by means of the rider.) This
sample is then weighed accurately and brushed into one of the beakers by
means of the small pencil brush of camel's hair. A second sample is weighed
and brushed into the second beaker. The weights are recorded in tho
proper places in the data book.
If the nature of the sample is such that it should not be unnwwsarily
exposed to air it must be weighed by difference. Place tho filled weighing
bottle on the left pan of the balance, using for this purpose a pair of crucible*
tongs having short pieces of clean tubing drawn over the tips, and carefully
weigh. Record this weight in the data book at the top of the «pn«* marked