FED-STD-228A
3.1.11 Ice bath.
3.1.12 Litmus paper.
3.1.13 Silver electrolysis apparatus. The apparatus shown on figure 7231A shall consist of two lead
anodes enclosed in Alundum shells 19 by 90 mm. and a single platinum gauze cathode 25 to 80 mm. in
diameter and 40 to 50 mm. in length to fit a 600-ml beaker. The three electrodes shall be connected to a
single binding post so that good electrical contact is made. The anodes shall be made by winding pure
lead wire, 2.5 mm. in diameter by 70 cm. in length, around 5-mm glass tubing to form a compact helix,
leaving sufficient wire at the top to form a lead to the binding post (copper and bismuth). For stirring the
catholyte, the apparatus shall be equipped with a glass corkscrew stirrer with tungsten shaft attached to a
1,000 RPM motor geared so that the solution is drawn towards the cathode. The apparatus shall be
equipped with an anolyte reservoir for flushing out the anode chambers during the electrolysis. The
apparatus shall be equipped with an anode of pure copper wire, 2 mm. in diameter, silver-free and
uncorroded, for use in determination of silver.
3.1.14 Silver reductor.
3.1.14.1 This apparatus shall be similar in design to figure 7231D, and shall conform to the following
dimensions:
Internal diameter - - - - - - - - - - - - - - - 15 to 20 mm.
Length of reductor column - - - - - - - - 120 to 150 mm.
Capacity of reservoir - - - - - - - - - - - - 50 to 75 mm.
3.1.14.2 Preparation of silver reductor. Sixty grams of silver nitrate shall be dissolved in 400 ml. of water
and a few drops of nitric acid added. A sheet of metallic copper, about 10 cm. square, shall be
suspended in the silver nitrate and the solution stirred mechanically until all the silver has precipitated, as
shown by the absence of silver chloride precipitate on addition of hydrochloric acid to a few milliliters of
the solution. The sheet of copper shall be removed from the solution and the precipitated silver washed
by decantation with sulfuric acid solution (1 to 199) until most of the dissolved copper is removed. Water
shall be added to the silver precipitate and the mixture poured into the reductor column, the lower end of
which contains a plug of glass wool placed over a few glass beads. After the solution has drained to the
top of the silver column, the silver precipitate shall be washed repeatedly with 1 to 199 sulfuric acid until
all the copper has been removed, as shown by the absence of a blue color on the addition of ammonium
hydroxide to the washings. The reductor shall then be filled with 1 to 9 hydrochloric acid solution. When
not in use, the reductor shall be kept full of hydrochloric acid solution (1 to 9).
3.1.14.3 Regeneration of silver. During the reduction of ferric iron by passing a hydrochloric acid solution
of iron through the reductor, silver chloride forms at the top of the column. This silver chloride darkens,
leaving a blackening around the column. When this dark layer has extended one-half to three-fourths of
the length of the tube down the column, the silver chloride and silver shall be transferred to a beaker and
covered with 1 to 199 sulfuric acid solution and a zinc rod placed in the beaker so that the zinc is in
contact with the silver chloride. The silver chloride is reduced to silver which shall be washed and
returned to the reductor column for further use.
3.1.15 Tin reduction apparatus. The apparatus shown on figure 7231B shall consist of a flask closed
with a three-hole rubber stopper containing an inlet tube for carbon dioxide, an air condenser, and a hole
for a burette (glass plugged).
200
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