FED-STD-228A
4.5 Zinc determination. The ammonical solution in a 600-ml. beaker reserved in 4.4.1 shall be made just
acid to litmus with 20 percent citric acid. The solution shall be neutralized with ammonium hydroxide, 25
ml. of formic acid added, the solution diluted to about 200 ml. and 1 ml. of talc suspension added. The
mixture shall be heated to about 95°C (203°F) and hydrogen sulfide passed through for 30 minutes while
allowing to cool. The beaker shall be set aside for about 1 hour, the precipitate filtered on a 9-cm. close-
texture paper, and washed with formic acid mixture wash solution. The zinc sulfide shall be dissolved
from the filter paper with 30 ml. of 1 to 9 hydrochloric acid, collecting the filtrate in a 100-ml. volumetric
flask. The filter paper shall be washed with hot water and the washings added to the volumetric flask.
The solution shall be cooled to room temperature, diluted to 100 ml., and mixed well. An aliquot
containing not more than 0.1 mg. of zinc shall be pipetted into a 50-ml. colorimetric tube and 1 to 9
hydrochloric acid added to make the total volume of acid equal to 15 ml. The solution shall be diluted to
45 ml., 5 ml. of 0.7 percent potassium ferrocyanide added, and the solution mixed well. The turbidity shall
be compared with a series of standards containing from 0.01 to 0.1 mg. of zinc. If available, a colorimeter
or spectrophotometer may be used for making the turbidity comparisons. A blank determination shall be
made using the same amounts of reagents and following the same procedure. The number of milliliters
of zinc standard required to match the specimen (A) is equal to the total volume minus the volume of zinc
required to match the blank.
4.6 Silver by iodide method. A specimen containing about 100 grams of the lead sheath shall be
transferred to a 800-ml. beaker, 1 gram of chloride-free tartaric acid, and 400 ml. of 1 to 4 nitric acid
added, and the mixture heated gently to dissolve the specimen. When reaction ceases, 10 ml. of nitric
acid shall be added and the heating continued until all the metal is dissolved and the brown fumes
expelled. The solution shall be cooled to room temperature, filtered through a Gooch crucible, and the
precipitate discarded. The filtrate shall be transferred to a 600-ml. beaker, diluted to 400 ml. and heated
to 55° ± 5°C (131° ± 9°F). Five ml. of 1 percent potassium iodide shall be added while stirring, the
solution digested at 50°C (122°F) for 15 minutes, avoiding contact with direct sunlight, and then filtered
through a weighed 15-ml. Gooch crucible. The precipitate on the filter shall be washed twice with hot
nitric tartaric acid wash solution, followed by washing with hot water (by filling the Gooch crucible and
draining by suction) until the filtrate shows no color on treatment with hydrogen sulfide. The silver iodide
precipitate in the crucible shall be dried to constant weight at 110° ± 1°C (230° ± 2°F) and weighed. The
weight of the crucible plus the precipitate minus the weight of the crucible is equal to the weight of the
silver iodide (A) in the specimen.
4.7 Silver by electrolytic method. A specimen containing about 100 grams of the lead sheath shall be
transferred to a 800-ml. beaker, 320 ml. of 1 to 3 nitric acid added, and the mixture heated on a steam
bath to dissolve the metal. If a dark residue remains, it shall be filtered off and dissolved in 5 ml. of 1 to 1
nitric acid and then added to the main solution. The solution shall be boiled to expel the brown fumes,
diluted to about 600 ml., and cooled. One percent potassium permanganate solution shall be added drop
wise until the solution turns a permanent pink color. Ten grams of tartaric acid shall be added, the
solution diluted to 700 ml., and heated to about 85°C (185°F). The solution shall be electrolyzed at about
85°C, while stirring, in a 0.1 percent copper nitrate solution as the anolyte, using the apparatus described
in 3.1.13. The anode chambers shall be flushed and the sides of the beaker washed down once or twice
during the electrolysis. After 30 minutes of electrolysis the solution shall be removed from the beaker by
siphoning while continuing the stirring and adding 1 liter of water. The electrode shall be washed in two
successive baths of ethyl alcohol or methyl alcohol, dried in an oven at 110° ± 1°C (230° ± 2°F), for 5
minutes, cooled, and weighed. Usually the deposit maintains a bright silvery appearance throughout the
electrolysis and is pure enough for direct weighing as metallic silver. If the deposit is dark, due to
contamination, it shall be dissolved in 5 ml. of 1 to 1 nitric acid and the silver determined by repeating the
electrolysis. The weight of the electrode plus the silver deposit minus the weight of the clean electrode is
equal to the weight of the silver deposit (A).
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