FED-STD-228A
4.4 Iron by volumetric or colorimetric method.
4.4.1 Preparation of solution of iron. A specimen containing about 50 grams of the lead sheath shall be
transferred to a 600-ml. beaker, 250 ml. of 1 to 4 nitric acid added, the mixture heated gently to dissolve
the lead, and then boiled to expel the brown fumes. The beaker shall be removed from the hot plate, 35
mil. of 1 to 1 sulfuric acid added, the solution filtered through a close-texture paper on a Buchner funnel,
and the beaker and paper washed with hot water. The filtrate and washings shall be collected in a 600-
ml. beaker, evaporated to white fumes, the heating continued until the volume of the sulfuric acid is
reduced to about 2 ml., and then cooled to room temperature. One hundred ml. of water shall be added,
the solution boiled for several minutes, cooled, made neutral with ammonium hydroxide, and 2 ml. of
hydrochloric acid, specific gravity 1.19, added. Hydrogen sulfide shall be passed into the solution for
about 15 minutes, the precipitate allowed to settle, and then filtered. The precipitate shall be washed with
hydrogen sulfide water and then discarded. The filtrate and washings shall be boiled to expel the
hydrogen sulfide and the iron oxidized by the addition of a few drops of 3 percent hydrogen peroxide.
The solution shall be evaporated to 50 to 75 ml., cooled slightly, and 5 ml. of ammonium alum solution
added. The solution shall be neutralized with ammonium hydroxide, adding 2 ml. in excess, boiled for 1
minute, and then set aside for about 1 hour. The precipitated iron shall be filtered on to rapid filter paper,
washed with hot 1 percent ammonium chloride, and the filtrate reserved for the determination of zinc in
4.5. The iron shall be determined on the precipitate by the volumetric method described in 4.4.2 or by the
colorimetric method as described in 4.4.3.
4.4.2 Iron by volumetric method. The ferric hydroxide precipitate from 4.4.1 shall be dissolved in 20 ml.
of 1 to 3 hydrochloric acid, the solution diluted to 50 ml., cooled, and then passed through the silver
reductor, 3.1.14, at the rate of 30 ml. per minute. The reductor shall be washed with 100 ml. of 1 to 30
hydrochloric acid, adding the solution in several portions, and allowing each addition to grain to the top of
the silver column before the next is added. Ten ml. of 1 to 1 sulfuric acid and 1 drop of ortho-
phenathroline indicator shall be added and the reduced iron solution titrated with standard ceric sulfate
solution to the disappearance of the pink color. A blank determination shall be made using the same
amounts of reagents and following the same procedure. The number of milliliters of ceric sulfate solution
required to titrate the iron (A) is equal to the total titration minus the blank titration.
4.4.3 Iron by colorimetric method. The ferric hydroxide precipitate from 4.4.1 shall be dissolved in 20 ml.
of hot 1 to 3 hydrochloric acid and cooled to room temperature. The solution shall be transferred to a
100-ml. colorimetric tube, 2 ml. of 20 percent ammonium thiocyanate added, diluted to 100 ml. and mixed
well. The color of the solution shall be compared with a standard containing a known amount of iron.
The standard shall be prepared by adding dropwise standard iron solution to a colorimetric tube
containing 75 ml. of distilled water, 2 ml. of 20 percent ammonium thiocyanate, and 20 ml. of 1 to 3
hydrochloric acid. The solution shall be mixed well and compared with the unknown by visual observation
against a white background. If available, a colorimeter or spectrophotometer may be used for making the
color comparisons. The standard iron solution shall be added until the color matches that of the
specimen, bringing the volume of the final comparison up to 100 ml. in the tube. A blank determination
shall be made using the same amounts of reagents and following the same procedure. The number of
milliliters of iron solution required to match the specimen (B) is equal to the total iron volume minus the
volume of iron solution required to match the blank.
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