O-S-801F
4.3.8 Fixed residue. As applicable, measure 10 ml of class 1, 24 ml of class 2, 36 ml of class 3, or 52 ml of class 4 specimen into a platinum dish, evaporate to dryness, and ignite at a dull red heat for five minutes. Cool in a desiccator and weigh the residue (save the residue for iron and copper determinations). Calculate the percent by weight fixed residue as follows:
Percent fixed residue = 100A BW
Where A = Weight of residue in grams, B = Milliliters of specimen, and
W = Specific gravity of the specimen determined in 4.3.5.
4.3.9 Sulfurous acid. As applicable, dilute 20 ml of class 1, 49 ml of class 2, or 72 ml of class 3 specimen to 80 ml with water; in the case of class 4 specimen, evaporate 104 ml to approximately 75 ml and dilute to 80 ml with water. Cool the 80 ml of solution to 25 °C and
titrate with 0.1 N potassium permanganate solution to a faint but permanent pink color. No more
than 0.46 ml of 0.1 N permanganate solution shall be required. Save the titrated solution for use in 4.3.10.
4.3.10 Preliminary test for antimony, copper, and selenium. Dilute the titrated solution from
4.3.9 with water to 150 ml and, if necessary, filter off any precipitate. Pass hydrogen sulfide through the solution for 5 minutes, let stand for 10 minutes, and filter without washing through a
9 cm quantitative filter paper to close texture (save the filtrate for 4.3.17). Unfold the paper and examine it closely, especially near the center, in daylight or equivalent artificial light. Absence of a black or brown color indicates that less than the permitted amount of copper is present and the determination of copper is to be omitted. If the paper, in the absence of a black or brown color, shows neither a light yellow nor an orange color, antimony and selenium are less than the permitted amounts and the determination of these impurities is to be omitted. (Note: If sulfur precipitates during the hydrogen sulfide treatment, the preliminary test for antimony may be meaningless. The precipitation of sulfur will not obscure copper or selenium, if present.)
4.3.11 Iron. (If the percent fixed residue determined in 4.3.8 does not exceed 0.0070 for class 1,
0.0038 for class 2, 0.0028 for class 3, or 0.0022 for class 4, the determination of iron is to be omitted.) Add 1 ml of hydrochloric acid to the residue retained in 4.3.8, warm on a steam bath to dissolve the residue, and dilute to 10 ml in a volumetric flask (solution A). Dilute a 1 ml aliquot of solution A to 100 ml in a volumetric flask (solution B). (Save the remaining 9 ml of solution
A for the copper determination.) To a 20 ml aliquot of solution B, add 1 ml of hydrochloric acid,
30 to 50 mg of ammonium persulfate, and 3 ml of 30 percent ammonium thiocyanate solution. The color developed shall not exceed that developed in a standard containing 0.018 mg of iron and the same quantities of reagents made up to an equal volume.
4.3.12 Copper. (If the percent fixed residue determination in 4.3.8 does not exceed 0.0060 for class 1, 0.0032 for class 2, 0.0024 for class 3, or 0.0019 for class 4, or if the preliminary test for copper in 4.3.10 does not indicate the presence of copper, the determination of copper is to be omitted.) Neutralize the remaining 9 ml of solution A from 4.3.11 with ammonium hydroxide and add 4 ml in excess. Heat sufficiently to coagulate any precipitate, filter into a Nessler tube,
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