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219. The Determination of Zinc in Ores.25 - For convenience, zinc ores may be divided into two classes, namely, those which are soluble in mineral acids and those which are only partially soluble. The most important ore of the soluble class is Sphalerite or Zinc blende, ZnS, and (of less importance) Zincite, red oxide of zinc, ZnO, Smithsonite, ZnCO3, Willemite, Zn2SiO4, and Calamine H2Zn2SiO 2 ZnO SiO2 H2O. These ores are likely to contain lead in considerable quantity, always iron and often copper, cadmium and sometimes antimony. The ores from Wisconsin, Missouri, Virginia, and the West all come under the class of soluble ores. The most important ore of the partially soluble class is Franklinite (Fe, Mn, Zn)O. (Fe, Mn)2O3, which occurs in large quantity at Franklin Furnace, N. J. Very often Franklinite will be accompanied by Willemite and Gahnite (Zinc Spinel) ZnO Al2O3, the latter ore being almost entirely insoluble. Franklinite contains large amounts of manganese but is practically free from members of the copper group.

220. Exercise No. 21. Determination of Zinc in Sphalerite (ZnS) or Other Soluble Zinc Ore. Treat 0.5 gram of the finely ground ore in a covered casserole with 10 c.c. 12 molar hydrochloric acid, heating gently until all violent action is over; add 5 c.c. 16 molar nitric acid and 5 c.c. 18 molar sulphuric acid and digest on hot-plate until the ore is completely decomposed. Evaporate to copious fumes of sulphuric anhydride in order to expel all traces of hydrochloric and nitric acids, but take care not to go to dryness; allow solution to cool, and, after estimating the amount of sulphuric acid which remains, add 50 c.c. of water and enough sulphuric acid more to make the total concentration of the acid about 1.5 to 2 molar. Introduce into the solution a piece of sheet aluminum about two inches square and bent up at its corners, and boil for about ten minutes, this will usually serve for the complete reduction to the metallic state of any lead, copper, arsenic and antimony 26 that might be present, but any cadmium or bismuth will be only partially reduced. Filter the solution through a filter paper in which is placed a piece of metallic aluminum, and receive the filtrate in an Erlenmeyer flask. After

25 "The Report of the Committee on Quantitative Methods," J. Ind. & Eng. Ch. 4, 468 (1912), gives a very thorough bibliography on zinc analyses.

26 Other metals completely reduced but very unlikely to be present are tin, cobalt and nickel.

washing the filter four or five times with small portions of hot water, cool the filtrate and washings to room temperature, add several drops of methyl orange indicator, and neutralize with 15 molar ammonium hydroxide; now add enough hydrochloric acid to make its concentration in the final volume equal to 0.3 molar, heat the solution to 80°-90°, and pass in hydrogen sulphide to precipitate any cadmium 27 or bismuth and any traces of copper. After the precipitate has settled, filter it off and wash with 0.3 molar hydrochloric acid saturated with hydrogen sulphide. The filtrate will now contain zinc, iron, manganese and perhaps calcium; it is boiled until free of hydrogen sulphide, and, if its volume is then greater than 125 c.c., the boiling is continued until the volume is reduced to this amount; allow solution to cool, add 2-3 drops methyl orange indicator, and 25 c.c. 1 molar citric acid; then carefully neutralize with 3 molar ammonium hydroxide, after which add 25 c.c. "formic mixture," and 25 c.c. 24 molar formic acid; heat to boiling and pass in hydrogen sulphide to precipitate the zinc as zinc sulphide, observing all the details of § 211. After the precipitate has settled, filter it off on filter paper and wash with 0.1 molar formic acid saturated with hydrogen sulphide. It is now ready for further treatment by any one of the methods described in §§ 215, 216 or 217. Calculate to the "moisture-free basis" the amount of zinc found in the sample, running the necessary determination of moisture for this purpose on a separate 2 gram sample and conducting the drying to constant weight at 100°-105°.

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221. Exercise No. 22. Determination of Zinc in Franklinite 28 (Fe, Mn,Zn)O Fe (Mn)2O3. As already mentioned in § 219, Franklinite is not completely soluble in acids; it is, moreover, usually accompanied by Gahnite (Zinc Spinel) ZnO Al2O3 which is almost entirely insoluble. It therefore becomes necessary to resort to fusion to decompose the insoluble residue which remains

27 It must be borne in mind that cadmium sulphide drags down zinc sulphide, so that if a considerable precipitate is obtained at this juncture it must be dissolved and reprecipitation effected.

28 Stone, School Mines Quarterly, 8, 150 (1887), reports the following analyses of samples of Franklinite:

(Mine Hill, N. J.)

Fe2O3
60.52

Mn2O3
6.79

ZnO

MnO

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after the acid treatment. If the residue contains considerable silica, it is best to use sodium carbonate as a flux, employing six parts by weight of the flux to one of residue; if the residue contains but little silica, it is necessary to use borax glass (fused sodium borate) along with the sodium carbonate, employing 6 parts by weight of sodium carbonate and 3 parts of borax glass to one of residue. It is to be remarked further that there is so much iron and manganese present in Franklinite that the chlorate and ammonia separations are not satisfactory for the subsequent removal of these elements.

Procedure. Weigh out one gram of the finely ground ore and treat it in a covered casserole with 10 c.c. of 12 molar hydrochloric acid and 10 c.c. of water, heating on the hot plate. Filter off the residue on filter paper and wash with small portions of hot water until free of chlorides; ignite in a platinum crucible and fuse with 6 parts by weight of sodium carbonate and 3 parts of borax glass to one of residue. Dissolve the melt in hydrochloric acid and join with the main solution. If either of the gravimetric methods ZnSO4 or ZnNH,PO4 is to be used subsequently, then the combined solutions must be evaporated to dryness and baked at 105°-110° for thirty minutes or so to dehydrate the silica, which must be removed by filtration after getting the rest of the constituents back in solution (see § 183 for details of procedure). If the volumetric method Zn,K2[Fe(CN)6]2 is to be used, the silica need not be removed. In any event the volume of the solution for further procedure should be about 125 c.c.; if greater than this it must be reduced by boiling. Allow solution to cool, add 2-3 drops methyl orange indicator and establish conditions for the hydrogen sulphide precipitation of the zinc as described in the previous paragraph, § 211. After the precipitate of zinc sulphide has been obtained, the determination can be completed by any one of the methods described in §§ 215, 216 or 217. Calculate to the "moisture-free basis" the amount of zinc found in the sample, running the necessary determination of moisture for this purpose on a separate 2 gram sample and conducting the drying to constant weight at 100°-105°.

222. Determination of Zinc in Alloys. The chief alloys of zinc are brass, zinc base alloys and German silver. Brass is an

alloy of copper and zinc with small amounts of lead, tin and iron; the percentages will usually be within the following limits:

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Zinc base alloys are alloys of zinc, copper and aluminum; or zinc, copper and tin. The composition varies considerably but the following percentages are typical:

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German silver is an alloy of copper, zinc and nickel; iron is usually present in small quantity as an impurity. The percentages usually lie within the following limits:

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In the analysis of zinc alloys which contain copper, the first thing to do after solution of the alloy is to remove the copper. As emphasized in § 208, the copper should not be removed by precipitation as the sulphide because of the fact that copper sulphide drags down zinc sulphide; the copper should be removed by electrolysis from a dilute nitric acid solution (0.2-0.3 molar HNO3) or by electro-deposition with metallic aluminum from a fairly concentrated sulphuric acid solution (1.5-2.0 molar H2SO4) which is free of nitrates or chlorides.

223. Exercise No. 23. Determination of Zinc in German Silver. Before being weighed, the sample should be carefully inspected and freed from any corrosion or foreign matter. Weigh out about 0.5 gram of sample and place in a 250-c.c. beaker. Add 40 c.c. of 8 molar nitric acid and cover with a watch-glass. Heat cautiously if necessary. When solution is complete wash down the sides of the flask with distilled water, cool, and add 10 c.c. of 9 molar sulphuric acid. Evaporate until dense fumes

of sulphuric acid are given off. Cool, add 50 c.c. of distilled water and one or two pieces of plate aluminum as described in 208. Boil gently, keeping the flask well covered, until all action ceases. Filter, wash carefully, adding the washings to the original solution, and reject the precipitate. This treatment removes the major part of the copper. Cool the solution to room temperature, and neutralize with 6 molar ammonium hydroxide, using methyl orange as indicator. Dilute to 100 c.c. and add just enough hydrochloric acid to make its final concentration 0.3 molar. Heat to 90°-95° and saturate the solution with hydrogen sulphide. Allow the precipitate of CuS, which should be very slight at most, to settle; filter through filter paper and wash the precipitate five or six times with 0.3 molar hydrochloric acid saturated with hydrogen sulphide; after being washed the precipitate can be rejected. Add the washings to the filtrate and boil the combined solution until the escaping steam no longer smells of hydrogen sulphide. If the volume of the solution is now greater than 125 c.c., continue the boiling until the volume has been reduced to this value. Allow solution to cool, add 2-3 drops methyl orange indicator and 25 c.c. 1 molar citric acid; then carefully neutralize with 3 molar ammonium hydroxide, after which add 25 c.c. "formic mixture" and 25 c.c. 24 molar formic acid; heat to boiling and pass in hydrogen sulphide to precipitate the zinc as zinc sulphide, observing all the details of § 211. After the precipitate has settled, filter it off on filter paper and wash with 0.1 molar formic acid saturated with hydrogen sulphide. It is now ready for further treatment by any one of the methods described in §§ 215, 216 or 217. Calculate the percent of zinc found in the sample.

224. Examples.

1. In determining the percentage of zinc in a sample of German Silver, 0.4952 g. of sample was taken for analysis, and the zinc determined by precipitation as sulphide with subsequent conversion to the sulphate; the weight of zinc sulphate found was 0.2194 g. What was the percentage of zinc?

Ans. 17.94%

2. In using a potassium ferrocyanide solution for zinc titrations the following results, inclusive of the amount of solution necessary to establish the end point, were obtained in the standardization:

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