previously dried at 100° C. (212° F.), is placed in a longnecked flask and dissolved in nitric acid. Every trace of nitrous acid is removed by boiling, and the fluid is then strongly evaporated, 30 cubic centimeters (1.83 cubic inches) of nitric acid and about 200 cubic centimeters (12.2 cubic inches) of water are added, and the solution is then precipitated with sulphuretted hydrogen without previous filtration. The whole is now filtered off (metallic sulphides, quartz, etc.) and washed. A flask is now placed beneath the funnel, and its contents is treated with hot nitric acid not too concentrated. The filter is then perforated, the undissolved residue rinsed off into the flask, and the filter washed out. The fluid is strongly reduced' by boiling, and then water and 30 cubic centimeters (1.83 cubic inches) of nitric acid are added. It is again precipitated with sulphuretted hydrogen, filtered, and the filtrate which carries some zinc is added to the principal zinc solution. The entire filtrate is then boiled nearly to dryness in the long-necked flask in order to remove the sulphuretted hydrogen, some potassium chlorate being added for the higher oxidation of any ferrous oxide. The mass is then supersaturated with pure ammonia, the iron precipitate filtered off, washed and dissolved in hot, medium strong nitric acid. It is now again precipitated with ammonia to extract any residue of zinc, and filtered through the same filter, these manipulations being repeated once or twice more. The entire filtrate is now acidulated with acetic acid, diluted to the volume of at least 2 liters (3.52 pints), sulphuretted hydrogen is introduced (in the absence of nickel and cobalt), and the liquid is then allowed to stand for 24 hours. The clear liquid is then poured off upon a filter, and finally the zinc sulphide. The flask is rinsed out with sulphuretted hydrogen water, the precipitate is also washed with it with the addition of some ammonium acetate. The filtrate from the zinc sulphide is supersaturated with ammonia and allowed to stand for at least 24 hours in a covered glass in order to see whether any zinc sulphide is still deposited. The filter is dried, and the zinc sulphide adhering to it detached by rubbing, the filter being entirely closed during the operation. The zinc sulphide, together with the ash of the filter and some distilled sulphur, is heated in one of Rose's crucibles (Fig. 52, p. 122) until it commences to frit, and then in a current of dry hydrogen until two weighings correspond (ZnS 67.01 Zn). The determi nation of zinc as zinc sulphide is very accurate. The 2. Determination of zinc as zinc oxide.-1 gramme (15.43 grains) of ore is dissolved in aqua regia, and ammonia in excess and ammonium carbonate are added, whereby zinc (and copper) passes into solution. resulting precipitate (iron, lead, etc.) is again dissolved and precipitated, and ammonia added to extract any remaining traces of zinc. The zinc (and copper) is then precipitated from the filtrate with sodium sulphide, and filtered. The zinc sulphide is separated from the copper sulphide upon the filter by treating with diluted hydrochloric acid, and the copper sulphide remaining behind is washed. The zinc is precipitated in the boiling filtrate by means of sodium carbonate, washed, dried, ignited, and weighed as zinc oxide containing 80.24 per cent. of zinc. The zinc may also be precipitated from the neutralized filtrate from the copper sulphide by means of sodium sulphide, and determined as zinc sulphide (p. 204). This assay is less accurate than the foregoing one. 3. Galvanic assay, according to Beilstein and Jawein.' 'Liebig's Jahresber. 1865, p. 686 (Luckow); Fresenius' Ztschr. xv. 303 (Wrightson); xvi. 469 (Parodi und Mascazzini). B. u. h. Ztg. 1878, p. 26 (Riche). Ber. d deutsch. chem. Ges. 1879, No. 5, p. 446 (Beilstein und Jawein). -0.5 to 1 gramme (7.71 to 15.43 grains) of ore is dissolved in nitric or sulphuric acid, and caustic soda added until a precipitate is formed. Solution of potassium cyanide is gradually added until the solution becomes clear, and then the platinum electrodes (p. 117) are immersed in the solution. The current of 4 Bunsen elements (the cylinder of zinc 15.5 centimeters (6.1 inches) high, the carbon dipping into nitric acid, with which 0.1 gramme (1.54 grains) of zinc will be precipitated per hour) is then passed through the liquid. The beaker-glass containing the solution should be placed in a dish of cold water, to prevent it, in case but a small quantity is being operated on, from becoming strongly heated by the action of the electric current. When precipitation is supposed to be complete, the electrodes are lifted out of the fluid, the zinc is first washed off with water, next with alcohol, and finally with ether, and then dried in the desiccator. After being weighed, the zinc is dissolved from the platinum with hydrochloric or nitric acid, and the electrodes are again placed in the fluid in order to test it for any residual zinc. Black stains, which may be perceived upon the electrodes, after the zinc has been removed, originate from finely divided platinum. When copper is present (as for instance with brass), the sample is dissolved in nitric acid and evaporated to dryness. The residue is taken up with water, and the copper precipitated by electrolysis from the solution previously acidulated with nitric acid, when lead will be deposited on the platinum spiral as peroxide. The zinc is then precipitated as above described. B. Volumetric assays.-Of the volumetric assays which have been recommended 1. Schaffner's assay with sodium sulphide' is used 1 B. u h. Ztg. 1856, p. 231, 306; 1857, p. 60 (Schaffner); 1876, p. 148, 174 (Laur); p. 225 (Thum); p. 304 (Tobler). Journ. f. prakt. Chem. more than any other. This is based upon the precipitation of zinc from ammoniacal solution by means of sodium sulphide. The termination of precipitation is indicated by the blackening of ferric hydrate as below described. This assay, if certain precautionary measures are adopted, allows of a determination of the zinc to within 0.5 per cent. The presence of metals soluble in ammonia (copper and manganese; nickel and cobalt occur but seldom) requires modifications of the method. 0.5 gramme (7.71 grains) of oxidized ores (smithsonite, calamine), with over 35 per cent. of zinc, and more, if the ore is poorer, are dissolved in heated hydrochloric acid, with an addition of a few drops of nitric acid, to oxidize the iron; the solution is then supersaturated with ammonia: or, raw, or roasted zinc blende is dissolved in aqua regia, evaporated to dryness, and the residue dissolved in 5 cubic centimeters (0.3 cubic inch) of hydrochloric acid and some water. Copper (also lead, antimony, etc.) being frequently present, the solution is precipitated with sulphuretted hydrogen, filtered, and the gas driven off by boiling. 10 cubic centimeters (0.61. cubic inch) of aqua regia are added (or some chlorine water, or a few drops of bromine may be added to the acid solution, or potassium permanganate to faint reddish coloration to the ammoniacal solution, and allowing it to stand for one hour) for the higher oxidation of the iron and manganese (if iron alone is present, an addition of nitric acid or potassium chlorate suffices), which, when ammonia in excess is added, are precipitated as hydrated lxxxviii. 486 (Küntzel). Fresenius' Ztschr. 1870, p. 465 (Deus); 1871, p. 209 (Schott). Mohr, Titrirmethode, 1874, p. 466 (F. Mohr). Dingler, cxlviii. 115 C. Mohr). Preuss. Ztschr. Bd. 25 (Hampe). Berggeist, 1874, No. 3 (Altenberger Pr.). p. 270 (Aarland). Berichte der deutsch. chem. Ges. 1879, No. 3, oxides, while the zinc remains in solution. The hydrated oxides are again dissolved in hydrochloric acid and precipitated with ammonia in excess, in order to extract any residue of zinc. Both filtrates are then united and diluted (to 500 cubic centimeters (30.51 cubic inches) if 5 grammes (77.16 grains), and to 175 to 225 cubic centimeters (10.67 to 13.72 cubic inches) if 0.5 gramme (7.71 grains) of ore have been used). 50 cubic centimeters (3.05 cubic inches) of the fluid are then placed in a beaker-glass, 1 to 2 drops of a solution of ferric chloride of the concentration given below are dropped into 1 cubic centimeter (0.061 cubic inch) of ammonia contained in a porcelain saucer. The precipitated hydrated ferric oxide is carefully rinsed into the beaker-glass, where it settles on the bottom. Titrated solution of sodium sulphide (1 cubic centimeter (0.061 cubic inch) 0.008 to 0.009 gramme (0.12 to 0.14 grain) of zinc) is then added, the contents of the beakerglass being given a spiral motion in the mean while, so that the flakes remain on the bottom. The addition of the sodium sulphide is continued until flakes of ferric oxide become discolored, and finally assume a brown color, which indicates that all the zinc has been precipitated.1 = The following have also been recommended as indicators for recognizing the final reaction; though none of them have taken the place of the hydrated ferric oxide. Porcelain saturated with ferric chloride (Barreswill), or paper (Streng) which is weighted with platinum · wire and laid upon the bottom of the beaker-glass; drop samples (Tupfproben) with nickel chloride (Küntzel), cobaltous chloride (Deus), alkaline solution of lead tartrate (F. Mohr); blotting-paper saturated with sugar of lead, and then treated with ammonium car It is advisable to have a stand with three burettes respectively for solution of zinc, sodium sulphide, and ferric chloride. |