dipped into fluid, is a spongy layer of zinc colored by a trace of sulphide of copper. Nickel, which is not thrown down by iron, is precipitated with zinc, but cobalt is not. Granulated zinc may be used instead of a strip, but the granules must be completely dissolved by the time the bubbles cease. The copper is then decanted, etc.—The cuprous fluid may be filtered into a platinum dish, and some hydrochloric acid added to it. It is then heated, and a few small pieces of zinc added to it, whereupon the copper will deposit itself firmly on the platinum, but loosely on the zinc. After precipitation is complete, which fact is to be tested 'with sulphuretted hydrogen, as above, the copper is rubbed and washed off from the zinc. It is then allowed to settle, is decanted, treated with hot water, to which some hydrochloric acid has been added; then quickly washed with hot water by decantation to prevent the loss of any of the copper by solution. It is finally moistened with some absolute alcohol, dried in a water-bath, or at 110° to 120° C. (230° to 248° Fahr.), and the tared platinum dish, which has been allowed to cool off in the desiccator, is weighed; or the copper, if the utmost accuracy is demanded, is heated in a stream of sulphuretted hydrogen. -Cuprous schist (Kupferschiefer):1 5 grammes (77.16 grains) are heated with 40 to 50 cubic centimeters (2.44 to 3.05 cubic inches) of hydrochloric acid. When the carbonic acid has been expelled, 6 cubic centimeters (0.36 cubic inch) of diluted acid, consisting of equal parts of nitric acid, of 1.2 specific gravity, and water, are added (to ores free from bitumen, or which have been ignited, 1 cubic centimeter (0.061 cubic inch) of nitric acid). This is digested for half an hour, then boiled for a quarter of an hour. The hot liquid, which should contain no free nitric acid, is filtered into a beaker-glass. A small rod of zinc upon a strip of platinum is placed in the filtrate, and the copper is precipitated, which will require fromto of an hour. The precipitated copper is then decanted and dissolved in nitric acid, together with that adhering to the platinum, and titrated with a solution of potassium cyanide (see p. 127). 2. Electrolytic assay.-The copper is precipitated in a coherent film upon a weighed platinum strip by the Fresenius' Ztschr. viii. 9. B. u. h. Ztg. 1869, p. 43, 2 Fresenius' Ztschr. iii. 334; vii. 253; ix. 102. 181; 1872, p. 251; 1875, p. 155; 1877, p. 5, 32. Preuss. Ztschr. xvii. Lief. 3; xx. Lief. 1. Grothe, polyt. Ztschr. 1877, p. 11. galvanic current, from the solution of the nitrate containing free nitric acid, which need not be filtered. From this solution, zinc, iron, nickel, cobalt, and chromium are not precipitated by galvanic action. The following are precipitated in the form of peroxides at the positive electrode: namely, lead, manganese, and (partially) silver. Mercury is precipitated at the negative electrode in the metallic state before copper; silver and bismuth at the same time as copper, and selenium, antimony, and arsenic only a considerable time later; the assay, therefore, as a general rule, requires the absence of the last-named metals and metalloids, which blacken the fine red color of the copper. Antimony remains behind, undissolved, when the assay sample is dissolved in nitric acid. A large number of assays can be carried out at the same time according to this very accurate and simple method, which is adapted for rich as well as for poor ores, etc. It is best to employ for the galvanic current, the Meidinger-Pinkus' battery' (with 6 large elements for richer copper ores, and 4 small elements for poor ores with less than 10 per cent. Cu); or Clamond's thermo-electric battery2 (consisting of a number of zinc and antimony elements arranged in the form of a ring, and heated by a gas-flame) may be used. If the current is too strong the copper does not deposit itself firmly upon the platinum. The copper is not precipitated in as pure and coherent a state, from the solution of the sulphate containing free sulphuric acid, and in case iron should be present a part of this is thrown down with the copper, while another part is reduced to protoxide. 1 gramme (15.43 grains) of the assay sample is dis 1 Fresenius' Ztschr. xi. 4 (Mansfeld). 2 B. u. h. Ztg. 1875, pp. 155, 251, 303. solved in strong nitric acid and evaporated to dryness in a porcelain dish. If necessary, the dish is heated over a lamp to burn off any separated sulphur (p. 39). The residuum is dissolved in 20 cubic centimeters (1.22 cubic inches) of nitric acid of 1.2 sp. gr., and filtered into a beaker-glass. The filtrate is diluted to 180 to 200 cubic centimeters (10.98 to 12.2 cubic inches), and stirred. The platinum spiral a (Fig. 50), weighing about 16 grammes (246.92 grains), is now placed in the beaker-glass. A cone of platinum foil (Fig. 49), weighing about 20 grammes (308.65 grains), is suspended over this from a stand in such a manner that, when rich ores are to be tested, the cone hangs, at the utmost, 1 centimeter (0.39 inch) above the foot ring b of the Fig. 49. Fig. 50. α spiral, but only 0.5 centimeter in case of poorer ores, and that a part of the cone shall project above the liquid. The cone is connected by means of a wire conductor with the negative electrode, and the spiral with the positive. The beaker-glass should be covered with a glass plate, cut into halves, and each half perforated with a hole for the wires to pass through. The strength of the galvanic current used is generally such that 90 to 100 cubic centimeters (5.49 to 6.1 cubic inches) of watergas will be developed in 30 minutes in the voltameter from diluted sulphuric acid (1:12), and as much as 180 cubic centimeters (10.98 cubic inches) for rich cupriferous substances. After an electrolytic action of 12 to 18 hours the liquid is examined in order to ascertain whether the whole of the copper has been precipitated, by adding some water, and stirring. In case it still contains copper, the bright portion of the platinum cone, now partly submerged, will be covered with a red film of copper. If no more copper is separated, the beaker-glass is placed in a spacious porcelain dish, and the acid liquid it contains is displaced by adding water until all acid reaction disappears (or the liquid is removed with a siphon, and water is added from a wash-bottle until gas ceases to develop at the positive electrode). The cone of platinum is then taken out and placed in a beakerglass with water. It is then rinsed off with hot water, next placed in a beaker-glass filled with absolute alcohol, or washed with it, and finally laid upon blotting paper, and dried in an air-bath at about 94° C. (201.2° Fahr.). (If the operator has some experience this can be done quicker upon a piece of sheet-iron heated over a lamp, or by holding the cone in the hot air arising from a large platinum or silver dish heated by the flame.) After it has cooled off the cone is weighed; and, as its weight had been accurately ascertained before the operation, the weight of the copper will be given by the increase in weight of the cone, from which the copper can then be dissolved by hot nitric acid. Dark spots upon the red copper indicate the presence of arsenic, antimony, or selenium. If only small quantities of the first two are present, they are very slowly precipitated, or not at all, from a strong acid solution, if the current is interrupted, while the fluid possesses still a faint bluish color. The presence of much iron prevents a complete precipitation of the copper, as it (Cu) is dissolved by the ferric sulphate while ferrous oxide is formed, the action of free nitric acid upon which, produces blackish-brown circles around the platinum cone. When this is observed it is a sure indication that the process of precipitating the copper has not taken place properly. In this case the assay sample is dissolved in 40 cubic centimeters (2.44 cubic inches) of nitric acid, and 360 cubic centimeters (21.96 cubic inches) of water, using a stronger galvanic current, giving 120 cubic centimeters (7.32 cubic inches) of water-gases in Volta's apparatus; or, what is still better, the copper is precipitated from an acid solution by sulphuretted hydrogen, and the copper sulphide dissolved in 30 cubic centimeters (1.83 cubic inches) of nitric acid of 1.2 specific gravity. It is then digested until the sulphur shows yellow. 200 cubic centimeters (12.2 cubic inches) of water are added, and the fluid is then electrolyzed. The solution is Fig. 51. D Herpin' dissolves 1 gramme (15.43 grains) or more of the assay sample in nitric acid, evaporates nearly to dryness, dissolves in a small quantity of dilute sulphuric acid, and dilutes the solution to 60 to 70 cubic centimeters (3.66 to 4.27 cubic inches). poured into the platinum dish A (Fig. 51), and the conducting stand B of the dish is connected with the negative electrode, the platinum spiral C with the positive electrode, and the liquid electrolyzed after the funnel D has been placed in position. When the copper has been precipitated, the fluid is poured from the dish. This is rinsed out first with water, and next with.alcohol, then dried and weighed, the copper being determined from the increase of weight.-Hampe's2 method of testing refined copper: 25 grammes of copper are dissolved in a beaker, at a moderate temperature, in 200 cubic centimeters (12.2 cubic inches) of water, and 175 to 180 cubic centimeters (10.68 to 10.98 cubic inches) of nitric acid of 1.2 specific gravity. To this are added 25 grammes of previously diluted sulphuric acid (about 4 cubic centimeters (0.24 cubic inch) more than is required for transforming the nitrate into sulphate). The liquid is then evaporated to dryness in a porcelain dish on the water-bath. 1 B. u. h. Ztg. 1875, p. 394. B B The dry mass is heated upon a Dingler, ccxvii. 440. Fresenius' Ztschr. xiii. 176. |