[11] In addition to what has been said (Chapter I., Note [65], and Chapter XXII., Note [35]) respecting the combination of CuSO4 with water and ammonia, we may add that Lachinoff (1893) showed that CuSO4,5H2O loses 4¾H2O at 180°, that CuSO4,5NH3 also loses 4¾NH3 at 320°, and that only ¼2O and ¼NH3 remain in combination with the CuSO4. The last ¼H2O can only be driven off by heating to 200°, and the last ¼NH3 by heating to 360°. Ammonia displaces water from CuSO4,5H2O, but water cannot displace the ammonia from CuSO4,5NH3. If hydrochloric acid gas be passed over CuSO4,5H2O at the ordinary temperature, it first forms CuSO4,5H2O,3HCl, and then CuSO4,2H2O,2HCl. When air is passed over the latter compound it passes into CuSO4H2O with a small amount of HCl (about ⅛HCl). At 100° CuSO4,5H2O in a stream of hydrochloric acid gas gives CuSO4,¼H2O,2HCl, and then CuSO4,¼H2O,HCl, whilst after prolonged heating CuSO4 remains, which rapidly passes into CuSO4,5H2O when placed under a bell jar over water. Over sulphuric acid, however, CuSO4,5H2O only parts with 3H2O, and if CuSO4,2H2O be placed over water it again forms CuSO4,5H2O, and so on.
[11 bis] Commercial blue vitriol generally contains ferrous sulphate. The salt is purified by converting the ferrous salt into a ferric salt by heating the solution with chlorine or nitric acid. The solution is then evaporated to dryness, and the unchanged cupric sulphate extracted from the residue, which will contain the larger portion of the ferric oxide. The remainder will be separated if cupric hydroxide is added to the solution and boiled; the cupric oxide, CuO, then precipitates the ferric oxide, Fe2O3, just as it is itself precipitated by silver oxide. But the solution will contain a small proportion of a basic salt of copper, and therefore sulphuric acid must be added to the filtered solution, and the salt allowed to crystallise. Acid salts are not formed, and cupric sulphate itself has an acid reaction on litmus paper.
[12] Among the alloys of copper resembling brass, delta metal, invented by A. Dick (London) is largely used (since 1883). It contains 55 p.c. Cu, and 41 p.c. Zn, the remaining 4 p.c. being composed of iron (as much as 3½ p.c., which is first alloyed with zinc), or of cobalt, and manganese, and certain other metals. The sp. gr. of delta metal is 8·4. It melts at 950°, and then becomes so fluid that it fills up all the cavities in a mould and forms excellent castings. It has a tensile strength of 70 kilos per sq. mm. (gun metal about 20, phosphor bronze about 30). It is very soft, especially when heated to 600°, but after forging and rolling it becomes very hard; it is more difficultly acted upon by air and water than other kinds of brass, and preserves its golden yellow colour for any length of time, especially if well polished. It is used for making bearings, screw propellers, valves, and many other articles. In general the alloys of Cu and Zn containing about ⅔ p.c. by weight of copper were for a long time almost exclusively made in Sweden and England (Bristol, Birmingham). These alloys for the most part are cheaper, harder, and more fusible than copper alone, and form good castings. The alloys containing 45–80 p.c. Cu crystallise in cubes if slowly cooled (Bi also gives crystals). By washing the surface of brass with dilute sulphuric acid, Zn is removed and the article acquires the colour of copper. The alloys approaching Zn2Cu3 in their composition exhibit the greatest resistance (under other equal conditions; of purity, forging, rolling, &c.) The addition of 3 p.c. Al, or 5 p.c. Sn, improves the quality of brass. Respecting aluminium bronze see Chapter XVII. p. [88].
[12 bis] Ball (also Kamensky), 1888, by investigating the electrical conductivity of the alloys of antimony and copper with lead, came to the conclusion that only two definite compounds of antimony and copper exist, whilst the other alloys are either alloys of these two together or with antimony or with copper. These compounds are Cu2Sb and Cu4Sb—one corresponds with the maximum, and the other with the minimum, electrical resistance. In general, the resistance offered to an electrical current forms one of the methods by which the composition of definite alloys (for example, Pb2Zn7) is often established, whilst the electromotive force of alloys affords (Laurie, 1888) a still more accurate method—for instance, several definite compounds were discovered by this method among the alloys of copper with zinc and tin; but we will not enter into any details of this subject, because we avoid all references to electricity, although the reader is recommended to make himself acquainted with this branch of science, which has many points in common with chemistry. The study of alloys regarded as solid solutions should, in my opinion, throw much light upon the question of solutions, which is still obscure in many aspects and in many branches of chemistry.
Fig. 97.—Cupel for silver assaying.
Fig. 98.—Clay muffle.