We may add that when equivalent aqueous solutions of NiX2 (green) and CoX2 (red) are mixed together they give an almost colourless (grey) solution, in which the green and red colour of the component parts disappears owing to the combination of the complementary colours.
A double salt NiKF3 is obtained by heating NiCl2 with KFHF in a platinum crucible; KCoF3 is formed in a similar manner. The nickel salt occurs in fine green plates, easily soluble in water but scarcely soluble in ethyl and methyl alcohol. They decompose into green oxide of nickel and potassium fluoride when heated in a current of air. The analogous salt of cobalt crystallises in crimson flakes.
If instead of potassium fluoride, CoCl2 or NiCl2 be fused with ammonium fluoride, they also form double salts with the latter. This gives the possibility of obtaining anhydrous fluorides NiF2 and CoF2. Crystalline fluoride of nickel, obtained by heating the amorphous powder formed by decomposing the double ammonium salt in a stream of hydrofluoric acid, occurs in beautiful green prisms, sp. gr. 4·63, which are insoluble in water, alcohol, and ether; sulphuric, hydrochloric, and nitric acids also have no action upon them, even when heated; NiF2 is decomposed by steam, with the formation of black oxide, which retains the crystalline structure of the salt. Fluoride of cobalt, obtained as a rose-coloured powder by decomposing the double ammonium salt with the aid of heat in a stream of hydrofluoric acid, fuses into a ruby-coloured mass which bears distinct signs of a crystalline structure; sp. gr. 4·43. The molten salt only volatilises at about 1400°, which forms a clear distinction between CoF2 and the volatile NiF2. Hydrochloric, sulphuric, and nitric acids act upon CoF2 even in the cold, although slowly, while when heated the reaction proceeds rapidly (Poulenc, 1892).
[34] Hydrated suboxide of cobalt (de Schulten, 1889) is obtained in the following manner. A solution of 10 grams of CoCl26H2O in 60 c.c. of water is heated in a flask with 250 grams of caustic potash and a stream of coal gas is passed through the solution. When heated the hydrate of the suboxide of cobalt which separates out, dissolves in the caustic potash and forms a dark blue solution. This solution is allowed to stand for 24 hours in an atmosphere of coal gas (in order to prevent oxidation). The crystalline mass which separates out has a composition Co(OH)2, and to the naked eye appears as a violet powder, which is seen to be crystalline under the microscope. The specific gravity of this hydrate is 3·597 at 15°. It does not undergo change in the air; warm acetic acid dissolves it, but it is insoluble in warm and cold solutions of ammonia and sal-ammoniac.
[34 bis] The following reaction may be added to those of the cobaltous and nickelous salts: potassium cyanide forms a precipitate with cobalt salts which is soluble in an excess of the reagent and forms a green solution. On heating this and adding a certain quantity of acid, a double cobalt cyanide is formed which corresponds with potassium ferricyanide. Its formation is accompanied with the evolution of hydrogen, and is founded upon the property which cobalt has of oxidising in an alkaline solution, the development of which has been observed in such a considerable measure in the cobaltamine salts. The process which goes on here may be expressed by the following equation; CoC2N2 + 4KCN first forms CoK4C6N6, which salt with water, H2O, forms potassium hydroxide, KHO, hydrogen, H, and the salt, K3CoC6N6. Here naturally the presence of the acid is indispensable in consequence of its being required to combine with the alkali. From aqueous solutions this salt crystallises in transparent, hexagonal prisms of a yellow colour, easily soluble in water. The reactions of double decomposition, and even the formation of the corresponding acid, are here completely the same as in the case of the ferricyanide. If a nickelous salt be treated in precisely the same manner as that just described for a salt of cobalt, decomposition will occur.
[35] The cobalt salts may be divided into at least the following classes, which repeat themselves for Cr, Ir, Rh (we shall not stop to consider the latter, particularly as they closely resemble the cobalt salts):—
(a) Ammonium cobalt salts, which are simply direct compounds of the cobaltous salts CoX2 with ammonia, similar to various other compounds of the salts of silver, copper, and even calcium and magnesium, with ammonia. They are easily crystallised from an ammoniacal solution, and have a pink colour. Thus, for instance, when cobaltous chloride in solution is mixed with sufficient ammonia to redissolve the precipitate first formed, octahedral crystals are deposited which have a composition CoCl2,H2O,6NH3. These salts are nothing else but combinations with ammonia of crystallisation—if it may be so termed—likening them in this way to combinations with water of crystallisation. This similarity is evident both from their composition and from their capability of giving off ammonia at various temperatures. The most important point to observe is that all these salts contain 6 molecules of ammonia to 1 atom of cobalt, and this ammonia is held in fairly stable connection. Water decomposes these salts. (Nickel behaves similarly without forming other compounds corresponding to the true cobaltic.)
(b) The solutions of the above-mentioned salts are rendered turbid by the action of the air; they absorb oxygen and become covered with a crust of oxycobaltamine salts. The latter are sparingly soluble in aqueous ammonia, have a brown colour, and are characterised by the fact that with warm water they evolve oxygen, forming salts of the following category: The nitrate may be taken as an example of this kind of salt; its composition is CoN2O7,5NH3,H2O. It differs from cobaltous nitrate, Co(NO3)2, in containing an extra atom of oxygen—that is, it corresponds with cobalt dioxide, CoO2, in the same way that the first salts correspond with cobaltous oxide; they contain 5, and not 6, molecules of ammonia, as if NH3 had been replaced by O, but we shall afterwards meet compounds containing either 5NH3 or 6NH3 to each atom of cobalt.
(c) The luteocobaltic salts are thus called because they have a yellow (luteus) colour. They are obtained from the salts of the first kind by submitting them in dilute solution to the action of the air; in this case salts of the second kind are not formed, because they are decomposed by an excess of water, with the evolution of oxygen and the formation of luteocobaltic salts. By the action of ammonia the salts of the fifth kind (roseocobaltic) are also converted into luteocobaltic salts. These last-named salts generally crystallise readily, and have a yellow colour; they are comparatively much more stable than the preceding ones, and even for a certain time resist the action of boiling water. Boiling aqueous potash liberates ammonia and precipitates hydrated cobaltic oxide, Co2O3,3H2O, from them. This shows that the luteocobaltic salts correspond with cobaltic oxide, Co2O3, and those of the second kind with the dioxide. When a solution of luteocobaltic sulphate, Co2(SO4)3,12NH3,4H2O, is treated with baryta, barium sulphate is precipitated, and the solution contains luteocobaltic hydroxide, Co(OH)3,6NH3, which is soluble in water, is powerfully alkaline, absorbs the oxygen of the air, and when heated is decomposed with the evolution of ammonia. This compound therefore corresponds to a solution of cobaltic hydroxide in ammonia. The luteocobaltic salts contain 2 atoms of cobalt and 12 molecules of ammonia—that is, 6NH3 to each atom of cobalt, like the salts of the first kind. The CoX2 salts have a metallic taste, whilst those of luteocobalt and others have a purely saline taste, like the salts of the alkali metals. In the luteo-salts all the X's react (are ionised, as some chemists say) as in ordinary salts—for instance, all the Cl2 is precipitated by a solution of AgNO3; all the (SO4)3 gives a precipitate with BaX2, &c. The double salt formed with PtCl4 is composed in the same manner as the potassium salt, K2PtCl4 = 2KCl + PtCl4, that is, contains (CoCl3,6NH3)2,3PtCl4, or the amount of chlorine in the PtCl4 is double that in the alkaline salt. In the rosepentamine (e), and rosetetramine (f), salts, also all the X's react or are ionised, but in the (g) and (h) salts only a portion of the X's react, and they are equal to the (e) and (f) salts minus water; this means that although the water dissolves them it is not combined with them, as PHO3 differs from PH3O3; phenomena of this class correspond exactly to what has been already (Chapter XXI., Note [7]) mentioned respecting the green and violet salts of oxide of chromium.
(d) The fuscocobaltic salts. An ammoniacal solution of cobalt salts acquires a brown colour in the air, due to the formation of these salts. They are also produced by the decomposition of salts of the second kind; they crystallise badly, and are separated from their solutions by addition of alcohol or an excess of ammonia. When boiled they give up the ammonia and cobaltic oxide which they contain. Hydrochloric and nitric acids give a yellow precipitate with these salts, which turns red when boiled, forming salts of the next category. The following is an example of the composition of two of the fuscocobaltic salts, Co2O(SO4)2,8NH3,4H2O and Co2OCl4,8NH3,3H2O. It is evident that the fuscocobaltic salts are ammoniacal compounds of basic cobaltic salts. The normal cobaltic sulphate ought to have the composition Co2(SO4)3 = Co2O3,3SO3; the simplest basic salts will be Co2O(SO4)2 = Co2O3)2SO3, and Co2O2(SO4) = Co2O3,SO3. The fuscocobaltic salts correspond with the first type of basic salts. They are changed (in concentrated solutions) into oxycobaltamine salts by absorption of one atom of oxygen, Co2O2(SO4)2. The whole process of oxidation will be as follows: first of all Co2X4, a cobaltous salt, is in the solution (X a univalent haloid, 2 molecules of the salt being taken), then Co2OX4, the basic cobaltic salt (4th series), then Co2O2X4, the salt of the dioxide (2nd series). The series of basic salts with an acid, 2HX, forms water and a normal salt, Co2X6 (in 3, 5, 6 series). These salts are combined with various amounts of water and ammonia. Under many conditions the salts of fuscocobalt are easily transformed into salts of the next series. The salts of the series that has just been described contain 4 molecules of ammonia to 1 atom of cobalt.