(e) The roseocobaltic (or rosepentamine), CoX2H2O,5NH3, salts, like the luteocobaltic, correspond with the normal cobaltic salts, but contain less ammonia, and an extra molecule of water. Thus the sulphate is obtained from cobaltous sulphate dissolved in ammonia and left exposed to the air until transformed into a brown solution of the fuscocobaltic salt; when this is treated with sulphuric acid a crystalline powder of the roseocobaltic salt, Co2(SO4)3,10NH3,5H2O, separates. The formation of this salt is easily understood: cobaltous sulphate in the presence of ammonia absorbs oxygen, and the solution of the fuscocobaltic salt will therefore contain, like cobaltous sulphate, one part of sulphuric acid to every part of cobalt, so that the whole process of formation may be expressed by the equation: 10NH3 + 2CoSO4 + H2SO4 + 4H2O + O = Co2(SO4)3,10NH3,5H2O. This salt forms tetragonal crystals of a red colour, slightly soluble in cold, but readily soluble in warm water. When the sulphate is treated with baryta, roseocobaltic hydroxide is formed in the solution, which absorbs the carbonic anhydride of the air. It is obtained from the next series by the action of alkalis.
(f) The rosetetramine cobaltic salts CoCl2,2H2O,4NH3 were obtained by Jörgenson, and belong to the type of the luteo-salts, only with the substitution of 2NH3 for H2O. Like the luteo- and roseo-salts they give double salts with PtCl4, similar to the alkaline double salts, for instance (Co2H2O,4NH3)2(SO4)2Cl2PtCl4. They are darker in colour than the preceding, but also crystallise well. They are formed by dissolving CoCO3 in sulphuric acid (of a given strength), and after NH3 and carbonate of ammonium have been added, air is passed through the solution (for oxidation) until the latter turns red. It is then evaporated with lumps of carbonate of ammonium, filtered from the precipitate and crystallised. A salt of the composition Co2(CO3)2(SO4),(2H2O,4NH3)2 is thus obtained, from which the other salts may be easily prepared.
(g) The purpureocobaltic salts, CoX3,5NH3, are also products of the direct oxidation of ammoniacal solutions of cobalt salts. They are easily obtained by heating the roseocobaltic and luteo-salts with strong acids. They are to all effects the same as the roseocobaltic salts, only anhydrous. Thus, for instance, the purpureocobaltic chloride, Co2Cl6,10NH3, or CoCl3,5NH3, is obtained by boiling the oxycobaltamine salts with ammonia. There is the same distinction between these salts and the preceding ones as between the various compounds of cobaltous chloride with water. In the purpureocobaltic only X2 out of the X3 react (are ionised). To the rosetetramine salts (f) there correspond the purpureotetramine salts, CoX3H2O,4NH3. The corresponding chromium purpureopentamine salt, CrCl3,5NH3 is obtained with particular ease (Christensen, 1893). Dry anhydrous chromium chloride is treated with anhydrous liquid ammonia in a freezing mixture composed of liquid CO2 and chlorine, and after some time the mixture is taken out of the freezing mixture, so that the excess of NH3 boils away; the violet crystals then immediately acquire the red colour of the salt, CrCl3,5NH3, which is formed. The product is washed with water (to extract the luteo-salt, CrCl3,6NH3), which does not dissolve the salt, and it is then recrystallised from a hot solution of hydrochloric acid.
(h) The praseocobaltic salts, CoX3,4NH3, are green, and form, with respect to the rosetetramine salts (f), the products of ultimate dehydration (for example, like metaphosphoric acid with respect to orthophosphoric acid, but in dissolving in water they give neither rosetetramine nor tetramine salts. (In my opinion one should expect salts with a still smaller amount of NH3, of the blue colour proper to the low hydrated compounds of cobalt; the green colour of the prazeo-salts already forms a step towards the blue.) Jörgenson obtained salts for ethylene-diamine, N2H4C2H4 which replaces 2NH3. After being kept a long time in aqueous solution they give rosetetramine salts, just as metaphosphoric acid gives orthophosphoric acid, while the rosetetramine salts are converted into prazeo-salts by Ag2O and NaHO. Here only one X is ionised out of the X3. There are also basic salts of the same type; but the best known is the chromium salt called the rhodozochromic salt, Cr2(OH)3Cl3,6NH3,2H2O, which is formed by the prolonged action of water upon the corresponding roseo-salt.
The cobaltamine compounds differ essentially but little from the ammoniacal compounds of other metals. The only difference is that here the cobaltic oxide is obtained from the cobaltous oxide in the presence of ammonia. In any case it is a simpler question than that of the double cyanides. Those forces in virtue of which such a considerable number of ammonia molecules are united with a molecule of a cobalt compound, appertain naturally to the series of those slightly investigated forces which exist even in the highest degrees of combination of the majority of elements. They are the same forces which lead to the formation of compounds containing water of crystallisation, double salts, isomorphous mixtures and complex acids (Chapter XXI., Note [8 bis]). The simplest conception, according to my opinion, of cobalt compounds (much more so than by assuming special complex radicles, with Schiff, Weltzien, Claus, and others), may be formed by comparing them with other ammoniacal products. Ammonia, like water, combines in various proportions with a multitude of molecules. Silver chloride and calcium chloride, just like cobalt chloride, absorb ammonia, forming compounds which are sometimes slightly stable, and easily dissociated, sometimes more stable, in exactly the same way as water combines with certain substances, forming fairly stable compounds called hydroxides or hydrates, or less stable compounds which are called compounds with water of crystallisation. Naturally the difference in the properties in both cases depends on the properties of those elements which enter into the composition of the given substance, and on those kinds of affinity towards which chemists have not as yet turned their attention. If boron fluoride, silicon fluoride, &c., combine with hydrofluoric acid, if platinic chloride, and even cadmium chloride, combine with hydrochloric acid, these compounds may be regarded as double salts, because acids are salts of hydrogen. But evidently water and ammonia have the same saline faculty, more especially as they, like haloid acids, contain hydrogen, and are both capable of further combination—for instance, ammonia with hydrochloric acid. Hence it is simpler to compare complex ammoniacal with double salts, hydrates, and similar compounds, but the ammonio-metallic salts present a most complete qualitative and quantitative resemblance to the hydrated salts of metals. The composition of the latter is MXnmH2O, where M = metal, X = the haloid, simple or complex, and n and m the quantities of the haloid and so-called water of crystallisation respectively. The composition of the ammoniacal salts of metals is MXnmNH3. The water of crystallisation is held by the salt with more or less stability, and some salts even do not retain it at all; some part with water easily when exposed to the air, others when heated, and then with difficulty. In the case of some metals all the salts combine with water, whilst with others only a few, and the water so combined may then be easily disengaged. All this applies equally well to the ammoniacal salts, and therefore the combination of ammonia may be termed the ammonia of crystallisation. Just as the water which is combined with a salt is held by it with different degrees of force, so it is with ammonia. In combining with 2NH3,PtCl2 evolves 31,000 cals.; while CaCl2 only evolves 14,000 cals.; and the former compound parts with its NH3 (together with HCl in this case) with more difficulty, only above 200°, while the latter disengages ammonia at 180°. ZnCl2,2NH3 in forming ZnCl2,4NH3 evolves only 11,000 cals., and splits up again into its components at 80°. The amount of combined ammonia is as variable as the amount of water of crystallisation—for instance, SnI48NH3, CrCl28NH3, CrCl36NH3, CrCl35NH3,PtCl2,4NH3, &c. are known. Very often NH3 is replaceable by OH2 and conversely. A colourless, anhydrous cupric salt—for instance, cupric sulphate—when combined with water forms blue and green salts, and violet when combined with ammonia. If steam be passed through anhydrous copper sulphate the salt absorbs water and becomes heated; if ammonia be substituted for the water the heating becomes much more intense, and the salt breaks up into a fine violet powder. With water CuSO4,5H2O is formed, and with ammonia CuSO4,5NH3, the number of water and ammonia molecules retained by the salt being the same in each case, and as a proof of this, and that it is not an isolated coincidence, the remarkable fact must be borne in mind that water and ammonia consecutively, molecule for molecule, are capable of supplanting each other, and forming the compounds CuSO4,5H2O, CuSO4,4H2O,NH3; CuSO4,3H2O,2NH3; CuSO4,2H2O,3NH3; CuSO4,H2O,4NH3, and CuSO4,5NH3. The last of these compounds was obtained by Henry Rose, and my experiments have shown that more ammonia than this cannot be retained. By adding to a strong solution of cupric sulphate sufficient ammonia to dissolve the whole of the oxide precipitated, and then adding alcohol, Berzelius obtained the compound CuSO4,H2O,4NH3, &c. The law of substitution also assists in rendering these phenomena clearer, because a compound of ammonia with water forms ammonium hydroxide, NH4HO, and therefore these molecules combining with one another may also interchange, as being of equal value. In general, those salts form stable ammoniacal compounds which are capable of forming stable compounds with water of crystallisation; and as ammonia is capable of combining with acids, and as some of the salts formed by slightly energetic bases in their properties more closely resemble acids (that is, salts of hydrogen) than those salts containing more energetic bases, we might expect to find more stable and more easily-formed ammonio-metallic salts with metals and their oxides having weaker basic properties than with those which form energetic bases. This explains why the salts of potassium, barium, &c., do not form ammonio-metallic salts, whilst the salts of silver, copper, zinc, &c., easily form them, and the salts RX3 still more easily and with greater stability. This consideration also accounts for the great stability of the ammoniacal compounds of cupric oxide compared with those of silver oxide, since the former is displaced by the latter. It also enables us to see clearly the distinction which exists in the stability of the cobaltamine salts containing salts corresponding with cobaltous oxide, and those corresponding with higher oxides of cobalt, for the latter are weaker bases than cobaltous oxides. The nature of the forces and quality of the phenomena occurring during the formation of the most stable substances, and of such compounds as crystallisable compounds, are one and the same, although perhaps exhibited in a different degree. This, in my opinion, may be best confirmed by examining the compounds of carbon, because for this element the nature of the forces acting during the formation of its compounds is well known. Let us take as an example two unstable compounds of carbon. Acetic acid, C2H4O2 (specific gravity 1·06), with water forms the hydrate, C2H4O2,H2O, denser (1·07) than either of the components, but unstable and easily decomposed, generally simply referred to as a solution. Such also is the crystalline compound of oxalic acid, C2H2O4, with water, C2H2O4,2H2O. Their formation might be predicted as starting from the hydrocarbon C2H6, in which, as in any other, the hydrogen may be exchanged for chlorine, the water residue (hydroxyl), &c. The first substitution product with hydroxyl, C2H5(HO), is stable; it can be distilled without alteration, resists a temperature higher than 100°, and then does not give off water. This is ordinary alcohol. The second, C2H4(HO)2, can also be distilled without change, but can be decomposed into water and C2H4O (ethylene oxide or aldehyde); it boils at about 197°, whilst the first hydrate boils at 78°, a difference of about 100°. The compound C2H3(HO)3 will be the third product of such substitution; it ought to boil at about 300°, but does not resist this temperature—it decomposes into H2O and C2H4O2, where only one hydroxyl group remains, and the other atom of oxygen is left in the same condition as in ethylene oxide, C2H4O. There is a proof of this. Glycol, C2H4(HO)2, boils at 197°, and forms water and ethylene oxide, which boils at 13° (aldehyde, its isomeride, boils at 21°); therefore the product disengaged by the splitting up of the hydrate boils at 184° lower than the hydrate C2H4(HO)2. Thus the hydrate C2H3(HO)3, which ought to boil at about 300°, splits up in exactly the same way into water and the product C2H4O2, which boils at 117°—that is, nearly 183° lower than the hydrate, C2H3(HO)3. But this hydrate splits up before distillation. The above-mentioned hydrate of acetic acid is such a decomposable hydrate—that is to say, what is called a solution. Still less stability may be expected from the following hydrates. C2H2(HO)4 also splits up into water and a hydrate (it contains two hydroxyl groups) called glycolic acid, C2H2O(HO)2 = C2H4O3. The next product of substitution will be C2H(HO)5; it splits up into water, H2O, and glyoxylic acid, C2H4O4 (three hydroxyl groups). The last hydrate which ought to be obtained from C2H6, and ought to contain C2(HO)6, is the crystalline compound of oxalic acid, C2H2O4 (two hydroxyl groups), and water, 2H2O, which has been already mentioned. The hydrate C2(HO)6 = C2H2O4,2H2O, ought, according to the foregoing reasoning, to boil at about 600° (because the hydrate, C2H4(HO)2, boils at about 200°, and the substitution of 4 hydroxyl groups for 4 atoms of hydrogen will raise the boiling-point 400°). It does not resist this temperature, but at a much lower point splits up into water, 2H2O, and the hydrate C2O2(HO)2, which is also capable of yielding water. Without going into further discussion of this subject, it may be observed that the formation of the hydrates, or compounds with water of crystallisation, of acetic and oxalic acids has thus received an accurate explanation, illustrating the point we desired to prove in affirming that compounds with water of crystallisation are held together by the same forces as those which act in the formation of other complex substances, and that the easy displaceability of the water of crystallisation is only a peculiarity of a local character, and not a radical point of distinction. All the above-mentioned hydrates, C2X6, or products of their destruction, are actually obtained by the oxidation of the first hydrate, C2H3(HO), or common alcohol, by nitric acid (Sokoloff and others). Hence the forces which induce salts to combine with nH2O or with NH3 are undoubtedly of the same order as the forces which govern the formation of ordinary ‘atomic’ and saline compounds. (A great impediment in the study of the former was caused by the conviction which reigned in the sixties and seventies, that ‘atomic’ were essentially different from ‘molecular’ compounds like crystallohydrates, in which it was assumed that there was a combination of entire molecules, as though without the participation of the atomic forces.) If the bond between chlorine and different metals is not equally strong, so also the bond uniting nH2O and nNH3 is exceeding variable; there is nothing very surprising in this. And in the fact that the combination of different amounts of NH3 and H2O alters the capacity of the haloids X of the salts RX2 for reaction (for instance, in the luteo-salts all the X3, while in the purpureo, only 2 out of the 3, and in the prazeo-salts only 1 of the 3 X's reacts), we should see in the first place a phenomenon similar to what we met with in Cr2Cl6 (Chapter XXI., Note [7 bis]), for in both instances the essence of the difference lies in the removal of water; a molecule RCl3,6H2O or RCl3,6NH3 contains the halogen in a perfectly mobile (ionised) state, while in the molecule RCl3,5H2O or RCl3,5NH3 a portion of the halogen has almost lost its faculty for reacting with AgNO3, just as metalepsical chlorine has lost this faculty which is fully developed in the chloranhydride. Until the reason of this difference be clear, we cannot expect that ordinary points of view and generalisation can give a clear answer. However, we may assume that here the explanation lies in the nature and kind of motion of the atoms in the molecules, although as yet it is not clear how. Nevertheless, I think it well to call attention again (Chapter [I.]) to the fact that the combination of water, and hence, also, of any other element, leads to most diverse consequences; the water in the gelatinous hydrate of alumina or in the decahydrated Glauber salt is very mobile, and easily reacts like water in a free state; but the same water combined with oxide of calcium, or C2H4 (for instance, in C2H6O and in C4H10O), or with P2O5, has become quite different, and no longer acts like water in a free state. We see the same phenomenon in many other cases—for example, the chlorine in chlorates no longer gives a precipitate of chloride of silver with AgNO3. Thus, although the instance which is found in the difference between the roseo- and purpureo-salts deserves to be fully studied on account of its simplicity, still it is far from being exceptional, and we cannot expect it to be thoroughly explained unless a mass of similar instances, which are exceedingly common among chemical compounds, be conjointly explained. (Among the researches which add to our knowledge respecting the complex ammoniacal compounds, I think it indispensable to call the reader's attention to Prof. Kournakoff's dissertation ‘On complex metallic bases,’ 1893.)
Kournakoff (1894) showed that the solubility of the luteo-salt, CoCl3,6NH3, at 0° = 4·30 (per 100 of water), at 20° = 7·7, that in passing into the roseo-salt, CoCl3H2O5NH3, the solubility rises considerably, and at 0° = 16·4, and at 20° = about 27, whilst the passage into the purpureo-salt, CoCl3,5NH3, is accompanied by a great fall in the solubility, namely, at 0° = 0·23, and at 20° = about 0·5. And as crystallohydrates with a smaller amount of water are usually more soluble than the higher crystallohydrates (Le Chatelier), whilst here we find that the solubility falls (in the purpureo-salt) with a loss of water, that water which is contained in the roseo-salt cannot be compared with the water of crystallisation. Kournakoff, therefore, connects the fall in solubility (in the passage of the roseo- into the purpureo-salts) with the accompanying loss in the reactive capacity of the chlorine.
In conclusion, it may be observed that the elements of the eighth group—that is, the analogues of iron and platinum—according to my opinion, will yield most fruitful results when studied as to combinations with whole molecules, as already shown by the examples of complex ammoniacal, cyanogen, nitro-, and other compounds, which are easily formed in this eighth group, and are remarkable for their stability. This faculty of the elements of the eighth group for forming the complex compounds alluded to, is in all probability connected with the position which the eighth group occupies with regard to the others. Following the seventh, which forms the type RX7, it might be expected to contain the most complex type, RX8. This is met with in OsO4. The other elements of the eighth group, however, only form the lower types RX2, RX3, RX4 … and these accordingly should be expected to aggregate themselves into the higher types, which is accomplished in the formation of the above-mentioned complex compounds.
[35 bis] Marshall (1891) obtained cobaltic sulphate, Co2(SO4)3,18H2O, by the action of an electric current upon a strong solution of CoSO4.
[36] The action of an alkaline hypochlorite or hypobromite upon a boiling solution of cobaltous salts, according to Schroederer (1889), produces oxides, whose composition varies between Co3O5 (Rose's compound) and Co2O3, and also between Co5O8 and Co12O19. If caustic potash and then bromine be added to the liquid, only Co2O3 is formed. The action of alkaline hypochlorites or hypo-bromites, or of iodine, upon cobaltic salts, gives a highly-coloured precipitate which has a different colour to the hydrate of the oxide Co2(OH)6. According to Carnot the precipitate produced by the hypochlorites has a composition Co10O16, whilst that given by iodine in the presence of an alkali contains a larger amount of oxygen. Fortmann (1891) re-investigated the composition of the higher oxygen oxide obtained by iodine in the presence of alkali, and found that the greenish precipitate (which disengages oxygen when heated to 100°) corresponds to the formula CoO2. The reaction must be expressed by the equation: CoX2 + I2 + 4KHO = CoO2 + 2KX + 2KI + 2H2O.
[37] Prior to Fortmann, Rousseau (1889) endeavoured to solve the question as to whether CoO2 was able to combine with bases. He succeeded in obtaining a barium compound corresponding to this oxide. Fifteen grams of BaCl2 or BaBr2 are triturated with 5–6 grams of oxide of barium, and the mixture heated to redness in a closed platinum crucible; 1 gram of oxide of cobalt is then gradually added to the fused mass. Each addition of oxide is accompanied by a violent disengagement of oxygen. After a short time, however, the mass fuses quietly, and a salt settles at the bottom of the crucible, which, when freed from the residue, appears as black hexagonal, very brilliant crystals. In dissolving in water this substance evolves chlorine; its composition corresponds to the formula 2(CoO2)BaO. If the original mass be heated for a long time (40 hours), the amount of dioxide in the resultant mass decreases. The author obtained a neutral salt having the composition CoO2BaO (this compound = BaO2CoO) by breaking up the mass as it agglomerates together, and bringing the pieces into contact with the more heated surface of the crucible. This salt is formed between the somewhat narrow limits of temperature 1,000°-1,100°; above and below these limits compounds richer or poorer in CoO2 are formed. The formation of CoO2 by the action of BaO2, and the easy decomposition of CoO2 with the evolution of oxygen, give reason for thinking that it belongs to the class of peroxides (like Cr2O7, CaO2, &c.); it is not yet known whether they give peroxide of hydrogen like the true peroxides. The fact that it is obtained by means of iodine (probably through HIO), and its great resemblance to MnO2, leads rather to the supposition that CoO2 is a very feeble saline oxide. The form CoO2 is repeated in the cobaltic compounds (Note [35]), and the existence of CoO2 should have long ago been recognised upon this basis.