Note 35 — Chapter XXII · IRON, COBALT, AND NICKEL

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that the first salts correspond with cobaltous oxide; they contain 5, and not 6, molecules of ammonia, as if NH₃ had been replaced by O, but we shall afterwards meet compounds containing either 5NH₃ or 6NH₃ to each atom of cobalt.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.

(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 MX_nmH₂O, 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 MX_nmNH₃. 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.