The researches of the same Dutch chemist upon the conditions of the formation of crystals from the double salt (NH4Cl)4Fe2Cl6,2H2O are even more perfect. This salt was obtained in 1839 by Fritsche, and is easily formed from a strong solution of Fe2Cl6 by adding sal-ammoniac, when it separates in crimson rhombic crystals, which, after dissolving in water, only deposit again on evaporation, together with the sal-ammoniac.

Roozeboom (1892) found that when the solution contains b molecules of Fe2Cl6, and a molecules of NH4Cl, per 100 molecules H2O, then at 15° one of the following separations takes place: (1) crystals, Fe2Cl6,12H2O, when a varies between 0 and 11, and b between 4·65 and 4·8, or (2) a mixture of these crystals and the double salt, when a = 1·36, and b = 4·47, or (3) the double salt, Fe2Cl6,4NH4Cl,2H2O, when a varies between 2 and 11·8, and b between 3·1 and 4·56, or (4) a mixture of sal-ammoniac with the iron salt (it crystallises in separate cubes, Retgers, Lehmann), when a varies between 7·7 and 10·9, and b is less than 3·38, or (5) sal-ammoniac, when a = 11·88. And as in the double salt, a : b :: 4 : 1 it is evident that the double salt only separates out when the ratio a : b is less than 4 : 1 (i.e. when Fe2Cl6 predominates). The above is seen more clearly in the accompanying figure, where a, or the number of molecules of NH4Cl per 100H2O, is taken along the axis of abscissæ, and b, or the number of molecules of Fe2Cl6, along the ordinates. The curves ABCD correspond to saturation and present an iso-therm of 15°. The portion AB corresponds to the separation of chloride of iron (the ascending nature of this curve shows that the solubility of Fe2Cl6 is increased by the presence of NH4Cl, while that of NH4Cl decreases in the presence of Fe2Cl6), the portion BC to the double salt, and the portion CD to a mixture of sal-ammoniac and ferric chloride, while the straight line OF corresponds to the ratio Fe2Cl6,4NH4Cl, or a : b :: 4 : 1. The portion CE shows that more double salt may be introduced into the solution without decomposition, but then the solution deposits a mixture of sal-ammoniac and ferric chloride (see Chapter XXIV. Note [9]bis). If there were more such well-investigated cases of solutions, our knowledge of double salts, solutions, the influence of water, equilibria, isomorphous mixtures, and such-like provinces of chemical relations might be considerably advanced.

[24] The normal ferric salts are decomposed by heat and even by water, forming basic salts, which may be prepared in various ways. Generally ferric hydroxide is dissolved in solutions of ferric nitrate; if it contains a double quantity of iron the basic salt is formed which contains Fe2O3 (in the form of hydroxide) + 2Fe2(NO3)6 = 3Fe2O(NO3)4, a salt of the type Fe2OX4. Probably water enters into its composition. With considerable quantities of ferric oxide, insoluble basic salts are obtained containing various amounts of ferric hydroxide. Thus when a solution of the above-mentioned basic acid is boiled, a precipitate is formed containing 4(Fe2O3)8,2(N2O5),3H2O, which probably contains 2Fe2O2(NO3)2 + 2Fe2O3,3H2O. If a solution of basic nitrate be sealed in a tube and then immersed in boiling water, the colour of the solution changes just in the same way as if a solution of ferric acetate had been employed (Note [22]). The solution obtained smells strongly of nitric acid, and on adding a drop of sulphuric or hydrochloric acid the insoluble variety of hydrated ferric oxide is precipitated.

Normal ferric orthophosphate is soluble in sulphuric, hydrochloric, and nitric acids, but insoluble in others, such as, for instance, acetic acid. The composition of this salt in the anhydrous state is FePO4, because in orthophosphoric acid there are three atoms of hydrogen, and iron, in the ferric state, replaces the three atoms of hydrogen. This salt is obtained from ferric acetate, which, with disodium phosphate, forms a white precipitate of FePO4, containing water. If a solution of ferric chloride (yellowish-red colour) be mixed with a solution of sodium acetate in excess, the liquid assumes an intense brown colour which demonstrates the formation of a certain quantity of ferric acetate; then the disodium phosphate directly forms a white gelatinous precipitate of ferric phosphate. By this means the whole of the iron may be precipitated, and the liquid which was brown then becomes colourless. If this normal salt be dissolved in orthophosphoric acid, the crystalline acid salt FeH3(PO4)2 is formed. If there be an excess of ferric oxide in the solution, the precipitate will consist of the basic salt. If ferric phosphate be dissolved in hydrochloric acid, and ammonia be added, a salt is precipitated on heating which, after continued washing in water and heating (to remove the water), has the composition Fe4P2O11—that is, 2Fe2O3,P2O5. In an aqueous condition this salt may be considered as ferric hydroxide, Fe2(OH)6, in which (OH)3 is replaced by the equivalent group PO4. Whenever ammonia is added to a solution containing an excess of ferric salt and a certain amount of phosphoric acid, a precipitate is formed containing the whole of the phosphoric acid in the mass of the ferric oxide.

Ferric oxide is characterised as a feeble base, and also by the fact of its forming double salts—for instance, potassium iron alum, which has a composition Fe2(SO4)3,K2SO4,24H2O or FeK(SO4)2,12H2O. It is obtained in the form of almost colourless or light rose-coloured large octahedra of the regular system by simply mixing solutions of potassium sulphate and the ferric sulphate obtained by dissolving ferric oxide in sulphuric acid.

[25] It would seem that all normal ferric salts are colourless, and that the brown colour which is peculiar to the solutions is really due to basic ferric salts. A remarkable example of the apparent change of colour of salts is represented by the ferrous and ferric oxalates. The former in a dry state has a yellow colour, although as a rule the ferrous salts are green, and the latter is colourless or pale green. When the normal ferric salt is dissolved in water it is, like many salts, probably decomposed by the water into acid and basic salts, and the latter communicates a brown colour to the solution. Iron alum is almost colourless, is easily decomposed by water, and is the best proof of our assertion. The study of the phenomena peculiar to ferric nitrate might, in my opinion, give a very useful addition to our knowledge of the aqueous solutions of salts in general.

[25 bis] The reaction FeX3 + KI = FeX2 + KX + I proceeds comparatively slowly in solutions, is not complete (depends upon the mass), and is reversible. In this connection we may cite the following data from Seubert and Rohrer's (1894) comprehensive researches. The investigations were conducted with solutions containing 1⁄10 gram—equivalent weights of Fe2(SO4)3 (i.e. containing 20 grams of salt per litre), and a corresponding solution of KI; the amount of iodine liberated being determined (after the addition of starch) by a solution (also 1⁄10 normal) of Na2S2O3 (see Chapter XX., Note [42]). The progress of the reaction was expressed by the amount of liberated iodine in percentages of the theoretical amount. For instance, the following amount of iodide of potassium was decomposed when Fe2(SO4)3 + 2nKI was taken:

n = 12361020
After15′ 11·426·340·673·591·696·0
„30′ 14·035·847·878·594·397·4
„1hour19·042·756·084·095·797·6
„10„32·656·075·793·296·597·6
„48„39·467·782·693·496·697·6

Similar results were obtained for FeCl3, but then the amount of iodine liberated was somewhat greater. Similar results were also obtained by increasing the mass of FeX3 per KI, and by replacing it by HI (see Chapter XXI., Note [26]).

[26] If chlorine be passed through a strong solution of potassium hydroxide in which hydrated ferric oxide is suspended, the turbid liquid acquires a dark pomegranate-red colour and contains potassium ferrate: 10KHO + Fe2O3 + 3Cl2 = 2K2FeO4 + 6KCl + 5H2O. The chlorine must not be in excess, otherwise the salt is again decomposed, although the mode of decomposition is unknown; however, ferric chloride and potassium chlorate are probably formed. Another way in which the above-described salt is formed is also remarkable; a galvanic current (from 6 Grove elements) is passed through cast-iron and platinum electrodes into a strong solution of potassium hydroxide. The cast-iron electrode is connected with the positive pole, and the platinum electrode is surrounded by a porous earthenware cylinder. Oxygen would be evolved at the cast-iron electrode, but it is used up in oxidation, and a dark solution of potassium ferrate is therefore formed about it. It is remarkable that the cast iron cannot be replaced by wrought iron.