Ferric chloride, FeCl₃

Ferric chloride, FeCl₃, occurs in nature in the lava of Vesuvius, as the mineral molysite. In the laboratory it is prepared in the anhydrous condition by passing a rapid current of dry chlorine through a retort over heated iron wire, advantageously cut into pieces some 6 mm. in length. The ferric chloride volatilises and condenses as beautiful crystals on the upper, cooler portions of the retort.

At the end of the operation the heating is discontinued and the chlorine expelled from the apparatus by a rapid current of carbon dioxide. The salt is now rapidly transferred to a tube and hermetically sealed.

Ferric chloride may also be obtained by passing a current of dry, .gaseous hydrogen chloride over heated amorphous ferric oxide; by passing chlorine over heated ferrous chloride; and by heating together ferrous sulphate and calcium chloride.

As prepared by any of these methods ferric chloride consists of dark, iridescent, hexagonal scales, which appear red by transmitted light, but exhibit a green lustre when viewed by reflected light. It melts under pressure at 301° C., but volatilises at 280° to 285° C., at atmospheric pressure, its real melting-point at 760 mm. being 303° C. Between 321° and 442° C. its vapour density in an atmosphere of chlorine is practically constant, and corresponds to the double formula Fe₂Cl₈.

At temperatures above 500° C. anhydrous ferric chloride dissociates into the ferrous salt and free chlorine, the equilibrium being represented by the equation: -

Fe₂Cl₆ ⇔ Fe₂Cl₄ + Cl₂.

The dissociation is already perceptible at 122° C., but is very small, becoming appreciable only at temperatures in the neighborhood of 500° C.

At higher temperatures still, the ferrous chloride dissociates into simple molecules of FeCl₂. These facts probably suffice to account for the low results obtained for the density of ferric chloride in an inert atmosphere, such as nitrogen, since under these conditions dissociation might well be expected to proceed to a greater extent at any given temperature than in an atmosphere of chlorine, by the law of Mass Action.

In boiling solutions of alcohol, ether, pyridine, and other organic solvents, ferric chloride appears to exist as simple molecules of FeCl₃, if the interpretation usually placed upon the results that have been obtained is regarded as correct.

When heated in a current of hydrogen, ferric chloride is reduced to the ferrous salt, provided the temperature is not allowed to rise too high; otherwise further reduction ensues. Traces of reduction can be detected after several hours at temperatures as low as 100° C. Heated in oxygen, chlorine is evolved, leaving a residue of ferric oxide; and, when heated in steam, gaseous hydrogen chloride and ferric oxide are produced. Anhydrous ferric chloride absorbs nitric oxide at ordinary temperatures, yielding a brown mass having the composition 2FeCl₃.NO. On raising the temperature to 60° C., the proportion of nitric oxide is reduced to one-half, a red powder, of composition corresponding to 4FeCl₃.NO, being obtained. At temperatures at which ferric chloride begins to volatilise reduction takes place, ferrous chloride being produced. In ethereal solution ferric chloride is reduced by nitric oxide at the ordinary temperature to the ferrous salt, which latter absorbs excess nitric oxide yielding a compound to which the formula FeCl₂.NO + 2H₂O has been given, which crystallises out at the ordinary temperature. If, however, the temperature is first raised to approximately 60° C., small yellow crystals of the anhydrous compound, FeCl₂.NO, are stated to result; but this is disputed, as has been already mentioned. With nitrogen peroxide in the cold, ferric chloride yields a brownish-yellow, deliquescent powder, of composition represented by the formula FeCl₃.NO₂. This substance is stable in air as also in a vacuum, but is decomposed by water, yielding nitrous acid.

FeCl₃.NOCl is obtained as a black, crystalline substance when ferric chloride is heated in the dried vapours from aqua regia. It is very hygroscopic and dissolves in water, evolving oxides of nitrogen. When heated it readily fuses and volatilises without decomposition. In a sealed tube it melts at 116° C.

Anhydrous ferric chloride readily absorbs ammonia at the ordinary temperature, yielding the hexammoniate, FeCl₃.6NH₃. This decomposes upon exposure to air, yielding the pentammoniate, FeCl₃.5NH₃, which is stable in a dry atmosphere. When heated to 100° C. the tetrammoniate, FeCl₃.4NH₃, results.

The anhydrous salt when heated to dull redness with metallic calcium is reduced to iron.

Ferric chloride combines with ether to form a dark red, highly deliquescent solid of composition FeCl₃.(C₂H₅)₂O. It is soluble in water and alcohol, and at 100° C. decomposes quantitatively, yielding the oxychloride FeOCl: -

FeCl₃.(C₂H₅)₂O = 2C₂H₅Cl + FeOCl.

The heats of formation of anhydrous ferric chloride and of its hydrates are as follow: -

2[Fe] + 2(Cl₂) + Aq. = 2FeCl₂.Aq. + 199,900 calories,
2FeCl₂.Aq. + (Cl₂) = 2FeCl₃. Aq. + 55,540 calories,
whence
2[Fe] + 3(Cl₂) + Aq. = 2FeCl₃.Aq. + 255,440 calories;
again,
2[FeCl₃] + Aq. = 2FeCl₃.Aq. + 63,360 calories,
whence, by subtraction,
2[Fe] + 3(Cl₂) = 2[FeCl₃] + 192,080 calories,
2[FeCl₃] + 5[H₂O] = [2FeCl₃.5H₂O] + 14,400 calories,
[2FeCl₃.5H₂O] + Aq. = 2FeCl₃.Aq. + 42,000 calories at 20° C.

Anhydrous ferric chloride is very deliquescent, and the study of its solubility in water is interesting, there being four distinct curves corresponding to the appearance of four hydrated salts, namely, 2FeCl₃.4H₂O (m. pt. 73.5° C.), 2FeCl₃.5H₂O (m. pt. 56° C.), 2FeCl₃.7H₂O (m. pt. 32.5° C;), and 2FeCl₃.12H₂O (m. pt. 37° C.) respectively. From the last point of discontinuity, namely F in figure 5 (66° C.), onwards the salt is anhydrous and is deposited from solution in that condition.

FeCl3 solubility

A study of the curves in fig. is particularly interesting from the point of view of the Phase Rule. AB represents the various states of equilibrium between ice and ferric chloride solutions, a minimum temperature being reached at the cryohydric point B, which is -55° C. At this point ice, solution, and the dodecahydrate of ferric chloride are in equilibrium. The number of degrees of freedom is nil - in other words, the system is invariant, and if heat be subtracted the liquid phase will solidify without change of temperature until the whole has become a solid mass of ice and dodecahydrate. Further abstraction of heat merely lowers the temperature of the system as a whole.

If, starting at the point B, heat be added to the system, ice will melt, and more of the dodecahydrate will dissolve in accordance with the equilibrium curve BCH, which is the solubility curve of this hydrate in water. At 37° C. the dodecahydrate melts, and if anhydrous ferric chloride be added to the system, the temperature at which the dodecahydrate remains in equilibrium with the solution is lowered until the eutectic point С is reached at 27.4° C. At this point the whole solidifies to a solid mixture of the dodecahydrate and heptahydrate.

The curve has been followed in the direction of the broken line CH to +8° C., the solution being supersaturated with respect to the dodecahydrate. Similarly the curve ED has been continued backwards until it intersects CH at H at 15° C. This is a metastable triple point or eutectic, and is capable of realisation experimentally on account of the fact that the heptahydrate is not so readily formed.

Curves EF and FG represent the solubilities of the tetrahydrate and the anhydrous salt respectively.

The following are the transition temperatures or eutectic points corresponding to the points В, С, H, D, E, and F in fig.: -

B	Ice - 2FeCl3.12H2O	-55° C
C	2FeCl3.12H2O - 2FeCl3.7H2O	+ 27.4°
H	2FeCl3.12H2O - 2FeCl3.5H2O	15°
D	2FeCl3.7H2O - 2FeCl3.5H2O	30°
E	2FeCl3.5H2O - 2FeCl3.4H2O	55°
F	2FeCl3.4H2O - FeCl3	66°

The dodecahydrate, 2FeCl₃.12H₂O, is obtained as deliquescent crystals by treating solid commercial ferric chloride with a current of hydrogen chloride, filtering the resulting liquid, and concentrating over potash in vacuo.

The same hydrate is obtained on allowing a concentrated solution of ferric chloride to evaporate slowly in the cold. It separates out as reniform masses of lemon-yellow crystals, or in opaque, yellow rhombic prisms, according to circumstances. This hydrate melts at 37° C.

The heptahydrate, 2FeCl₃.7H₂O, first obtained by Roozeboom, yields monoclinic crystals, somewhat darker than the preceding hydrate, but readily distinguished by their dichroism, the colours ranging from yellow to blue. When exposed to the air at room temperature, the crystals become coated with the yellow dodecahydrate. They melt at 32.5° C.

The pentahydrate, 2FeCl₃.5H₂O, may be prepared by heating the preceding hydrate to 100° C. for several hours, when hydrogen chloride is evolved. Upon slowly cooling deep red crystals of the pentahydrate are deposited. It also results on keeping crystals of the dodecahydrate in vacuo over sulphuric acid. Liquefaction to a brown solution at first takes place, but finally deep red, deliquescent crystals of the pentahydrate separate out. These melt at 56° C. and deliquesce upon exposure to air.

When treated with a current of dry hydrogen chloride, the pentahydrate readily liquefies, and if saturated with the gas at 25° C. and then cooled to 0° C. it deposits yellow lamellae of the acid salt FeCl₃.HCl.2H₂O.

The tetrahydrate, 2FeCl₃.4H₂O, first obtained by Roozeboom, crystallises in the rhombic system. The crystals appear pleochroic in polarised light, the colours ranging from yellow to brown. They melt at 73.5° C.

Aqueous solutions of ferric chloride are conveniently prepared by dissolving iron in hydrochloric acid and subsequently saturating the solution with chlorine to oxidise the ferrous salt to the ferric condition. After standing, the solution should still smell of chlorine, otherwise sufficient of the gas has not been added. Excess may now be removed by bubbling carbon dioxide through the warm solution. Other methods of preparation consist in dissolving ferric hydroxide in aqueous hydrochloric acid; and by oxidation of ferrous chloride in the presence of hydrochloric acid by some oxidiser such as nitric acid.

In concentrated solution ferric chloride is somewhat oily in appearance, and dark brown in colour. When such a solution is diluted with water, a considerable amount of heat is liberated in consequence of hydrolysis; thus

FeCl₃ + 3H₂O ⇔ Fe(OH)₃ + 3HCl.

Very dilute solutions of ferric chloride are practically colourless when freshly prepared, but become brownish yellow on keeping, owing to the separation of ferric hydroxide in accordance with the above equation.

For example, fresh solutions containing less than 11 per cent, of ferric chloride appear colourless in a 40-cm. tube, but after several hours become yellow, the colour intensifying during forty-eight hours after preparation.

Excessively dilute solutions of ferric chloride give no coloration with potassium ferrocyanide, the salt being completely hydrolysed and converted into colloidal ferric hydroxide.

The hydrolysis of ferric chloride may be illustrated for lecture purposes by filling a tube to about three-fourths of its height with a 5 to 10 per cent, solution of gelatin rendered pink with faintly alkaline phenolphthalein. When the gelatin has solidified, a 10 per cent, solution of ferric chloride is added. As diffusion proceeds downwards, two layers become increasingly distinct - namely, the lower, colourless layer, due to the more rapid diffusion of the acid liberated by hydrolysis; and the upper, opaque layer of brown ferric hydroxide.

An interesting lecture experiment to illustrate suppression of hydrolysis of ferric chloride under certain conditions consists in diluting a solution of the salt until it is practically colourless. Concentrated hydrochloric acid is now added, and the solution assumes a yellow colour, characteristic of the un-ionised FeCl₃-molecule. The addition of glycerol to a solution likewise intensifies the colour, and this is attributed to diminished dissociation consequent upon the introduction of a substance possessing a lower dielectric constant.

Measurement of the electric resistance of aqueous solutions of ferric chloride indicates a gradual increase in conductivity after dilution, a definite maximum value ultimately being reached for each concentration of the salt. The time required to reach a final stage of equilibrium varies with the concentration of the salt. For a 0.0001-normal solution some three hours are required, whilst a week is usual for a 0.0006- normal solution. This increase in conductivity is usually attributed to the gradual liberation of hydrochloric acid in accordance with the

equation

FeCl₃ + 3H₂O → Fe(HO)₃ + 3HCl.

The difficulty, however, is to understand the extreme slowness with which equilibrium is attained, for the hydrolysis should take place with great rapidity. In order to account for this, the change has been regarded as taking place in stages as follows: -

FeCl₃^FeCl₂(OH) → FeCl(OH)₂ → Fe(OH)₃.

This theory, however, cannot be regarded as altogether satisfactory.

Spring, on the other hand, holds that ferric chloride in solution dissociates into ferrous chloride and chlorine, in the same manner as when heated in the gaseous state: -

FeCl₃ ⇔ FeCl₂ + Cl.

The chlorine then reacts with water, yielding hydrogen chloride and oxygen, which latter combines with the ferrous chloride to yield the oxychloride, Fe₂Cl₄O, until the equilibrium represented by the following equation is attained: -

2FeCl₂ + H₂O + 2Cl ⇔ Fe₂Cl₄O + 2HCl.

A suggestive theory, supported by ultra-microscopic examination of dilute ferric chloride solutions has been advanced by Wagner, according to which hydrolysis is instantaneous, but the gradual change in electric conductivity is due to changes in the superficial magnitude of the colloid particles. At first the colloid particles are small, and thus present in toto an enormous surface which adsorbs practically the whole of the liberated acid. Gradually the particles increase in size, becoming less numerous, so that the total superficial area falls, liberating proportional amounts of the adsorbed acid. Wagner's theory appears to the present author to be the most satisfactory that has as yet been advanced.

Ferric chloride decomposes sodium nitrite in aqueous solution with evolution of oxides of nitrogen. The reaction is believed to take place in two stages, namely: -

2FeCl₃ + 6NaNO₂ = 2Fe(NO₂)₃ + 6NaCl,
2Fe(NO₂)₃ + 3H₂O = 2Fe(HO)₃ + 3NO₂ + 3NO.

A solution of ferric chloride decomposes lead sulphide or powdered galena with ease on warming, the products being ferrous chloride, lead chloride, and sulphur.

2FeCl₃ + PbS = 2FeCl₂ + PbCl₂ + S.

A similar reaction takes place with copper pyrites or with copper sulphides. Thus: -

2FeCl₃ + CuS = 2FeCl₂ + CuCl₂ + S,
and 2FeCl₃ + Cu₂S = 2FeCl₂ + 2CuCl + S.

This reaction has been utilised in the separation of copper from pyrites, a solution of ferric chloride being allowed to slowly percolate through the ore raised in heaps, the residual ferrous chloride being oxidised to ferric and used over again.

Ferric chloride is readily reduced by suitable reagents to the ferrous salt. Metallic zinc or iron, or even nascent hydrogen, effects the reduction in aqueous solution. Alkali sulphides reduce it with deposition of sulphur, and alkali iodides with liberation of iodine, thus: -

2FeCl₃ + 2KI = 2FeCl₂ + 2KCl + I₂.

Alcoholic solutions of ferric chloride are reduced by light, which acts, not as a catalyst, but as a generator of the necessary chemical energy. Ferrous chloride, hydrogen chloride, and formaldehyde are the primary products of the reaction.

Dilute solutions of ferric chloride in pure anhydrous ether are rapidly reduced to ferrous chloride upon exposure to direct sunlight. The chlorine is used up, partly in chlorinating the ether and partly in oxidation processes, so that the reaction is not reversible in the dark. More concentrated solutions yield ferrous chloride and a black organic compound containing iron.

Ferric chloride is reduced by aqueous stannous chloride solution . in accordance with the following equation: -

2FeCl₃ + SnCl₂ = 2FeCl₂ + SnCl₄.

Several acid salts of ferric chloride have been described. On saturating the pentahydrate, 2FeCl₃.5H₂O, with hydrogen chloride at 25° C. and cooling the liquid so obtained to 0° C., the compound, FeCl₃.HCl.2H₂O, is obtained in the form of yellow, crystalline lamellae. The compounds, FeCl₃.HCl.4H₂O and FeCl₃.HCl. 6H₂O, have been obtained respectively as greenish crystals, melting at - 3° C., and yellow crystals, melting at - 6° C.