Chemical elements
  Iron
    History of Iron
    Mineralogy
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    Physical Properties
    Chemical Properties
      Iron Hydride
      Ferrous fluoride
      Aluminium pentafluoferrite
      Ferric fluoride
      Ammonium ferrifluoride
      Barium ferrifluoride
      Potassium ferrifluoride
      Sodium ferrifluoride
      Thallous ferrifluoride
      Ferrous diferrifluoride
      Ferrous monoferrifluoride
      Ferrous chloride
      Ammonium tetrachlorferrite
      Ferric chloride
      Tetrachlorferrates
      Pentachlorferrates
      Ferroso-ferric chloride
      Ferrous perchlorate
      Ferric perchlorate
      Ferrous chlorate
      Ferric chlorate
      Ferrous Oxychlorides
      Ferrous bromide
      Ferric bromide
      Ferric chloro-bromide
      Ferrous bromate
      Ferrous iodide
      Ferric iodide
      Ferric iodate
      Ferrous oxide
      Ferrous hydroxide
      Triferric tetroxide
      Ferric oxide
      Ferrous acid
      Calcium ferrite
      Cobalt ferrite
      Cupric ferrite
      Cuprous ferrite
      Magnesium ferrite
      Nickel ferrite
      Potassium ferrite
      Sodium ferrite
      Zinc ferrite
      Barium ferrate
      Strontium ferrate
      Barium perferrate
      Calcium perferrate
      Potassium perferrate
      Sodium perferrate
      Strontium perferrate
      Iron Subsulphides
      Ferrous sulphide
      Ferric sulphide
      Potassium ferric sulphide
      Sodium ferric sulphide
      Cuprous ferric sulphide
      Iron disulphide
      Ferrous sulphite
      Ferric sulphite
      Potassium ferri-tetrasulphite
      Potassium ferri-disulphite
      Potassium ferri-sulphite
      Ammonium ferri-sulphite
      Sodium ferri-disulphite
      Sodium hydrogen ferri-tetrasulphite
      Ferrous sulphate
      Ferrous copper sulphate Fe
      Ferrous ammonium sulphate
      Ferrous potassium sulphate
      Ferrous aluminium sulphate
      Basic ferrous sulphate
      Ferric sulphate
      Ammonium ferri-disulphate
      Trisodium ferri-trisulphate
      Ferric Alums
      Ferric ammonium alum
      Ferric potassium alum
      Ferric rubidium alum
      Ferroso-ferric sulphate
      Ferrous amido-sulphonate
      Ferric amido-sulphonate
      Ferrous thiosulphate
      Ferrous pyrosulphate
      Ferrous tetrathionate
      Ferric selenide
      Iron diselenide
      Iron Selenites
      Ferrous selenate
      Ferric rubidium selenium alum
      Ferric caesium selenium alum
      Ferric tellurite
      Ferrous chromite
      Ferrous chromate
      Iron nitride
      Nitro-Iron
      Ferrous nitrate
      Ferric nitrate
      Ferrous Nitroso Salts
      Potassium ferro-heptanitroso sulphide
      Sodium ferro-heptanitroso sulphide
      Ammonium ferro-heptanitroso sulphide
      Tetramethyl ammonium ferro-heptanitroso sulphide
      Ferro-dinitroso Sulphides
      Potassium ferro-dinitroso thiosulphate
      Triferro phosphide
      Diferro phosphide
      Iron monophosphide
      Iron sesqui-phosphide
      Ferrous hypophosphite
      Ferric hypophosphite
      Ferrous phosphite
      Ferric phosphite
      Ferrous orthophosphate
      Ferrous hydrogen orthophosphate
      Ferrous dihydrogen orthophosphate
      Ferric orthophosphate
      Sodium ferri-diorthophosphate
      Ammonium ferri-diorthophosphate
      Sodium ferri-triorthophosphate
      Ferric dihydrogen orthophosphate
      Acid ferric orthophosphate
      Ferrous metaphosphate
      Ferric metaphosphate
      Ferrous pyrophosphate
      Ferric pyrophosphate
      Hydrogen ferri-pyrophosphate
      Sodium ferro-pyrophosphate
      Ferrous thio-orthophosphite
      Ferrous thio-orthophosphate
      Ferrous thio-pyrophosphite
      Ferrous thio-pyrophosphate
      Iron sub-arsenide
      Iron mon-arsenide
      Iron sesqui-arsenide
      Iron di-arsenide
      Iron thio-arsenide
      Ferrous met-arsenite
      Ferric arsenite
      Ferrous ortho-arsenate
      Ferric ortho-arsenate
      Ferro mono-antimonide
      The di-antimonide
      Ferrous thio-antimonite
      Ferric ortho-antimonate
      Triferro carbide
      Diferro carbide
      Iron dicarbide
      Iron pentacarbonyl
      Diferro nonacarbonyl
      Iron tetracarbonyl
      Ferrous carbonate
      Ferrous bicarbonate
      Ferrous potassium carbonate
      Complex Iron Carbonates
      Ferrous thiocarbonate
      Ferrous thiocarbonate hexammoniate
      Ferrous cyanide
      Ferro-cyanic acid
      Aluminium ferrocyanide
      Aluminium ammonium ferrocyanide
      Ammonium ferrocyanide
      Barium ferrocyanide
      Calcium ferrocyanide
      Calcium ammonium ferrocyanide
      Cobalt ferrocyanide
      Copper ferrocyanide
      Ammonium cuproferrocyanide
      Barium cuproferrocyanide
      Lithium cuproferrocyanide
      Magnesium cuproferrocyanide
      Potassium cuproferrocyanide
      Sodium cuproferrocyanide
      Ammonium cupriferrocyanide
      Potassium cupriferrocyanide
      Potassium ferrous cupriferrocyanide
      Sodium cupriferrocyanide
      Strontium cupriferrocyanide
      Lithium ferrocyanide
      Magnesium ferrocyanide
      Magnesium ammonium ferrocyanide
      Manganese ferrocyanide
      Nickel ferrocyanide
      Potassium ferrocyanide
      Potassium aluminium ferrocyanide
      Potassium barium ferrocyanide
      Potassium calcium ferrocyanide
      Potassium cerium ferrocyanide
      Potassium magnesium ferrocyanide
      Potassium mercuric ferrocyanide
      Silver ferrocyanide
      Sodium ferrocyanide
      Sodium cerium ferrocyanide
      Strontium ferrocyanide
      Thallium ferrocyanide
      Zinc potassium ferrocyanide
      Ferricyanic acid
      Ammonium ferricyanide
      Barium ferricyanide
      Barium potassium ferricyanide
      Calcium ferricyanide
      Calcium potassium ferricyanide
      Cobalt ferricyanide
      Copper ferricyanide
      Lead ferricyanide
      Magnesium ferricyanide
      Mercuric ferricyanide
      Mercurous ferricyanide
      Potassium ferricyanide
      Sodium ferricyanide
      Strontium ferricyanide
      Zinc ferricyanide
      Ferrous hydrogen ferrocyanide
      Ferrous potassium ferrocyanide
      Prussian Blues
      Ferrous ferrocyanide
      Ferric ammonium ferrocyanide
      Nitroprussic acid
      Sodium nitroprusside
      Ammonium nitroprusside
      Barium nitroprusside
      Cobalt nitroprusside
      Nickel nitroprusside
      Potassium nitroprusside
      Carbonyl Penta-Ferrocyanides
      Carbonyl ferrocyanic acid
      Barium carbonyl ferrocyanide
      Copper carbonyl ferrocyanide
      Ferric carbonyl ferrocyanide
      Potassium carbonyl ferrocyanide
      Silver carbonyl ferrocyanide
      Sodium carbonyl ferrocyanide
      Strontium carbonyl ferrocyanide
      Uranyl carbonyl ferrocyanide
      Sodium ammonio ferrocyanide
      Potassium aquo ferrocyanide
      Potassium aquo ferricyanide
      Sodium aquo penta-ferricyanide
      Potassium sulphito ferrocyanide
      Ferrous thiocyanate
      Ferric thiocyanate
      Sodium ferrothiocyanate
      Sodium ferrithiocyanate
      Potassium ferrithiocyanate
      Iron subsilicide
      Iron monosilicide
      Iron disilicide
      Triferro disilicide
      Ferrous orthosilicate
      Ferrous magnesium orthosilicate
      Ferrous metasilicate
      Ferric silicate
      Diferro boride
      Iron monoboride
      Iron diboride
      Ferrous chlorborate
      Ferrous bromborate
    Corrosion
    Iron Salts
    PDB 101m-1aeb
    PDB 1aed-1awd
    PDB 1awp-1beq
    PDB 1bes-1c53
    PDB 1c6o-1ci6
    PDB 1cie-1cry
    PDB 1csu-1dfx
    PDB 1dgb-1dry
    PDB 1ds1-1e08
    PDB 1e0z-1ehj
    PDB 1ehk-1f5o
    PDB 1f5p-1fnp
    PDB 1fnq-1fzi
    PDB 1g08-1gnl
    PDB 1gnt-1h43
    PDB 1h44-1hdb
    PDB 1hds-1i5u
    PDB 1i6d-1iwh
    PDB 1iwi-1jgx
    PDB 1jgy-1k2o
    PDB 1k2r-1kw6
    PDB 1kw8-1lj0
    PDB 1lj1-1m2m
    PDB 1m34-1mko
    PDB 1mkq-1mun
    PDB 1muy-1n9x
    PDB 1naz-1nx4
    PDB 1nx7-1ofe
    PDB 1off-1p3t
    PDB 1p3u-1pmb
    PDB 1po3-1qmq
    PDB 1qn0-1ra0
    PDB 1ra5-1rxg
    PDB 1ry5-1smi
    PDB 1smj-1t71
    PDB 1t85-1u8v
    PDB 1u9m-1uyu
    PDB 1uzr-1vxf
    PDB 1vxg-1wri
    PDB 1wtf-1xlq
    PDB 1xm8-1y4r
    PDB 1y4t-1ygd
    PDB 1yge-1z01
    PDB 1z02-2a9e
    PDB 2aa1-2azq
    PDB 2b0z-2boz
    PDB 2bpb-2ca3
    PDB 2ca4-2cz7
    PDB 2czs-2dyr
    PDB 2dys-2ewk
    PDB 2ewu-2fwl
    PDB 2fwt-2gl3
    PDB 2gln-2hhb
    PDB 2hhd-2ibn
    PDB 2ibz-2jb8
    PDB 2jbl-2mgh
    PDB 2mgi-2o01
    PDB 2o08-2ozy
    PDB 2p0b-2q0i
    PDB 2q0j-2r1h
    PDB 2r1k-2spm
    PDB 2spn-2vbd
    PDB 2vbp-2vzb
    PDB 2vzm-2wiv
    PDB 2wiy-2xj5
    PDB 2xj6-2ylj
    PDB 2yrs-2zon
    PDB 2zoo-3a17
    PDB 3a18-3aes
    PDB 3aet-3bnd
    PDB 3bne-3cir
    PDB 3ciu-3dax
    PDB 3dbg-3e1p
    PDB 3e1q-3eh4
    PDB 3eh5-3fll
    PDB 3fm1-3gas
    PDB 3gb4-3h57
    PDB 3h58-3hrw
    PDB 3hsn-3ir6
    PDB 3ir7-3k9y
    PDB 3k9z-3l4p
    PDB 3l61-3lxi
    PDB 3lyq-3mm8
    PDB 3mm9-3n62
    PDB 3n63-3nlo
    PDB 3nlp-3o0f
    PDB 3o0r-3p6o
    PDB 3p6p-3prq
    PDB 3prr-3sel
    PDB 3sik-3una
    PDB 3unc-4blc
    PDB 4cat-4erg
    PDB 4erm-4nse
    PDB 4pah-8cat
    PDB 8cpp-9nse

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: -

Fe2Cl6Fe2Cl4 + 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.


Solubility of Ferric Chloride in Water

FeCl3 solubility
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.: -

BIce - 2FeCl3.12H2O-55° C
C2FeCl3.12H2O - 2FeCl3.7H2O+ 27.4°
H2FeCl3.12H2O - 2FeCl3.5H2O15°
D2FeCl3.7H2O - 2FeCl3.5H2O30°
E2FeCl3.5H2O - 2FeCl3.4H2O55°
F2FeCl3.4H2O - FeCl366°


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 + 3H2OFe(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 + 3H2OFe(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)2Fe(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: -

FeCl3FeCl2 + 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 + 2ClFe2Cl4O + 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.

Acid Chlorides

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.
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