Ferric chloride
Ferric chloride (Iron(III) chloride, perchlorure de fer, ou trichlorure de fer).
Molecular formula (anhydrous) : FeCl3
Molar mass FeCl3 = 162,2 g·mol-1
(Cl = 66,57 %, Fe= 34,43 %)

Anhydrous iron(III) chloride is deliquescent, forming hydrated hydrogen chloride mists in moist air. It is rarely observed in its natural form, mineral molysite, known mainly from some fumaroles. Odor : slight HCl (hydrochloric acid).
The colour of iron(III) chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red.
Hexahydrated FeCl3 , 6H2O (M=270.3 g·mol-1) : yellow.
When dissolved in water, iron(III) chloride undergoes hydrolysis and gives off heat in an exothermic reaction. The resulting brown, acidic, and corrosive solution is used as a flocculant in drinking water production and sewage treatment, and as an etchant for copper-based metals in printed circuit boards. Anhydrous iron(III) chloride is a fairly strong Lewis acid, and it is used as a catalyst in organic synthesis.

Anhydrous iron(III) chloride may be prepared by union of the elements:

2 Fe (solid) + 3 Cl2 (gas) >>> 2 FeCl3 (solid)

Solutions of iron(III) chloride are produced industrially both from iron and from ore, in a closed-loop process.

  1. Dissolving pure iron in a solution of iron(III) chloride
    Fe(s) + 2 FeCl3(aq) >>> 3 FeCl2(aq)
  2. Dissolving iron ore in hydrochloric acid
    Fe3O4(s) + 8 HCl(aq) >>> FeCl2(aq) + 2 FeCl3(aq) + 4 H2O
  3. Oxidation of iron (II) chloride with chlorine
    2 FeCl2(aq) + Cl2(g) >>> 2 FeCl3(aq)
  4. Oxidation of iron (II) chloride with oxygen
    FeCl2(aq) + ¼O2 + HCl >>> FeCl3(aq) + ½H2O

Like many other hydrated metal chlorides, hydrated iron(III) chloride can be converted to the anhydrous salt by refluxing with thionyl chloride.
Conversion of the hydrate to anhydrous iron(III) chloride is not accomplished by heating, as HCl and iron oxychlorides are produced.

It is available in various solid or liquid forms (depending on the manufacturer):

Main properties :

Solide form (anhydrous)
% FeCL3
40 (about)
% Fe
Volumetric mass density ( 20 °C)
2804 kg/m³ (±7) - 1820 kg/m3 (hexahydrate)
1400 kg/m³
Chloride (Cl), by weight
26.2 %
Melting point
306 °C (579 K) - 37 °C (hexahydrate)
Solidification point
-50 °C
Dynamic Viscosity (at 20 °C)
12 cP (Pure water : 1.0020 cP)

Solubility in water (soft) depending the temperature :
Temperature (°C)
Max. Solubilité(g/L)
(g/L = g as FeCl3 per liter)

Chemical action in the water.
The chemical action of the coagulant causes:

A formulation for water treatment.
Ferric chloride react with dissolved calcium bicarbonate.
.....2 FeCl3.......+ ........3 Ca[HCO3]2 ...........>>>.... 2 Fe[OH]3 .... + ...6CO2......+........3CaCl2
(2x162,2)=324,4...........(3x162)=486 or 30°F as TAC . ................(2x106,85)=213,7............(6x44)=264.........(3x2x35,5 de Cl)=213

Per 1 mg/L FeCl3 added there (30/324,4)= 0.092 °F lower Alkalinity (TAC), and is formed (264/324,4)= 0.814 mg of free CO2, and is also formed (213,7/324,4) = 0.659 mg of ferric hydroxide (as insoluble flocs), and(213/324,4)=0.657 mg of dissolved chloride.

Standard products used for the production of drinking water: Official French Bulletin - Chlorure ferrique : NF EN 888.


Other uses :


Effects on the environment.
Ferric chloride may decrease the pH of stream, thus representing a potential threat to aquatic fauna and flora (stream, river, groundwater).

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