Potassium ferrioxalate

Chembox new
Name = Potassium ferrioxalate
ImageFile = Potassium-ferrioxalate-2D.png ImageSize = 250px
ImageName = Potassium ferrioxalate
IUPACName = Potassium ferric(III) oxalate
OtherNames = Potassium ferrioxalatePotassium trioxalatoferrate (III)
Section1 = Chembox Identifiers
CASNo = 5936-11-8
RTECS =
Section2 = Chembox Properties
Formula = K₃ [Fe(C₂O₄)₃] K₃ [Fe(C₂O₄)₃] .3H₂O
MolarMass = 437.20 g/mol
Appearance = emerald green hydrated crystals
Density = 2.13 g/cm³, solid
Solubility = ?M (20 °C)

MeltingPt = 230 °C (493.15 K)
Section3 = Chembox Structure
Coordination = octahedral
CrystalStruct =
Dipole = 0 D
Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = Corrosive. Eye, respiratory and skin irritant.
RPhrases = R20 R21 R22 R34 R36 R37 R38
SPhrases =
Section8 = Chembox Related
OtherAnions =
OtherCations =
Function = compounds
OtherFunctn = Fe(C₂O₄)₂

Potassium ferrioxalate, is a chemical compound with the formula K₃ [Fe(C₂O₄)₃] , where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron centre. Potassium acts as a counterion, balancing the 3- charge of the complex. Crystals of the trihydrated form of the complex, K₃ [Fe(C₂O₄)₃] .3H₂O, are emerald green in colour. In solution the complex dissociates to give the ferrioxalate anion, [Fe(C₂O₄)₃] ^3-, which appears fluorescent green in colour. Potassium ferrioxalate is often used in chemical actinometry.

Preparation

The complex can be synthesised from the reaction between iron(III) sulfate, barium oxalate and potassium oxalate:

Fe₂(SO₄)₃ + 3 Ba(C₂O₄) + 3 K₂(C₂O₄) → 2 K₃ [Fe(C₂O₄)₃] + 3 BaSO₄

The reactants are dissolved in water and heated for around 1.5 hours. BaSO₄ precipitates out leaving behind the newly formed complex in solution. The complex can then be obtained by filtering off the BaSO₄ and cooling the solution so that it crystallises out.

Isomerism

The ferrioxalate complex demonstrates optical activity since there are two non-superimposable stereoisomers of the complex. In accordance with the IUPAC convention, the isomer with the left handed screw axis is assigned the Greek symbol Λ (lambda). Its mirror image with the right handed screw axis is given the Greek symbol Δ (delta) [Greenwood&Earnshaw] .

Photoreduction

In solution the ferrioxalate complex is decomposed by light. The complex reacts with a photon of light to form Fe(C₂O₄)₂^2- and CO₂. Iron is reduced (gains an electron) from the +3 oxidation state to +2. This process is called photoreduction:

2 [Fe(C₂O₄)₃] ^3- + "hν" → 2 [Fe(C₂O₄)₂] ^2- + C₂O₄^2- + 2 CO₂

If a solution of green K₃ [Fe(C₂O₄)₃] is left in sunlight for a few hours it turns orange due to the formation of Fe(C₂O₄)₂^2- ions.

References