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Sodium ferrioxalate

Sodium ferrioxalate
Sodium ferrioxalate
Names
IUPAC name
Sodium iron(III) oxalate
Other names
Sodium ferrioxalate

Sodium ferric oxalate
Sodium trisoxalatoferrate (III)

Sodium oxalatoferrate
Identifiers
Properties
Na3[Fe(C2O4)3]

Na3[Fe(C2O4)3].xH2O

Molar mass 388.88 g/mol - anhydrous sodium trioxalatoferrate (III)
388.88 + x(18.01) g/mol - hydrated sodium trioxalatoferrate (III)
Appearance lime green hydrated crystals
Density 1.97 g/cm3 at 17 °C
32.5pts per 100pts solvent, cold water, 182pts per 100pts, boiling water
Structure
octahedral
0 D
Hazards
Main hazards Corrosive. Eye, respiratory and skin irritant.
R-phrases (outdated) R20 R21 R22 R34 R36 R37 R38
Related compounds
Other anions
Potassium ferrioxalate
Related compounds
Iron(II) oxalate
Iron(III) oxalate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY  (what is YesYN ?)
Infobox references

Sodium ferric oxalate
Sodium trisoxalatoferrate (III)

Na3[Fe(C2O4)3].xH2O

Sodium ferrioxalate, also known as sodium oxalatoferrate, is a chemical compound with the formula Na3[Fe(C2O4)3], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions act as ligands bound to an iron centre. Sodium acts as a counterion, balancing the -3 charge of the complex. Crystals of the hydrated form of the complex, Na3[Fe(C2O4)3].xH2O, are lime green in colour. In solution the complex dissociates to give the ferrioxalate anion, [Fe(C2O4)3]3−, which appears a deep apple green in colour.

The bonds to the iron atom are dative covalent bonds where the ligands, (oxalate ions, blue), donate a lone pair into the empty p and d orbitals of the transition metal (iron, red), atom. The three oxalate ions donate 12 electrons in all and Fe-III has three electrons in the d orbitals leaving 13 empty places in the remaining d and p orbitals.

This compound is very soluble in hot water, (182 parts per 100 parts solvent by mass), but a lot less soluble in cold water, (32 parts per 100 parts solvent), about the solubility of sodium chloride. It is not appreciably soluble in ethanol or ethanol water mixtures which are more than 50% ethanol by mass. It is somewhat more soluble in water than the corresponding potassium salt.

The crystals pictured were synthesised by mixing solutions of sodium oxalate and ferric oxalate and waiting a few hours for the brown colour of the ferric oxalate to be replaced with the green colour of the complex anion. This complex is relatively inert and the equilibrium is attained only slowly at room temperature. The ferric oxalate was made by dissolving rust in oxalic acid and filtering off any residual insolubles. The solution was evaporated at just below boiling until small crystals appeared on the bottom indicating the solution was then hot and saturated. The solution was allowed to cool in a beaker sitting on a large aluminium block. The thermal mass of the block allowed sufficiently slow cooling over night to produce crystals a few millimetres long. These larger crystals are pictured at the upper left.


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