Green Salt

INTRODUCTION

A hydrated iron (III) oxalate complex will be synthesized, isolated, purified and analyzed to determine its chemical formula. The complex has the general formula, Kx[Fey(C2O4)z]• n H2O (Green Salt). The values of x, y, z, and n must be experimentally determined. Oxalate ions will be analyzed by titration with permanganate ions, iron(III) ions by atomic absorption spectroscopy, the number of potassium ions will be derived using charge balance, and the number of water molecules will be obtained by calculating the molar mass of both the hydrated and anhydrous salt and taking their difference.

BACKGROUND

Synthesis of Potassium Iron (III) Oxalate Hydrate Salt

The iron(II) ions from of Fe(NH4)2(SO4)2• 6H2O will be precipitated as iron(II) oxalate.

Fe2+ (aq) + C2O42- (aq) ⇋ FeC2O4 (s) | Eqn 1 |

The supernatant liquid, containing the ammonium and sulfate ions, as well as excess oxalate ions and oxalic acid will be decanted and discarded. The solid will then be re-dissolved and the iron(II) ions will be oxidized to iron(III) ions by reaction with hydrogen peroxide.

2 Fe2+ (aq) + H2O2 (aq) ⇋ 2 Fe3+ (aq) + 2 OH- | Eqn 2 |

The iron(III) ions will then be precipitated to form the iron(III) potassium oxalate hydrate salt. Since the resulting salt is soluble in water, a different solvent will be used to precipitate the green salt. Ethanol is used as it is less polar than water and the green salt is not soluble in ethanol.

Analysis of Oxalate in the Complex Iron Salt

The potassium iron (III) oxalate hydrate salt is treated with 6 M H2SO4 to release the oxalate ion from the salt as oxalic acid. (Note: This should be done in the fume hoods as irritating fumes may be emitted.)

The oxalate ions (in the form of oxalic acid) are then titrated with potassium permanganate (a strong oxidizing agent) to determine the concentration of the oxalate. (The endpoint will be detected when a faint pink color (MnO4- ion) remains for at least 15 seconds.)

C2O42-(aq) + MnO4-...