Trioxalatoferrato potasico

Experiment 7 Revision 1.0

Synthesis of Potassium Ferric Oxalate Trihydrate
• • • • To learn about Coordination Compounds. To learn about metal ion - ligand complexes. To learn about chemical stoichiometry and the Theoretical Yield of a Reaction. To learn about Hydrates.

In this Laboratory exercise we will synthesize the Coordination Compound Potassium Ferric Oxalate Trihydrate,K3[Fe(C2O4)3]•3H2O. This compound is the Potassium salt (K+) of the Trioxalatoferrate(III) complex ion, Fe(C2O4)33-. This Complex is itself a result of the bonding between the Ferric Ion (Fe3+) and three Oxalate (C2O42-) ligands. And, in its crystalline form, the compound solidifies with three associate Water molecules as a Hydrate. Coordination Compounds are substances that contain a Complex Ion, a speciesconsisting of a central metal atom, a transition metal or Main Group metal, bound to several ligands. An example is Hexaammine Cobalt (III) Chloride, [Co(NH3)6]Cl3, which contains the complex ion Co(NH3)63+. This salt, just like simpler salts (e.g., NaCl, CaF2, etc.) involves Ionic Bonding between the complex ion and the three chloride, Cl-, ions. Further, like many simpler salts, it is soluble inwater: [Co(NH3)6]Cl3(s) Co(NH3)63+(aq) + 3 Cl-(aq)

In this example, the complex ion contains the Co3+ transition metal ion and six NH3 ligands. Each ligand forms a single bond to the central metal ion, resulting in an octahedrally bound complex:

The number of bonds formed between the metal center of the Complex and its ligands is referred to as the Coordination Number. In the above example,the Coordination Number is six. The coordination number determines the geometry of these complexes. Typical geometries are:

Coordination Number 2 4

6

Geometry Linear Square Planar or Tetrahedral Octahedral

In this case the NH3 ligand is monodentate, meaning that it forms a single bond with the metal ion. Ligands can be multidentate, forming multiple bonds to the metal center. TheOxalate Ion (C2O42-) is an example of a bidentate ligand; having two bonding points for the metal ion:

In this laboratory we will synthesize, in two steps, the Coordination Compound Potassium Ferric Oxalate Trihydrate; K3[Fe(C2O4)3]•3H2O, a salt which contains the Trioxalatoferrate(III) complex ion, Fe(C2O4)33-. In this complex, the 3 Oxalate ions, C2O42-, form 6 bonds to the Fe3+ center. Thisgives the complex an octahedral shape, much like that pictured above for Co(NH3)63+. The first reaction is the conversion of Ferrous Ammonium Sulfate Hexahydrate, a pale green crystalline solid, to Ferrous Oxalate Dihydrate, a yellow solid: Fe(NH4)2(SO4)2•6H2O(s)
(Ferrous Ammonium Sulfate Hexahydrate)

+ H2C2O4(aq)
(Oxalic Acid)

FeC2O4•2H2O(s) (Ferrous Oxalate Dihydrate)

+

(NH4)2SO4(aq)+ (Ammonium Sulfate)

H2SO4(aq) +

4H2O

The main product of the first reaction, Ferrous Oxalate Dihydrate, will be separated from the other reaction products, which will remain in solution. In the second reaction, the Ferrous Oxalate Dihydrate will be converted to Potassium Ferrous Oxalate Trihydrate according to: 2FeC2O4•2H2O(s) + H2C2O4(aq) + H2O2(aq) + 3K2C2O4(aq) (Hydrogen (PotassiumOxalate) Peroxide)

2K3[Fe(C2O4)3]•3H2O(s)
(Potassium Ferric Oxalate Trihydrate)

This particular product is a specialty chemical in the sense that it has only a few commercial uses. Most abundantly, it is used in Platinum/Palladium photographic printing. An aqueous solution of the potassium ferric oxalate is first converted to the acidic hydrogen ferric oxalate: K3[Fe(C2O4)3](aq) + 3 H+(aq)H3[Fe(C2O4)3](aq) + 3 K+(aq)

The resulting Acid is then used as a sensitizer in developing a photographic print. Another, unrelated, use is as a desensitizer in patients that exhibit a hypersensitive dental condition.

Pre-Lab Questions
1. What is the Theoretical Yield of the synthesis of Potassium Ferric Oxalate Trihydrate if we start with 5.3242g of the starting material, Ferrous...