Inorganic Synthesis

Potassium ferricyanide

Theory:

Potassium ferricyanide is a water soluble coordination compound with a formula K3[Fe(CN)6]. The compound forms well shaped crystals that are bright red colored. As for many complexes of the type MI3[MIII(CN)6] two structural polytypes (monoclinic and orthorhombic) that are stable at room temperature exist in the case of potassium ferricyanide [1]. Other polytypes stable at lower temperatures include three- and seven-layer monoclinic structure of the compound. At about 130K the crystals of the compound undergo a second-order phase transition that results in slightly changed lattice parameters [2].

The compound is prepared by dissolving iron in air-free water solution of potassium cyanide, yielding potassium ferrocyanide. Chlorine is then passed through the solution of potassium ferrocyanide. The iron atoms are oxidized by chlorine gas from Fe(II) oxidation state to Fe(III) state. Precautions must be taken when working with potassium cyanide as it is highly toxic substance!

6 KCN + Fe + 2 H2O → K4[Fe(CN)6] + 2 KOH + H2

2 K4[Fe(CN)6] + Cl2 → 2 K3[Fe(CN)6] + 2 KCl

The solubility of the compound was determined by Friend and Smirles in 1928. A break found in solubility curve previously by other groups was not observed thus no hydrates exist in a system potassium ferricyanide/water. Solubility curve conforms to the equation Sw = 30.4 + 0.80t – 0.0020t2 (Sw being grams of anhydrous salt per 100 g of water) [3].

Graph1

phf2

phf1

Potassium ferricyanide crystals on matrix [photo: Michal Hegedus]

Literature:

[1] B. N. Figgis, B. W. Skelton and A. H. White, Aust. J. Chem. Vol. 31, p. 1195 (1978).
[2] Y. Morioka, K. Yoriumi, T. Ito, A. Saito and I. Naka-gawa, J. Phys. Soc. Jpn. Vol. 54, p. 2184 (1985).
[3] J. A. N. Friend & W. N. Smirles, J. Chem. Soc. Vol. 1, p. 2242-2245 (1928).