Лако-красочные материалы — производство

Prussian blue (C. I. Pigment Blue 27), known also as iron blue or Milori blue, is the longest established of all synthetic colorants still in use and retains moderate importance as a low cost blue pigment. On the basis of single crystal X-ray diffraction studies, it has been concluded that Prus­sian blue is best represented as the hydrated iron(iii) hexacyanoferrate(ii), Fe4[Fe(CN)6]3-nH2O. However, when precipitated in the fine particle size essential for its use as a pigment, significant and variable amounts of potassium or ammonium ions are incorporated into the product by surface adsorption or occlusion. In addition, the commercial products may contain indefinite amounts of water and they can exhibit variable stoichiometry and a degree of structural disorder. In the crystal structure of Prussian blue, the Fe(ii) atoms are bonded exclusively to carbon atoms in FeC6 octahedra and the Fe(iii) atoms are bonded exclusively to the nitrogen atoms. Many of the pigmentary properties of Prussian blue have been explained on the basis of its crystal structure. For example, the extreme insolubility of the material has been attributed to the fact that the complex is polymeric as a result of the — Fe(ii)-C-N-Fe(iii)- bonding sequence. The colour is due to metal-metal electron transfer from an Fe(ii) atom to an adjacent Fe(iii) atom, a phenomenon commonly en­countered in mixed oxidation state compounds of this type.

The industrial production of Prussian blue is based on the reaction in aqueous solution of sodium hexacyanoferrate(ii), Na4Fe(CN)6, with iron(ii) sulfate, FeSO47H2O in the presence of an ammonium salt, which results initially in the formation of the colourless insoluble iron(ii) hexa — cyanoferrate(ii) (Berlin white). Prussian blue is generated by subsequent oxidation with a dichromate or chlorate.