THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUE6.5.3. Reactivity of Trivalent Iron6.5.3.1. OverviewThe solubility of trivalent iron diminishes rapidly with increasingalkalinity (rising pH value). Even in a pH neutral environment, almostall iron is bound as rust. 343 The reaction between iron compounds andcyanide resulting in the formation of the intermediate product iron(III)-cyanide, [Fe(CN) 6 ] 3– , is therefore largely a reaction on the solid-liquidinterface, that is, between the iron adhering to the solid body and thecyanide ion in solution. This reaction occurs considerably more slowlythan the same reaction in an aqueous solution. The fastest possible reactionrequires a large surface area on the solid-fluid phase boundary,that is, a large, interior, microscopically rough surface and a fine,highly porous solid body, since in such cases, a lot of the iron compoundslie on the surface and are therefore less solidly bound and canquickly combine with cyanide.In an increasingly alkaline environment, only decreasingly smallamounts of ‘rust’ can slowly be converted into iron(II)-cyanide, butcannot react with iron(III)-ions to form Iron Blue.6.5.3.2. ExcursusEven in an alkaline environment, it must be expected that rust, inthe presence of perceptible cyanide concentrations, will be quite slowlytransformed into iron(III)-cyanide and finally into iron(II)-cyanide. 344The last step required for the formation of Iron Blue, however, thecombination of iron(II)-cyanide with iron(III), will not occur due to thelack of dissolved iron(III)-ions. In a strongly alkaline environment, anincreasing concentration of iron(II)-cyanide, which is chemically stable,can slowly accumulate. It remains in a stand-by position, waitingfor the pH value to drop.Iron salts generally tend to incorporate water, and Iron Blue is noexception to this. A higher water content in the solid body results in343 Fe 2 O (3-x)(OH) 2x · x H 2 O344 Naturally, the equilibrium of the reaction Fe(OH) 3 + 6 CN – ⇀↽ [Fe(CN) 6 ] 3- + 3 OH – undersuch conditions is strongly on the left hand side. However, this does not mean, as is wellknown, that a minute quantity of iron(III)-cyanide will not be formed. The latter, however, iswithdrawn from the equilibrium in alkaline medium in the presence of excess cyanide, by beingreduced by the latter to iron(II)-cyanide, which is considerably more stable in alkaline mediumthan iron(III)-cyanide; for further details, see also chapter 6.6.1.163