THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUEconcentration). 356At neutral pH1pH = 10values, equilibriumconcentrations0.1pH = 9of CN – are0.01within the rangeof 3×10 –4 0.001topH = 8pH = 7on the temperature.An increase1×10 –3 mol perliter, depending0.00010.00001pH = 6pH = 50.000001in the pH value0 5 10 15 20by one point resultsTemperature [°C]25 30in a ten-foldGraph 5: Cyanide equilibrium concentration in waterincrease in the as a function of the temperature and pH value atcyanide equilibrium1mol-% HCN in the air.concentra-tion. The actual cyanide concentration in masonry is determined by thevelocity of absorption of the gas, adsorption effects within the solidmaterial, and possible reactions of the cyanide.The result of all these factors is that slightly alkaline pH values arefavorable to the formation of the pigment.The individual parameters and their influence on the formation ofIron Blue are summarized in the following table:Table 4: Formation of Iron BlueParameter EffectWater contentReactivity ofthe ironc(CN-)sat. [mol/l]Increase in water content results in the following: increased absorptionof hydrogen cyanide; long-term retention of ad-/absorbed hydrogencyanide; increased mobility of reaction partners; increased reactivityof iron oxide; water is the basic precondition for disassociationand redox reactions; generally positive influence with increasingwater content. The water content is dependent above all upon thetemperature.Factor determining reaction velocity, apart from the type of materialand pH value (see below), positively influenced by increasing watercontent.356 Valid for ideal solutions.169