THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTParameter EffectTemperature Increased ad-/absorption of hydrogen cyanide as well as—under otherwiseidentical conditions—decreased velocity of individual reactionswith falling temperature; strong increase on water content, andtherefore a strongly positive net influence upon all other factors witha falling temperature.pH value Increased iron reactivity with falling pH, as well as a massive reductionin cyanide accumulation and redox reactivity of iron(III)-cyanide; compromise between iron reactivity and cyanide formation/Fe3+ reduction: A weakly alkaline pH value is favorable toabsorption of hydrogen cyanide and accumulation of cyanide as wellas for the reduction in iron(III)-cyanide, which determines the velocityof the reaction. Although more strongly alkaline media can accumulateiron(II)-cyanide over longer periods of time, no Iron Blue canform under such circumstances.6.6. Stability of Iron Blue6.6.1. pH SensitivityIron Blue is an extremely acid-resistant, but base-decomposingpigment. 357 Hydrogen cyanide is only released by warm, diluted sulfuricacid, while hydrochloric acid, by contrast, has no effect. 358 In aclearly alkaline environment, i.e., in the presence of high concentrationsof OH – ions, these displace the cyanide ion from the iron(III)-ion.Fe(OH) 3 is then precipitated (‘rust sludge’), and the Iron Blue is destroyed.359357 The hexacyanoferrate acids are very strong acids: J. Jordan, G.J. Ewing, Inorg. Chem. 1(1962), pp. 587-591. The findings of analyses of disassociation constants show, for hexacyanoferrate(III):K1 III >K III2 >K III3 >0.1; hexacyanoferrate(II): K1>K II2>0.1; II K3=6×10 II -3 ; K4=6.7×10 II-5 .Thus, hexacyanoferrate(III) is still almost completely disassociated at pH=1, hexacyanoferrate(II)doubly, from pH=3 triply, from pH=5 complete.358 G.-O. Müller, Lehrbuch der angewandten Chemie, vol. I, Hirzel, Leipzig 1986, p. 108; thepigment is, however, reversibly soluble in concentrated hydrochloric acid, i.e., the pigment isnot decomposed, but merely physically brought into solution; there is therefore no release ofhydrogen cyanide; see also H.J. Buser et al., op. cit. (note 329); see also chapter 8.2.: analyticalmethod for total cyanide content according to DIN: the pigment is destroyed by boilingHCl aq. . Iron Blue suspensions (see note 331) have an acid pH value of approximately 4. At thisslightly acid eigen pH, as is formed, for example, by acid rain in surface waters, Iron Blue isat its most stable: H. Ferch, H. Schäfer, op. cit. (note 333). In technical applications, the alkalineresistance is increase by adding nickel, cf. R.E. Kirk, D.F. Othmer, op. cit. (note 332);J.A. Sistino, op. cit. (note 332); E. Elsermann, Deutsche Farben-Z. 5 (1951), pp. 419ff.; R.Beck, Deutsche Farben-Z. 6 (1952), p. 231.359 Iron(III)-hydroxide is even less soluble in this range than Iron Blue; on the solubility ofFe(OH) 3 see chapter 6.6.3.; to be exact, Iron Blue is not totally destroyed at a high pH; rather,170