THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUEbroad pH-spectrum, partly as polymer hydroxo-aquo-complexes (comparechapter 6.5.3.).Graph 6 shows the correlation between the pH value of the freeFe 3+ -concentration in a hypothetical saturated solution of Iron Blue andthe respectively resulting minimal pK S values possible for Iron Blue,which it must possess, should stability prevail at the given pH-reading.Tananaev’s pK S value given, it is obvious that the pigment would remainstable only up to pH 3. Accordingly, it would dissociate itself byits eigen pH value of 4 (see chapter 6.6.1., note 358), which is formedin its own dispersion. Thus the magnitude of error in the results ofTananaev et al. and Krleza et al. is apparent.These reflections show that iron, bound as hydroxides or oxides insolid materials, tends to dissolve in a neutral medium more readily thanIron Blue, since its equilibrium concentration must be higher than thatof Iron Blue.6.6.3. Excursus: Competing LigandsAs shown, OH – -ions may, due to the low solubility of Fe(OH) 3 ,noticeably precipitate the Fe 3+ of Iron Blue in pH media above 9 to 10.The residual hexacyanoferrate(II), on the other hand, would only decomposein strongly alkaline media, because Fe(OH) 2 is simply moresoluble (compare Table 5). 369Tartrate 370 has, in contrast to oxalate, hardly any effects so thatTable 5: Dissociation constants and solubility productsof iron compoundsCompound Constant SourceK S (Fe 4 [Fe(CN) 6 ] 3 ) 4.1 · 10 -187 mol 7 l -7K D(6) ([Fe(CN) 6 ] 4- ) 10 -24 -1mol l368K D(6) ([Fe(CN) 6 ] 3- ) 10 -31 -1mol l368K S (Fe(OH) 2 ) 4.79×10 -17 mol 3 -3l367K S (Fe(OH) 3 ) 2.67×10 -39 mol 4 -4l367K S (FeCO 3 ) 3.13×10 -11 mol 2 -2l367368 C. Wilson, Wilson & Wilson’s Comprehensive Inorganic Chemistry, Vol. 1B, Elsevier, Amsterdam1960, p. 162.369 In absence of free cyanide ions, the pH stability limit of hexacyanoferrate(II) (total dissociation)is at 11.8, but already very small amounts of free cyanide (10 -10 mol l -1 ) push the limit upto pH=13.370 Tartrate, corresponding base of tartaric acid. The mixed potassium-sodium-salt is the famoustartrate (potassium bitartrate), which crystallizes on the cork of wine bottles (Seignette salt).175