Excursus Competing Ligands

As shown, OH -ions may, due to the low solubility of Fe(OH)3, noticeably precipitate the Fe3+ of Iron Blue in pH media above 9 to 10. The residual hexacyanoferrate(II), on the other hand, would only decompose in strongly alkaline media, because Fe(OH)2 is simply more soluble (compare Table 5).369 

Tartrate370 has, in contrast to oxalate, hardly any effects so that 

Wilson, Wilson Wilson s Comprehensive Inorganic Chemistry, Vol. IB, Elsevier, Amsterdam 1960, 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 (1010 mol l 1) push the limit up to pH=13. 

370 Tartrate, corresponding base of tartaric acid. The mixed potassium-sodium-salt is the famous tartrate (potassium bitartrate), which crystallizes on the cork of wine bottles (Seignette salt). 

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