Hexacyanoferrate ll ions

Solubility the alkali and alkaline earth hexacyanoferrate(II)s are soluble in water those of the other metals are insoluble in water and in cold dilute acids, but are decomposed by alkalis. 

Use a 0.025m solution of potassium hexacyanoferrate(II), (often called potassium ferrocyanide), K4[Fe(CN)6]-3H20, to study these reactions. 

A little sulphur dioxide may also be produced, due to the oxidation of iron(II) with sulphuric acid  

With dilute sulphuric acid, little reaction occurs in the cold, but on boiling, a partial decomposition of hexacyanoferrate(II) occurs with the evolution of hydrogen cyanide (POISON)  

Iron(II) ions, formed in this reaction, react with some of the undecomposed hexacyanoferrate, yielding initially a white precipitate of potassium iron(II) hexacyanoferrate(II)  

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Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

FIGURE 16.16 When potassium cyanide is added to a solution of iron(ll) sulfate, the cyanide ions replace the H.O ligands of the [Fe(H20), - + complex (left and produce a new complex, the hexacyanoferrate(ll) ion, Fe(CN)(l 4 (right). The blue color is due to the polymeric compound called Prussian blue, which forms from the cyanoferrate ion. 

The richness of coordination chemistry is enhanced by the variety of shapes that complexes can adopt. The most common complexes have coordination number 6. Almost all these species have their ligands at the vertices of a regular octahedron, with the metal ion at the center, and are called octahedral complexes (1). An example of an octahedral complex is the hexacyanoferrate(ll) ion, [Fe(CN)f, 4. ... 

Cobalt(II) oxide and hydroxide are insoluble, the acetate and nitrate dehques-cent the sulfate, efflorescent the chloride, hygroscopic. The borate, carbonate, cyanide, oxalate, phosphates, sulfide and hexacyanoferrate(ll and III) are insoluble. The ordinary cobalt(II) ammines and the hexacyanocobaltate(III) salts of Alk and Ae are soluble, those of the d-block and Ag" ions insoluble. 

Compounds based around the hexacyanoferrate ion (principally the iron(ll) ion, [Fe(ll)(CN)6] ) form a significant and historically important group, not least because so-called Prussian blue q.v.) is a member, a term which might reasonably be considered applicable to any of the blue hexacyanoferrate(ll) pigments. 

Z. Gao, X. Zhou, G. Wang, P. li, and Z. Zhao, Potassium ion-selective electrode based on a cobalt(ll) -hexacyanoferrate film-modified electrode. AnaZ. Chim. Acta 244, 39-48 (1991). 

The fact that Prussian blue is indeed ferric ferrocyanide (Fe4 [Fe (CN)6]3) with iron(in) atom coordinated to nitrogen and iron(ll) atom coordinated to carbon has been established by spectroscopic investigations [4]. Prussian blue can be synthesized chanicaUy by the ntixing of ferric (ferrous) and hexacyanoferrate ions with different oxidation state of iron atoms either Fe + [Fe kCN) ]" or Fe " + [Fe lCN) ] . Alter ntixing, an immediate formation of the dark blue colloid is observed. However, the mixed solutions of ferric (ferrous) and hexacyanoferrate ions with the same oxidation state of iron atoms are apparently stable. 

Kumar S, Mathur PC (1978) Kinetics and mechanism of oxidation of citric acid by alkaline hexacyanoferrate(III) catalysed by Cu(II) ions. J Inorg Nucl Chem 40 581-584 Lin LL, Liu GG, Lv WY (2012) Simultaneous oxidation of citric acid and reduction of copper ion by TiOj. Adv Mater Res Durnten-Ztlrich 485 253-256... 

Poly(diallyldimethylammonium chloride) (PDADMAC) and ferri-/ferrocyanide show a similar behavior [233]. In most cases, the ferri-/ferrocyanide (trivalent/ tetravalent hexacyanoferrate[lll]/[ll]) couple exhibits an unexpected interaction pattern with strong cationic polyelectrolytes. Hereby, the trivalent ferricyanide favors complexation compared with the tetravalent ferrocyanide [234, 235]. Although the higher charged ferrocyanide is supposed to release a higher number of monovalent counterions, this entropic contribution is less dominant for the ferrocyanide. This conclusion was derived from entropy measurements [232], clearly emphasizing the enthalpic repulsion between the highly hydrated, weakly polarizable ferrocyanide and the somewhat hydrophobic polymeric backbone [236]. This leads to pronounced ion-specific effects. In contrast, some systems... 

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