Hexacyanoferrate II ions

ll HEXACYANOFERRATE(II) IONS, [Fe(CN)6]4- 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. 

Concentrated sulphuric acid complete decomposition occurs on prolonged boiling with the evolution of carbon monoxide, which burns with a blue flame  

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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The stability of complex ions varies within very wide limits. It is quantitatively expressed by means of the stability constant. The more stable the complex, the greater is the stability constant, i.e. the smaller is the tendency of the complex ion to dissociate into its constituent ions. When the complex ion is very stable, e.g. the hexacyanoferrate(II) ion [Fe(CN)6]4", the ordinary ionic reactions of the components are not shown. 

It would be expected that iodine would quantitatively oxidise hexacyanoferrate(II) ions ... 

In strongly acid solution the reaction proceeds from left to right, but is reversed in almost neutral solution. Oxidation also proceeds quantitatively in a slightly acid medium in the presence of a zinc salt. The very sparingly soluble potassium zinc hexacyanoferrate(II) is formed, and the hexacyanoferrate(II) ions are removed from the sphere of action ... 

The Lewis bases attached to the central metal atom or ion in a d-metal complex are known as ligands they can be either ions or molecules. An example of an ionic ligand is the cyanide ion. In the hexacyanoferrate(II) ion, [Fe(CN)6]4, the CN- ions provide the electron pairs that form bonds to the Lewis acid Fe2+. In the neutral complex Ni(CO)4, the Ni atom acts as the Lewis acid and the ligands are the CO molecules. 

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(II), Cu(II), Fe(II), Hg(II), Ni, Pd(II), Pt(II), Tl(III), and Zn. The alkaline earths, Mn(II), 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(III) is also masked by cyanide. However, as the hexacy-anoferrate(III) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(II) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Figure 10.2 Data and simulation of the impedance behaviour of hexacyanoferrate(ii) ion at a gold disc electrode. The experimentally obtained data are represented with circles while the simulation, performed with the Eco Chimie GPES software, is shown as a continuous line. Reprinted with permission of Dr K Dawes, Windsor Scientific, Slough, UK.

This system can be taken as an example of so-called reversible, diffusion-controlled electrochemical processes. In short, during the initial anodic scan, an electron-transfer process between the ferrocyanide (or hexacyanoferrate(II)) ions, [Fe(CN)6] ", and the working electrode occurs. This can be represented by means of the equation (here, aq denotes species in aqueous solution) ... 

FIGURE 16.20 When cyanide ions (in the form of potassium cyanide) are added to a solution of iron(II) sulfate, they replace the HzO ligands of the [Fe(H20)6]2+ complex and produce a new complex, the more strongly colored hexacyanoferrate(II) ion,... 

Silver nitrate Iron (III) chloride Hexacyanoferrate (II) Ions, [Fe(CN)6]4 White precipitate of silver hexacyanoferrate (II) Prussian blue is formed in neutral or acid conditions, which is decomposed by alkali bases... 

Yellowish brown precipitate of iron (II) cyanide soluble in excess reagent, forming the hexacyanoferrate (II) ion In complete absence of air, white precipitate of potassium iron (II) hexacyanoferrate if air is present, a pale blue precipitate is formed... 

Similarly, from the Tables 1.17 and 1.18 we can see, for example, that permanganate ions (in acid medium) can oxidize chloride, bromide, iodide, iron(II), and hexacyanoferrate(II) ions, also that iron(III) ions may oxidize arsenite or iodide ions but never chromium(III) or chloride ions etc. It must be emphasized that the standard potentials are to be used only as a rough guide the direction of a reaction will depend on the actual values of oxidation-reduction potentials. These, if the concentrations of the species are known, can be calculated easily by means of the Nernst equation. 

The hexacyanoferrate(II) ion being a complex ion does not give the typical reactions of iron(II) (cf. Sections 1.31 to 1.33). The iron present in such solutions may be detected by decomposing the complex ion by boiling the solution with concentrated sulphuric acid in a fume cupboard with good ventilation, when carbon monoxide gas is formed (together with hydrogen cyanide, if potassium cyanide is present in excess) ... 

The precipitate is insoluble in dilute acids, but decomposes in concentrated hydrocholoric acid. A large excess of the reagent dissolves it partly or entirely, when an intensive blue solution is obtained. Sodium hydroxide turns the precipitate to red, as iron(III) oxide and hexacyanoferrate(II) ions are formed ... 

Prussian blue test This is a delicate test and is carried out in the following manner. The solution of the cyanide is rendered strongly alkaline with sodium hydroxide solution, a few millilitres of a freshly prepared solution of iron(II) sulphate added (if only traces of cyanide are present, it is best to use a saturated (25 %) solution ofiron(II) sulphate) and the mixture boiled. Hexacyanoferrate(II) ions are thus formed. Upon acidifying with hydrochloric acid (in order to neutralize any free alkali which may be present), a clear solution is obtained, which gives a precipitate of Prussian blue upon the addition of a little iron(III) chloride solution. If only a little cyanide was used, or is present, in the solution to be tested, a green solution is obtained at first this deposits Prussian blue on standing. 

It is difficult to filter this precipitate, as it tends to form a colloid. The reaction can be used to distinguish hexacyanoferrate(II) ions from hexacyanoferrate(III) and thiocyanate, which do not react. 

Ammonium molybdate solution from a solution of potassium hexacyano-ferrate(II), acidified with dilute hydrochloric acid, a brown precipitate of molybdenyl hexacyanoferrate(II) is formed. The exact composition of the precipitate is not known. The precipitate is insoluble in dilute acids, but soluble in solutions of alkali hydroxides. The test can be applied to differentiate hexacyanoferrate(II) ions from hexacyanoferrate(III) and thiocyanate, which do not react. 

The precipitate is insoluble in 6m hydrochloric acid. Hexacyanoferrate(III) ions do not react. Oxidizing anions (like chromate, arsenate or nitrite) should be absent, as these oxidize hexacyanoferrate(II) ions in acid medium. 

Reducing agents (sulphides, thiosulphates, etc.) interfere since they yield molybdenum blue hexacyanoferrate(II) ions give a red colouration. Arsenates (warming is usually required), arsenites, chromates, oxalates, tartrates, and silicates give a similar reaction with some variation in the colour of the precipitate. All should be removed before applying the test. 

Detection of oligosaccharides (e.g., stachyose, raffinose, sucrose, and fructose) in a soybean extract using invertase hydrolysis of p-o-fructo-fructoside to fructose, and further oxidation of this sugar by hexacyanoferrate (III) ion in the presence of fructose dehydrogenase (FDH). This analysis is based on a coimmobilization of invertase from Candida utilis and FDH from Gluconobacter on poly(vinyl alcohol) (PVA) beads and coulometric quantification of the hexacyanoferrate(II) ions formed. 

If the complex is an anion, its name ends in -ate. For example, in K4[Fe(CN)6] the anion [FeCCN) ]" " is called hexacyanoferrate(II) ion. Note that the Roman numeral II indicates the oxidation state of iron. Table 22.5 gives the names of anions containing metal atoms. 

IV) and hexacyanoferrate(II) ions reduce the nickel(IV) complex quickly to nickel(II) but with no evidence for formation of species analogous to the green reduced manganate. 

Treadwell and Huber reported the electrolytic reduction of aqueous K4[Fe(CN)s] in the presence of excess KCN yielding a colourless solution which reduces one mole of [Fe(CN)6] . This was interpreted as indicating the formation of an iron(I) cyano complex, although the possible formation of a hydrido species such as [Fe(CN)5H] has not been ruled out by subsequent chemical studies. The complex ions [Fe(CN)6H] " and [Fe(CN)5] have been observed in pulse radiolysis studies of aqueous hexacyanoferrate(II) solutions. y-Irradiation of single crystals of alkali halides doped with hexacyanoferrate(II) ions have yielded ESR spectra characteristic of a number of iron(I) species. - These include [Fe(CN)5r -, [Fe(CN)5(H20)] -, [Fe(CN)5Cl] -, [Fe(CN)4Cl2] ", [Fe(CN)5Br] and [Fe(CN)4Br2] . The spectra of the pentacyano species contain some evidence for a bent cyanide ligand. ... 

Challier and Slade [175] reported the synthesis of nanocoinposite materials consisting of polyaniline molecules encapsulated between ultra-thin mixed metal hydroxide sheets which are propped apart by spacers of terephthalate or hexacyanoferrate(II) ions acting as pillars. The layered double hydroxides (LDHs) were prepared by the method of Drezdon [176] which were refluxed with aniline to synthesize aniline intercalated LDHs. In thermo-gravimetric studies, terephthalate/Cu/Cr LDHs as well as hexa-cyanoferrate(lI)/Cu/Al LDHs showed weight losses in two steps attributed to the removal of trapped water and thermal breakdown of the intercalated systems. The former material exhibited somewhat better thermal stability than the latter one. 

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