Ferricyanic acid

The parent acid of the hexakiscyanoferrate(3—) salts is ferricyanic acid [17126464] (trihydrogen hexakiscyanoferrate). Red-brown needles are obtained by evaporation of solutions prepared by adding sulfuric acid to tribarium bis(hexakiscyanoferrate). The acid is used to prevent metal surface corrosion. 

Such cyanide complexes are also known for several other metals. All the fer-rocyanide complexes may be considered as the salts of ferrocyanic acid H4Fe(CN)e and ferricyanide complexes are that of ferricyanic acid, H3Fe(CN)e. The iron-cyanide complexes of alkali and alkaline-earth metals are water soluble. These metals form yellow and ruby-red salts with ferro-cyanide and ferricyanide complex anions, respectively. A few of the hexa-cyanoferrate salts have found major commercial applications. Probably, the most important among them is ferric ferrocyanide, FeFe(CN)e, also known as Prussian blue. The names, formulas and the CAS registry numbers of some hexacyanoferrate complexes are given below. Prussian blue and a few other important complexes of this broad class of substances are noted briefly in the following sections ... 

H3Fe(CN)e (aq.). Berthelot27 measured the heat of oxidation of aqueous ferrocyanic acid with bromine and with chlorine, his data yielding, for the ferricyanic acid, HsFe(CN)e (aq.), Qf= —148.1 and —148.4, respectively. 

Hydrogen ferricyanide, Ferricyanic acid, H3[Fe(CN)6], maybe obtained as crystalline brown needles by the action of dilute sulphuric acid upon lead ferricyanide and subsequent concentration of the... 

Barium ferricyanide, Ba3[Fe(CN)6]2.20H2O, is prepared by neutralising ferricyanic acid with barium carbonate and evaporating to crystallisation in vacuo.2. It also results on boiling a solution of barium ferrocyanide with lead dioxide.4 It yields reddish brown crystals. These are soluble in water without decomposition. The salt is decomposed on warming with acids. The compound Ba3[Fe(CN)6]2.2BaBr2. 20H2O, has been obtained.5... 

Calcium ferricyanide, Ca3 [Fe(CN) 6] 2( )12H20, may be obtained 9 by oxidising aqueous calcium ferrocyanide with calcium plumbate and carbon dioxide under pressure, at the ordinary temperature. The solid salt is separated by evaporation under reduced pressure. The salt may also be obtained by oxidation of calcium ferrocyanide with chlorine 10 and by neutralising ferricyanic acid with calcium carbonate. It yields needle-shaped crystals, which are stable in dry air. 

Hydro-ferrocyanic and Hydro-ferricyanic Acids.—The acids which are the mother substances of these complex cyanides may be obtained from the potassium salts by treating with strong acids. They are ... 

The iron salts of ferro and ferricyanic acid are the compounds to which the names cyanogen and cyanide are due. Two of these salts are of deep blue color and the Greek word from which cyanogen and cyanide are derived is cyanos which means blue. The ferric ferro-cyanide, Fe4 "(Fe"(CN6)3, is known as Prussian blue and the ferrous ferri-cyanide, Fe3"(Fe" (CN)6)2, is Turnbull s blue. These compounds are formed when ferric salts in solution are treated with potassium ferro-cyanide and when ferrous salts in solution are treated with potassium ferricyanide. They are common qualitative tests for the two forms of iron salts. The compounds are also used as laundry blueing and are formed in the blue print process of photography. 

Treatment of the complex salt with acid liberates the corresponding complex ferricyanic acid, an oxidant which is rather endothermic (AHy (aq) -1-640.5 kJ/mol, 2.96 kJ/g). and which forms solid complexes with ether, etc. 

In 1935, Kolthoff and Tomsicek also reported that the fourth ionization constant for ferrocyanic acid, H4Fen(CN)6, is 5.6 x 10 5 at 25°C.64 This means that HFen(CN)63- is a weak acid. At pH values of 6 and below, substantial concentrations of HFen(CN)63- are present. In 1962, Jordan and Ewing showed that H2Fen(CN)62- is the dominant species at pH l.65 They also reported that ferricyanic acid (H3Fein(CN)6) is much more extensively dissociated, such that it is effectively completely deprotonated to the free anion at pH values larger than 1. 

POTASSIUM FERRICYANATE, POTASSIUM FERRICYANIDE, or POTASSIUM HEXACYANOFERRATE(IH) (13746-66-2) Mixtures with water, acids, alcohols cause slow decomposition, producing hydrocyanic acid. Explosive reaction with ammonia. Violent reaction with copp)er(II) nitrate, trihydrate. Incompatible with chromium trioxide, sodium nitrite. 

M. Abe, Application of synthetic inorganic ion exchangers to analytical chemistry. I. Hydroxides and hydrated oxides, Bunseki Kagaku (Anal. Chem.), 1974, 23, 1254 M. Abe, Application of synthetic inorganic ion exchanger to analytical chemistry. II. Acidic salts, heteropolyacid salts and insoluble ferricyanates, II. ibid., 1974, 23, 1254, 1561. 

Narcotine and hydrastine yield sparingly soluble precipitates with a number of inorganic complex acids, such as silicotungstic (199-206), phosphotungstic (199), ferro- and ferricyanic (207-210), chloroplatinic (92, 211-214), and many others including double compounds with heavy metals (213, 215-218). Many color reactions have been observed, but a large number of these depend upon some tjipe of condensation with one of the fission products, and few if any are exclusively diagnostic or specific (160, 167, 219-224). 

These equations only summarize the course of the reaction. It is likely that the ferrocyanide acts as a reducer, so that Tumbuirs blue (the ferrous salt of ferricyanic acid) KFeii[Fem(CN)e], is also formed. ... 

D. Ferricyanic Acid and Ferricyanides [GyanoferratesHi] (1) Test with benzidine or o-tolidine... 

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