Ferrocyanic acid

AH of the [Fe(CN)3] salts maybe considered salts of ferrocyanic acid or tetrahydrogen hexakiscyanoferrate [1712647-5], H4[Fe(CN)3], a strongly acidic, air-sensitive compound. It is soluble in water and alcohol but is insoluble in ether. It can be prepared by precipitation of an etherate by adding ether to a solution of [Fe(CN)3] that was acidified with concentrated sulfuric acid. Removal of the ether of solvation affords a white powder which is stable when dry but slowly turns blue in moist air because of Pmssian Blue formation. 

The aqueous ferricyanide oxidation of 2-mercaptoethanol to the disulphide is also complex kinetically" . In the pH range used (l.S. l) no complication from ionisation of the thiol is expected. Individual decays of oxidant concentrations are initially second-order but eventually become almost zero-order. For both second-and zero-order paths the rate depends on the first power of the thiol concentration and the former path is retarded by increasing the acidity, an approximately inverse relation existing above pH 3.2. Addition of ferrocyanide transforms the kinetics the rapid, second-order path is inhibited and the zero-order path is accelerated until, at 10 M ferrocyanide, the whole of the disappearance of oxidant is zero-order. Addition of Pb(C104)2, which removes product ferrocyanide, greatly enhances the oxidation rate and the consumption of oxidant becomes rs/-order. Two routes are considered to co-exist (taking due account of the acidity of ferrocyanic acid), viz. 

Contact with acids liberates the solid complex ferrocyanic acid which is endothermic (AH°f (aq) +534.7 kJ/mol, 2.48 kJ/g), and forms complexes with diethyl ether etc. 

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 ... 

Ferrocyanic acid [tetrahydrogen hexakis(cyanoferrate(4—)] H4Fe(CN)6 [17126-47-5]... 

Hexacyanoferric(II) acid, ferrocyanic acid or tetrahydrogen hexakis(cyanate-(4-)) [17126-47-5] is used to prepare many adducts with oxygen-containing organics. Also addition compounds with inorganic salts are known. This acid may be obtained as a white precipitate upon addition of potassium ferrocyanide to concentrated hydrochloric acid ... 

Methylamine occurs in herring brine 2 in crude methyl alcohol from wood distillation,3 and in the products obtained by the dry distillation of beet molasses residues.4 It has been prepared synthetically by the action of alkali on methyl cyanate or iso-cyanurate 5 by the action of ammonia on methyl iodide,6 methyl chloride,7 methyl nitrate,8 or dimethyl sulfate 9 by the action of methyl alcohol on ammonium chloride,10 on the addition compound between zinc chloride and ammonia,11 or on phos-pham 12 by the action of bromine and alkali on acetamide 13 by the action of sodamide on methyl iodide 14 by the reduction of chloropicrin,15 of hydrocyanic or of ferrocyanic acid,16 of hexamethylenetetramine,17 of nitromethane,18 or of methyl nitrite 19 by the action of formaldehyde on ammonium chloride.20... 

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. 

Fe4(Fe(CN)6)3 (c). Berthelot27 measured the heat of reaction of aqueous ferrocyanic acid with ferric hydroxide to be 76.6, and the heat of reaction of aqueous KCN with aqueous ferrous and ferric sulfates to be 225.0 whence, for ferric ferrocyanide, Qf=— 322.5 and —312. 

A saturated solution of ferrocyanide is used at a temperature of 20° C. The Deutsche G-old und Silber Scheidean-stalt modify the process by addition of calcium ferrocyanide which prevents the contamination of the end-product with alkali. H. von Hayek3 has examined the process, and shown that a 100 per cent, yield may be obtained if a high current density be used and the anode rotated, whilst the electrolyte is kept alkaline to prevent the formation of free ferrocyanic acid. 

Ferrocyanic acid is also obtained when a solution of insoluble Prussian blue in hot, concentrated hydrochloric acid is allowed to stand (see p. 227). 

Potassium ferrocyanide, K4Fe(CN)6.3H20, is the most important salt of ferrocyanic acid, and is known in commerce by the more familiar name of yellow prussiate of potash.1 It results when a solution of... 

It must not be overlooked, however, that isomerism has been definitely discovered3 in the case of tetramethyl ferrocyanide, (CH3)4Fe(CN)6, and it is always possible that isomerides of inorganic salts of ferrocyanic acid may be capable of existence. 

Hydrogen carbonyl ferrocyanide, or carbonyl ferrocyanic acid,... 

The heat of combustion of carbonyl ferrocyanic acid is 809,800 calories, the products being nitrogen, carbon dioxide, water, and ferric oxide.4 The heat of formation of the anhydrous acid is —48,600 calories 5 the acid is thus less strongly endothermic than ferrocyanic acid (—122,000 calories). 

Quince Ferrocyunus (Paris Codex) [Ferrocyanate of quinine, ferrocyanic acid H4 Fe(CN)g], one to five grains. 

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. 

Ferrocyanic acid H fefCNlg A colorless, crystalline acid obtained by treating ferrocyanldes with acids.Paris blue. ... 

Gay-Lussac said T consider this acid [ferrocyanic acid] as a true hydracid, the radical of which is formed from i atom of iron and 3 atoms of cyanogen , i.e. with modern atomic weights Fe(CN)g. He called this radical cyanoferre , the acid being acide hydrocyanoferrique and the salts cyanoferrures . This theory, he says, is the same as that of hydrochloric acid and the chlorides. 

Benzidine acetate is oxidized by soluble ferricyanides, with formation of insoluble blue meri-quinoid compounds (see page 283). This redox reaction permits the detection of ferricyanides in the absence of other oxidizing compounds (chromates, peroxo-compounds, etc.). The test can also be used in the presence of ferrocyanides. However, it should be noted that the benzidine salt of ferrocyanic acid separates as a white precipitate, similar to benzidine sulfate. More reagent is consumed, and the detection of very small amounts of ferricyanide is rendered more difficult. To detect very small amounts of ferricyanide in the presence of large amounts of ferrocyanide, it is necessary to add sufficient lead salt to precipitate lead ferrocyanide ferricyanides remain in solution. Addition of benzidine, then causes the white Pb2[Fe(CN)e] to turn blue, because of the formation and adsorption of benzidine blue. 

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