Potassium Ferrocyanide Hydrate

When a solution of potassium ferrocyanide reacts with rather less than one equivalent of a ferric salt, a blue hydrated precipitate of a-soluble Prussian blue, or ferric potassium ferrocyanide Fe K[Fe (CN)6], is obtained. Now, Hofmann and his co-workers 5 have shown that this precipitate is identical with that prepared under precisely similar conditions by the addition of a ferrous salt to potassium fcrricyanide, although in this case ferrous potassium ferricyanide, Fe K[Fe (CN)6], might be expected. It is therefore assumed that the latter salt is unstable, and, at the moment of formation, undergoes intramolecular rearrangement to the former complex. 

Ferric ammonium ferrocyanide, FeNH4[Fe(CN)6], results when potassium ferrocyanide is heated with a solution of hydroxylamine hydrochloride. It is a deep blue insoluble powder.7 The hydrated salt FeNH4[Fe(CN)6].H20 is known as Monthiers9 blue,8 having been obtained by Monthier as the result of oxidising the white precipitate thrown down by the action of potassium ferrocyanide upon ammoniacal ferrous chloride solution. It may also be prepared by dissolving iron wire in a solution containing ammonia, ammonium chloride, and... 

For this the easiest method is to use potassium ferrocyanide and sponge or spray it with a dilute solution of iron sulphate or use copper sulphate and sponge or spray with ammonium hydrate. If we use acetate of cobalt—then this when heated gives a very similar result. 

Citrates.—Ferric Citrate—Ferri citras (XT. S.)—(Fea)(C H60T)a- -6Aq—489.8-1-108—is in garnet-colored scales, obtained by dissolving ferric hydrate in solution of citric acid, and evaporating the solution at about C0° (140° F.). It loses 3 Aq at 120° (248° F.), and the remainder at 150° (302° F.). If a small quantity of ammonium hydroxid be added, before the evaporation, the product consists of the modified citrate = ferri et ammonii citras (XT. S.), which only reacts with potassium ferrocyanid after addition of HCl. 

Analytical Characters.—Fbrbous—Are acid colorless when anhydrous pale green when hydrated oxidized by air to basic ferric compounds. (1.) Potash greenish-white ppt. insoluble in excess changing to green or brown in air. (2.) Ammonium hy-droxid greenish ppt. soluble in excess not formed in presence of ammoniacal salts. (3.) Ammonium sulfhydrate black ppt. insoluble in excess soluble in acids. (4.) Potassium ferrocyanid in absence of ferric salts) white ppt. turning blue in air. (5.) Potassium ferricyanid blue ppt. soluble in KHO insoluble in HCl. 

Characters and Tests.—Blue crystalline salt, in oblique prisms, soluble in water, forming a pale blue solution which strongly reddens blue litmus. The aqueous solution gives with barium chloride a white precipitate (barium sulphate), insoluble in hydrochloric acid, showing the salt to be a sid-phate and with potassium ferrocyanide a maroon-red precipitate, indicating the presence of copper (cupric ferrocyanide). If an aqueous solution of the salt be mixed with twice its volume of chlorine water, to peroxidize any iron that may be present, and solution of ammonia be added, the precipitate (cupric hydrate) formed by the first addition of the ammonia will be dissolved by a further and sufficient addition of the alkali, and a violet blue solution (copper ammonio-sulphate) will be produced, leaving nothing un ssolved unless iron be present, in which case a reddish-brown precipitate will be left. 

Characters and Tests.—Soft pasty mass, of a reddish-brown colour. Dissolves readily in diluted hydrochloric add without the aid of heat, and the solution gives a copious blue precipitate (ferric ferrocyanide, Prussian blue, Fe4 Fcyg) with potassium ferrocyanide, but none with potassium ferridcyanide. A little of it dried at 212° P. until it ceases to lose weight gives off water of hydration when heated to dull redness in a test-tube. 

Potassium ferrocyanide trihydrate [14459-95-1] M 422.4, piq 2.57, pK 4.35 (for ferrocyanide). It is purified by repeated crystallisation from distilled water, and never heating above 60 . The anhydrous salt is prepared by drying at 110 over P2O5 in a vacuum desiccator. To obtain the trihydrate, it is necessary to equilibrate the salt in a desiccator over a saturated aqueous solution of sucrose and NaCl. It can also be precipitated from a saturated solution at 0° by adding an equal volume of cold 95% EtOH, setting aside for several hours, then centrifuge and wash with cold 95% EtOH. It is finally sucked air dry with water-pump vacuum. The anhydrous salt is obtained by drying the hydrate in a platinum boat at 90° in a slow stream of N2 [Loftfield Swift Chem Soc 60 3083 7958]. 

Copper sulphate and ammonium hydrate produce a blue solution also potassium ferrocyanide and weak solution of iron sulphate also ferrocyanide of jjotassium and ferrous sulphate (protosulphate of iron)—the last named being a deep blue. 

Several types of corrosion inhibitors have been investigated in the last 20 years [53-55] these include calcium and sodium nitrites, sodium benzoate, sodium/potassium chromate, sodium salts of silicates and phosphates, stannous chloride, hydrazine hydrate, sodium fluorophosphate, permanganate, aniline and related compounds, alkalis, azides, ferrocyanide, EDTA and many chelating compounds. However, in terms of field practice and research data, nitrite-based compounds occupy a dominant position. 

If sodium ferrocyanide 10-hydrate is not available, it may be prepared as follows from the potassium salt ... 

The chemical dosimeter that is used most frequently is the thiocyanate dosimeter [119]. Other chemical dosimeters for pulse radiolysis are ferrocyanide [119], modified Fricke (Super-Fricke) [119], hydrated electron [120], 02-saturated solutions of potassium iodide [112], and N20-saturated solutions of methylviologen and formate [118]. The C(N02)4 (tetranitromethane, TNM) dosimeter is used in pulse radiolysis experiments with simultaneous optical and conductometric detection [121-124]. The composition and characteristics of the various chemical dosimeters used for pulse radiolysis with optical detection are listed in Table 8. 

Ferrocyanides are easily produced when solutions of potassium or sodium cyanides are brought into contact with ferrous hydrate or ferrous sulphide —... 

Solutions of oxide of zinc are recognised by the following characters. Caustic alkalies produce a bulky white precipitate of hydrate, soluble in an excess of the alkali. The alkaline carbonates precipitate a bulky white carbonate of zinc, permanent when carbonate of soda or potash is employed, but soluble in excess of carbonate of ammonia. Sulphuretted hydrogen has no action if the solution be acid but if it be quite neutral forms a white precipitate of hydrated sulphuret of zinc, which is best formed by hydrosulphuret of ammonia. Ferrocyanide of potassium also causes a white precipitate. 

Supposedly there was also a wet process involving mixing a dilute solution of cupric sulphate and ferrous sulphate, in proper proportions. .. with a quantity of ferrocyanide of potassium, not in excess (Salter, 1869), though clearly this is giving one of the copper hexacyanofenate(II) pigments copper dipotassium hexacyanoferrate(II) or dicopper hexacyanoferrate(II) hydrate. Terry (1893) also discriminates between Prussian black (calcined Prussian blue) and Prussiate black, the carbonaceous residue from making yellow prussiate of potash . 

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