Prussian yellow

At high anodic potentials Prussian blue converts to its fully oxidized form as is clearly seen in cyclic voltammograms due to the presence of the corresponding set of peaks (Fig. 13.2). The fully oxidized redox state is denoted as Berlin green or in some cases as Prussian yellow . Since the presence of alkali metal ions is doubtful in the Prussian blue redox state, the most probable mechanism for charge compensation in Berlin green/Prussian blue redox activity is the entrapment of anions in the course of oxidative reaction. The complete equation is ... 

PB also can be oxidized to give first a green material, Berlin Green (BG), in the partially oxidized state, and then finally a yellow material, Prussian Yellow (PY), in the fully oxidized state. This oxidation typically occurs between 0.8 and 1.0 V vs. SCE. The complete oxidation of Prussian Blue to Prussian Yellow is shown in the following equation. 

Prussian yellow. This irreversible binding of thallium(I) ions is probably related to the well-known use of Prussian blue to bind thallium ions following poisoning. 

Prussian blue films can be reduced to the colourless form, called Everitt s salt (KFe4[Fe (CN)g]3 or K2FeFe (CN)g), or oxidized to the yellow form called Prussian yellow (Fe4[Fe (CNigJaCl or KFeFe (CNlgCl). These electrochemical processes can be easily detected by cyclic voltammetry of Prussian blue films in KCl solutions. The reduction process for the soluble Prussian blue structure has been described as "... 

Hydrochloric acid should not be used for acidifying the alkaline solution since the yellow colour, due to the ferric chloride formed, causes the Prussian blue to appear greenish. For the same reason, ferric chloride should not be added—as is frequently recommended a sufficient concentration of ferric ions is produced by atmospheric oxidation of the hot alkaline solution. The addition of a little dfiute potassium fluoride solution may be advantageous in assisting the formation of Prussian blue in a readily filterable form. 

Since ferrous iron usually colors minerals green, and ferric iron yellow or brown, it may seem rather remarkable that the presence of both together should give rise to a blue color, as in the case of vivianite. It may be pointed out, however, that this is by no means a unique instance of such an effect. Even apart from the artificial substances, Prussian and Turnbull s blues, which are complex cyanides containing both ferric and ferrous iron, there are several blue minerals in which the color seems explainable only on this basis. The most noteworthy of these are crocidolite and related amphiboles iolite and the blue tourmaline or indicolite. Other instances may perhaps be discovered, should this subject ever be investigated as it deserves to be . 

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 ... 

Balsam of copaiba, nine ounces lamp-black, three ounces Indigo and Prussian blue, ground together in equal proportions, one and a quarter ounce Indian red, throe-fburths of au ounce thy, yellow turpentine soap, three ounces r this mixture Is to be ground, by means of a slab and mullor, to an impalpable smoothness, It is stated that this iuk possesses a beautiful color, but appears to work foul. 

With sesquioxide of iron, the red prussiate gives no precipitate the yellow prussiate, a dark-blue precipitate, which Is prussian bbte—... 

Tha orude solution of prussiate of potassa, and even the prussiate purified by a single crystallization, still contain, as has been stated, more or less carbonate of potassa, so that, besides the prussian blue, there is precipitated at the same time yellow oxide of iron, which deteriorates its tint. To obviate tills inconvenience, the carbonate of potassa is neutralized by sulphuric acid, which may he added cither to the solution of prussiate of potassa or to that of the protosnlphate of iron. In the manufacture of the common Berlin blues, the carbonate of potassa is neutralized by alum thoro results a precipitate of alumina, which mixes intimately with the prussian blue, and greatly augjnente... 

The precipitate formed by adding a solution of femdeyanide of potassium—the red prussiate—to a protosalt of iron, constitutes the odor known in commerce as Turnbull s blue. It is produced by the substitution of three atoms of iron for the three of potassium in the red salt. Its color is a little brighter than common prussian blue. It is prepared in the arts by adding to a solution of copperas a mixture of yellow prussiate of potassa and chloroxide of sodium,... 

Alkali or alkaline-earth salts of both complexes are soluble in water (except for Ba2[Fe(CN)6]) but are insoluble in alcohol. The salts of hexakiscyanoferrate(4—) are yellow and those of hexakiscyanoferrate(3—) are mby red. A large variety of complexes arise when one or more cations of the alkali or alkaline-earth salts is replaced by a complex cation, a representative metal, or a transition metal. Many salts have commercial applications, although the majority of industrial production of iron cyanide complexes is of iron blues such as Prussian Blue, used as pigments (see PIGMENTS, inorganic). Many transition-metal salts of [Fe(CN) J4"" have characteristic colors. Addition of [Fe(CN)6]4 to an unknown metal salt solution has been used as a qualitative test for those transition metals. 

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