Ferric ferrocyanide, detection

Detection of cyanide ion and other tests for nitrogen. The cyanide is converted first to ferrocyanide ion by boiling with ferrous sulfate, then to ferric ferrocyanide, or Prussian blue, by addition of ferric chloride and addition of acid. The Prussian blue is detected by its blue color, which is intensified by addition of fluoride ion ... 

Perhaps the most spectacular secret ink is Prussian blue, which forms by means of a chemical reaction between ferric sulfate and potassium ferrocyanide. Generally, a message written with ferric sulfate solution will be revealed when it is sprayed with ferrocyanide. A spy can soak fabric with each of these solutions and transport secret information without detection. During World War ii a German spy named George Vaux Bacon made notations on his socks and cloth buttons with the secret ink reagents. He, too, was caught and executed. 

Wet Tests.—The presence of iron in solution may readily be detected by a considerable number of sensitive reactions. Thus ferrous iron gives a green precipitate of ferrous hydroxide upon addition of excess of ammonium hydroxide. With potassium ferricyanide and a trace of acid, a deep blue precipitate—Turnbull s blue—is obtained. With potassium ferrocyanide a white precipitate is obtained in the entire absence of any ferric salt. Ferric iron, on the other hand, is usually characterised by its deep yellow or brown colour. Addition of concentrated hydrochloric acid deepens the colour. With excess of ammonium hydroxide, brown flocculent ferric hydroxide is precipitated. With potassium ferrocyanide solution, a deep blue colour is obtained in acid solution, whilst with potassium ferricyanide there is no action. Potassium thiocyanate gives in acid solution a deep red colour, which is not d troyed by heat. Salicylic acid gives a violet colour, provided no free mineral acid is present. 

Altschul et al. (1, 2) originally discovered that cytochrome c peroxidase reacts with a stoichiometric amount of hydroperoxide to form a red peroxide compound, which will be referred to hereafter as Compound ES. It has a distinct absorption spectrum, as shown in Fig. 2. The formation of Compound ES from the enzyme and hydroperoxides is very rapid (fci > 10 10 sec"M. No intermediate, which precedes Compound ES, has been thus far detected. In the absence of reductants, or S2, Compound ES is highly stable. The rate constant of its spontaneous decay is of the order of 10 sec 22). The primary peroxide compound (Compound I) of horseradish peroxidase decays much faster at a rate of 10 sec (6). This unusual stability of Compound ES allows one to determine various physical and chemical parameters quantitatively and reliably. Titrations of Compound ES with reductants such as ferrocjHio-chrome c Iff, 20) and ferrocyanide 18, 34) have established that Compound ES is two oxidizing equivalents above the original ferric nnzyme. The absorption spectrum of Compound ES is essentially identical to that of Compound II of horseradish peroxidase which contains one oxidizing equivalent per mole in the form of Fe(IV). In addition, EPR examinations have revealed that Compound ES contains a stable free radical, the spin concentration of which is approximately one equivalent per mole (Fig. 3). Therefore, it is reasonable to conclude that two oxidiz-... 

The following spraying reagents have been reported (185,136-146) for the detection of anions saturated AgN03 solution in methanol, 0.2-0.5% diphenylamine solution in 4M H2SO4,10% ferric chloride solution in 2 M HCl, 0.5% alcoholic solution of pyrogallol, and 1% aqueous solution of potassium ferrocyanide. 

The following should be noted with reference to the detection of aluminum in the presence of other metals When much iron is present, it may happen that the potassium ferrocyanide in the paper in the area of the drop is insufficient to bind all the iron. When the fleck is then moistened with the wash water, unchanged ferric chloride diffuses outward, extends the Prussian blue fleck, and renders the detection of the aluminum difficult. It is best to use very small drops, so that the fleck is at most 5 mm in diameter, and also to carry out a comparative test on an iron solution under exactly the same conditions on the same paper. The difference is then quite easy to see. 

By mixing a drop of potassium ferrocyanide and iron chloride on a spot plate or watch glass, 1.3 y potassium ferrocyanide may be detected. The test is appreciably more sensitive when carried out on paper impregnated with ferric chloride since the capillary localization of the Prussian blue then comes into play. As little as 0.07 y K4Fe(CN)4 may be detected. 

Thiocyanates and/or iodides interfere with the sensitive Pmssian blue test for ferrocyanides when carried out in a micro test tube or on a spot plate. There is simultaneous formation of red ferrithiocyanate or free iodine, which interfere with the decisive detection of small amounts of Prussian blue. If, however, the test is carried out on filter paper impregnated with ferric chloride, no difficulty is experienced because of thiocyanates or iodides. A capillary separation succeeds through the fact that ferric thiocyanate is readily decomposed by mercuric chloride, sodium fluoride, or sodium thiosulfate. If there is danger of interference by ferricyanide, the second procedure should be followed. 

A far better sensitivity is attained for the detection of ferrocyanide in the presence of thiocyanate when use is made of the fact that ferric thiocyanate is decomposed by mercuric chloride (formation of [Hg(CNS)4] ions), sodium fluoride (production of [FeFe] ions), or sodium thiosulfate (reduction to Fe+ ions). 

To detect ferrocyanide in the presence of both thiocyanate and iodide, a drop of the test solution is placed on ferric chloride paper. In the presence of much iodide, only a deep brown fleck of free iodine is formed. On addition of a drop of a concentrated sodium thiosulfate solution, the iodine, the ferric thiocyanate and also the yellow paper are completely decolorized. The fleck of Prussian blue, which was previously hidden by iodine and ferric thiocyanate, is now easily seen against the white background. 

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