Iron complexes with thiocyanate, formation

While the liquid-liquid extraction of inorganic elements as coordination complexes with thiocyanate ions can be traced back to Skey (1867), the extraction from hydrochloric acid into ether of iron(III) (J. W. Rothe, 1892) or gallium (E. H. Swift, 1924) depends on the formation of solvated acido complexes derived from HMC14 extractions of metal complexes from nitric, thio-cyanic, hydrofluoric, hydrochloric and hydrobromic acids were studied exhaustively by Bock and his collaborators (1942—1956).6... 

The mechanism of the reaction has not been elucidated. Presumably several reactions occur simultaneously. Thiocyanates react with iron(III) salts with the formation of red-colored complexes. In sulfuric acid medium nitrate or nitrite coddize diphenylamine to... 

The non-aqueous medium favors the formation of the red-violet complex. The yellow complex is poorly soluble in a wide variety of organic solvents. Molybdenum, uranium, and iron interfere with the determination since these elements form colored thiocyanate complexes. Disadvantages of this method are the long time required and the formation of the yellow thiocyanate complex which cannot be excluded. 

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

Another important application is the test for iron(III) ions with thiocyanate. In slightly acid medium a deep-red colouration is formed, owing to the stepwise formation of a number of complexes ... 

This chargeless molecule can be extracted by ether or amyl alcohol. In addition to this a set of complex ions, such as [Fe(SCN)]2+, [Fe(SCN)2]+, [Fe(SCN)4]-, [Fe(SCN)5]2-, and [Fe(SCN)6]3 are also formed. The composition of the product in aqueous solution depends mainly on the relative amounts of iron and thiocyanate present. Phosphates, arsenates, borates, iodates, sulphates, acetates, oxalates, tartrates, citrates, and the corresponding free acid interfere due to the formation of stable complexes with iron(III) ions. 

Mercuryil) nitrate was introduced as a reagent for Fe(III), with thiocyanate as indicator. The Fe(III)-Hg(I) reaction has been studied for the direct determination of iron and the indirect determination of oxidants that react quantitatively with Fe(II). For example, hydroxylamine and Hg(I) can be determined by adding an excess of Fe(III) and back-titrating the excess. An unusual example is the reduction of Cu(II) to Cu(I) by Fe(II) in the presence of thiocyanate. In this reaction, complex formation causes the usual direction of reaction to be reversed. 

Halide impurities are probably the most studied of the four general categories of impurities common to ionic liquids and, besides electrochemical analysis, two methods are currently being used to determine the level of residual halide impurities in ionic liquids [12]. The titration of the ionic hquid vnth AgN O3 is still widely used but suffers from a certain solubility of AgQ in the ionic hquid under investigation. This method can be enhanced by the Vollhard method for chlorine determination where the chloride is first precipitated with excess AgNO3 followed by back-titration of the mother liquor with aqueous potassium thiocyanate [13]. This method uses a visual endpoint through the formation of a complex between thiocyanate and an iron (III) nitrate indicator. 

Coupling of Several Pairs of Quantities Predicting behavior becomes more difficult when dealing with a coupling not just between two pairs of quantities, but maybe between three such pairs. When a dissolved substance is added, the solvent is also added at the same time so that the potentials of all other dissolved substances change. We saw such behavior in Sect. 6.6 with the formation of the red iron thiocyanate complex (Experiment 6.1) ... 

Iron Complex formation with thiocyanate 1.0-30 lron(ll III) can be determined using SF... 

Iron salts yield a red-violet color with thioglycolic add and ammonia probably because of the formation of inner complex anions containing iron. This color reaction is more sensitive than the thiocyanate reaction with iron it is not impaired by materials which form complexes with iron If thioglycolic add containing a,a -dipyridyl or a,a -phenanthroline is used, the sensitivity is still better. The reaction described on page 263 occurs. 

The thermal decomposition of N,N-diethyl-N -benzoylthiourea complexes with nickel, cobalt, and iron (II) rmder dynamic nitrogen atmosphere induces the formation of NiS, CoS, and FeS [128], Metal thiocyanate is formed in one, two, and three steps as decomposition intermediate, due to the evolving of diethylbenzamide. 

An interesting set of reactions involves the complex ions [Fe(CN)6] and [Fe(CN)6] . These ions are commonly called ferrocyanide and ferricyanide, respectively. Fe (aq) yields a dark blue precipitate called Prussian blue when treated with potassium ferrocyanide, K4[Fe(CN)6](aq), whereas Fe (aq) yields a similar blue precipitate, called Turnbull s blue, when treated with potassium ferricyanide, K3[Fe(CN)6](aq). Together, these two and similar related precipitates are known commercially as iron blue. Iron blue is used as a pigment for paints, printing inks, laundry bluing, art colors, cosmetics (eye shadow), and blueprinting. An additional sensitive test for Fe " (aq) is the formation of a blood-red complex ion with thiocyanate ion, SCN (aq). 

Discussion. Potassium may be precipitated with excess of sodium tetraphenyl-borate solution as potassium tetraphenylborate. The excess of reagent is determined by titration with mercury(II) nitrate solution. The indicator consists of a mixture of iron(III) nitrate and dilute sodium thiocyanate solution. The end-point is revealed by the decolorisation of the iron(III)-thiocyanate complex due to the formation of the colourless mercury(II) thiocyanate. The reaction between mercury( II) nitrate and sodium tetraphenylborate under the experimental conditions used is not quite stoichiometric hence it is necessary to determine the volume in mL of Hg(N03)2 solution equivalent to 1 mL of a NaB(C6H5)4 solution. Halides must be absent. 

The frequent occurrence of thiocyanate in spin cross-over complexes is documented, in Section 5.4.1.4 above and in connection with diimine and with triazole ligands in Sections 5.4.3.5.9 and 5.4.3.6.3 below, while complex formation with iron(III) appears in Section 5.4.5.1.4. 

The content of acid-soluble iron in paper is determined by TAPPI standard T434 (iron combined in clay and other complex compounds is presumed to be nonreactive). The presence of iron can be shown by the color produced upon wetting the paper briefly with warm 6N hydrochloric acid and then adding a solution of potassium ferrocyanide or thiocyanate localized specks of iron or rust are indicated by the more intense color formation. Complete analysis of paper for metallic elements has been accomplished by chemical procedures, emission spec-trography, scanning electron microscopy/x-ray, and neutron activation. 

Thiocyanate is used in some cases for the detection of ions. Its reaction with iron(III) ion is characteristic and can be used for detecting both ions. The deep-red colour observed is due to the formation of a number of thiocyanato-ferrate(III) ions and also of the chargeless molecule [Fe(SCN)3], The blue tetrathiocyanatocobaltate(II) [Co(SCN)4]2 complex is sometimes used for the detection of cobalt. 

Thiocyanate ions react with Fe(III) in a moderately acidic medium to yield a red colour which, for a long time, has been the basis for the determination of iron(III), or total Fe after oxidation of Fe(II) to Fe(IIl) [21-23]. Owing to stepwise complex formation in solution, FeCSCN) ", Fe(SCN)2, and further complexes can be formed. The concentrations of the reagents and the pFl of the medium determine which complexes are prevalent. In general, only FelSCN) " is formed at microgram concentrations of Fe(III). The higher complexes are more intensely coloured. 

The stopped-flow technique was introduced by Chance in 1940. Earlier applications to kinetic analysis were concerned with studies on kinetics and reaction mechanisms (e.g., the formation of the iron(ni)-thiocyanate complex, that of 12-molybdo-phosphoric acid, the redox reaction between 2,6-di-chlorophenolindophenol and ascorbic acid, etc.) as well as the resolution of mixtures of metal ions using substitution reactions. On the other hand, the inception of commercially available stopped-flow instruments and inexpensive modular mixing systems for adaptation to existing detectors have led to a broad use of this technique in routine kinetic determination of individual species and mixtures in a variety of samples of clinical, pharmaceutical, nutritional, and environmental interest. The analytical features of the methods developed for this purpose usually surpass those of the equilibrium counterparts, as shown by the selected examples given in Table 2. In addition, stopped-flow systems accelerate some slow reactions relative to the conventional kinetic technique as a... 

Very similar expressions are obtained for the formation and dissociation of metal complexes or complex ions formed by the reaction of a metal ion in solution with a complexing agent or ligand, both of which are capable of independent existence in solution (see Section 19.5.6). This can be shown by the reaction of iron(IIl) ion and thiocyanate ligand... 

Rate studies on the formation of the iron(m) complex in the presence of nitrite ion confirm that the iron(n)-thiocyanate complex is a precursor to the oxidized species. In the corresponding reaction with thiourea in basic solution the order with respect to the thiol is complex, and plots of log Arobs (observed rate constant) against pH pass through a maximum at pH ll.S. The violet-coloured adduct formed in the reaction is considered to incorporate two pentacyanoferrate(n) groups and one mole of thiourea, but on addition of acid the reversible formation of a blue colour is observed. 

Many investigators have studied substitution at iron(II)-diimine complexes in binary aqueous mixed solvents and other investigators in aqueous salt solutions. Some years ago the results of addition of salts and a cosolvent were assessed, for [Fe(5N02phen)3] in water, t-butyl alcohol, acetone, dimethyl sulfoxide, and acetonitrile mixtures containing added potassium bromide or tetra-n-butylammonium bromide. " Now the effects of added chloride, thiocyanate, and perchlorate on dissociation and racemization rates of [Fe(phen)3] in water-methanol mixtures have been established. The main explanation is in terms of increasing formation of ion pairs as the methanol content of the medium increases, but it is somewhat spoiled by the (unnecessary) assumption of a mechanism involving interchange within the ion pairs. Kip values (molar scale) of 11,18, and 25 were estimated for perchlorate, chloride, and thiocyanate in 80% (volume) methanol at 298.2 K. These values may be compared with values of 20, 7, and 4 for association between [Fe(phen)3] and iodide, " [Fe(bipy)3] and iodide, " and [Fe(phen)3] and cyanide " " in aqueous solution (at 298.2,... 

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