Iron complexes colour

Discussion. Salicylic acid and iron(III) ions form a deep-coloured complex with a maximum absorption at about 525 nm this complex is used as the basis for the photometric titration of iron(III) ion with standard EDTA solution. At a pH of ca 2.4 the EDTA-iron complex is much more stable (higher stability constant) than the iron-salicylic acid complex. In the titration of an iron-salicylic acid solution with EDTA the iron-salicylic acid colour will therefore gradually disappear as the end point is approached. The spectrophotometric end point at 525 nm is very sharp. 

Symmetrical premetallised 1 2 metal-dye complexes of unsulphonated monoazo structures with aluminium (5.57) or trivalent iron (5.58) have been patented recently for use as solvent dyes [36]. They contain a polar methoxypropylaminosulphone grouping in each diazo component and are marketed as alkylamine salts. It remains to be seen, however, whether a full colour gamut of bright aluminium and iron complex dyes can be discovered with light fastness performance equivalent to that of currently available chromium and cobalt complex dyes. 

When injected, it forms a stable water-soluble iron complex (ferrioxamine) that prevents the iron from entering into further chemical reactions and is readily excreted in the urine giving the urine a characteristic reddish colour. Some of it is also excreted in the faeces via the bile. It can also chelate aluminium and thus is useful in aluminium overload. It is primarily a chelator used in acute iron poisoning and chronic iron overload as in thalassemia patients needing multiple transfusions. 

Fig. 13.1 The general structure of an octahedral complex (a) and the structure of a red-coloured iron complex (b).

A new approach for the generation of NHCs has been reported using a cyclopentadi-ene(arene)iron complex (17).22 The method converts imidazolium to the free carbene in the presence of oxygen (peroxyradical anion is the base deprotonating the imidazolium salt). A colour change and precipitation of the oxidized iron complex are evidence for the reaction outcome. 

In order to prevent the reduction between iron(II) and formaldoxime occurring, another iron complexing agent (potassium cyanide) was used in the presence of a reductant (ascorbic acid) that reduces iron(III) to iron(II). Aluminium, titanium, uranium, molybdenum and chromium also form light-coloured complexes that normally do not interfere in the determination of manganese in water or plant material by this method. If the aluminium or titanium concentrations are higher than 40 ppm an additional masking flow of tartrate is recommended [31]. 

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. 

This reaction of the two octahedral complexes occurs without any change in the coordination spheres, or ligand sets, of either metal. However, if you inspect the two metal centres, you will note that the iron complex (the reductant) is oxidized from Fe(II) to Fe(III) and at the same time the iridium complex (the oxidant) is reduced from Ir(IV) to Ir(III) - an electron has been transferred from one metal to the other. This reaction can be conveniently followed, since the colour of each species changes as it is converted from one oxidation state to another. 

The thiocyanate ion SCN forms an intensely red-coloured complex (most simply represented as [Fe(SCN)(H20)5] ) which is a test for iron(III). However, unlike cobalt(III), iron(lll) does not form stable hexammines in aqueous solution, although salts containing the ion [FefNHj) ] can be obtained by dissolving anhydrous iron(III) salts in liquid ammonia. 

On sheet iron a groundcoat, including cobalt and nickel, is generally used, but for mass production (e.g. cookers) use of decarbonised steel and direct application of colours is more common. This involves a more complex steel pretreatment. 

Discussion. Molybdenum(VI) in acid solution when treated with tin(II) chloride [best in the presence of a little iron(II) ion] is converted largely into molybdenum(V) this forms a complex with thiocyanate ion, probably largely Mo(SCN)5, which is red in colour. The latter may be extracted with solvents possessing donor oxygen atoms (3-methylbutanol is preferred). The colour depends upon the acid concentration (optimum concentration 1M) and the concentration of the thiocyanate ion (1 per cent, but colour intensity is constant in the range 2-10 per cent) it is little influenced by excess of tin(II) chloride. The molybdenum complex has maximum absorption at 465 nm. 

The almost colourless leuco form of the base passes upon oxidation into the strongly coloured indamine. When titrating iron(III) at a pH of about 3 and the colourless hydrochloride of the leuco base is added, oxidation to the violet-blue indamine occurs with the formation of an equivalent amount of iron(II). At the end point of the EDTA titration, the small amount of iron(II) formed when the indicator was introduced is also transformed into the Fe(III)-EDTA complex FeY-, whereupon the blue indamine is reduced back to the leuco base. 

After dissolution of the alloy in a mixture of concentrated nitric and hydrochloric acids the iron is masked with triethanolamine in an alkaline medium, and the manganese titrated with standard EDTA solution using thymolphthalexone as indicator. The amount of iron(III) present must not exceed 25 mg per 100 mL of solution, otherwise the colour of the iron(III)-triethanolamine complex is so intense that the colour change of the indicator is obscured. Consequently, the procedure can only be used for samples of ferro-manganese containing more than about 40 per cent manganese. 

Nickel may be determined in the presence of a large excess of iron(III) in weakly acidic solution by adding EDTA and triethanolamine the intense brown precipitate dissolves upon the addition of aqueous sodium hydroxide to yield a colourless solution. The iron(III) is present as the triethanolamine complex and only the nickel is complexed by the EDTA. The excess of EDTA is back-titrated with standard calcium chloride solution in the presence of thymolphthalexone indicator. The colour change is from colourless or very pale blue to an intense blue. The nickel-EDTA complex has a faint blue colour the solution should contain less than 35 mg of nickel per 100 mL. 

One of the best oxidation-reduction indicators is the 1,10-phenanthroline-iron(II) complex. The base 1,10-phenanthroline combines readily in solution with iron(II) salts in the molecular ratio 3 base l iron(II) ion forming the intensely red l,10-phenanthroline-iron(II) complex ion with strong oxidising agents the iron(III) complex ion is formed, which has a pale blue colour. The colour change is a very striking one ... 

The standard redox potential is 1.14 volts the formal potential is 1.06 volts in 1M hydrochloric acid solution. The colour change, however, occurs at about 1.12 volts, because the colour of the reduced form (deep red) is so much more intense than that of the oxidised form (pale blue). The indicator is of great value in the titration of iron(II) salts and other substances with cerium(IV) sulphate solutions. It is prepared by dissolving 1,10-phenanthroline hydrate (relative molecular mass= 198.1) in the calculated quantity of 0.02M acid-free iron(II) sulphate, and is therefore l,10-phenanthroline-iron(II) complex sulphate (known as ferroin). One drop is usually sufficient in a titration this is equivalent to less than 0.01 mL of 0.05 M oxidising agent, and hence the indicator blank is negligible at this or higher concentrations. 

Discussion. Minute amounts of beryllium may be readily determined spectrophotometrically by reaction under alkaline conditions with 4-nitrobenzeneazo-orcinol. The reagent is yellow in a basic medium in the presence of beryllium the colour changes to reddish-brown. The zone of optimum alkalinity is rather critical and narrow buffering with boric acid increases the reproducibility. Aluminium, up to about 240 mg per 25 mL, has little influence provided an excess of 1 mole of sodium hydroxide is added for each mole of aluminium present. Other elements which might interfere are removed by preliminary treatment with sodium hydroxide solution, but the possible co-precipitation of beryllium must be considered. Zinc interferes very slightly but can be removed by precipitation as sulphide. Copper interferes seriously, even in such small amounts as are soluble in sodium hydroxide solution. The interference of small amounts of copper, nickel, iron and calcium can be prevented by complexing with EDTA and triethanolamine. 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

A. Thiocyanate method Discussion. Iron(III) reacts with thiocyanate to give a series of intensely red-coloured compounds, which remain in true solution iron(II) does not react. Depending upon the thiocyanate concentration, a series of complexes can be obtained these complexes are red and can be formulated... 

Sulphuric acid is not recommended, because sulphate ions have a certain tendency to form complexes with iron(III) ions. Silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury (>lgL-1), zinc, cadmium, and bismuth interfere. Mercury(I) and tin(II) salts, if present, should be converted into the mercury(II) and tin(IV) salts, otherwise the colour is destroyed. Phosphates, arsenates, fluorides, oxalates, and tartrates interfere, since they form fairly stable complexes with iron(III) ions the influence of phosphates and arsenates is reduced by the presence of a comparatively high concentration of acid. 

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