Cobalt complexes colour

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. 

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. 

The formazans, which were first discovered in 1892, are derivatives of the hypothetical parent compound (163). Various methods are available for their preparation and a wide range of l,3,S-sub tituted formazans (164) has been synthesized. These are usually brightly and intensely coloured and, following the observation that 1,3,5-triphenylformazan readily formed copper, nickel and cobalt complexes, considerable effort was devoted to developing metal complex formazans as dyestuffs. Very few such compounds have found commercial application, largely because of the ease with which formazans are oxidized to the corresponding tetrazolium salts (165). This property has also led to some confusion regarding the nature of the metal complexes formed by variously-substituted formazans, which is not yet fully clarified. 

The red colour of the complex in which ammonia replaces chloride, and the yellow colour in which NO2 replaces chloride appear very quickly, indicating the original complex is labile in these reactions. For other reactions of the cobalt complex see Sec. 14.8.2. 

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. 

These are of two general kinds octahedral, pink complexes and tetrahedral, blue complexes. If cobalt(II) chloride is dissolved in aqueous solution, the predominant species is the hexaaquo-ion [ColHjO) ] (pink). If this solution is heated, it becomes blue, and the same effect is observed if chloride ion is added in excess. This colour change is associated with the change... 

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. 

Murexide forms complexes with many metal ions only those with Cu, Ni, Co, Ca and the lanthanides are sufficiently stable to find application in analysis. Their colours in alkaline solution are orange (copper), yellow (nickel and cobalt), and red (calcium) the colours vary somewhat with the pH of the solution. 

In the back-titration small amounts of copper and zinc and trace amounts of manganese are quantitatively displaced from the EDTA and are complexed by the triethanolamine small quantities of cobalt are converted into a triethanolamine complex during the titration. Relatively high concentrations of copper can be masked in the alkaline medium by the addition of thioglycollic acid until colourless. Manganese, if present in quantities of more than 1 mg, may be oxidised by air and forms a manganese(III)-triethanolamine complex, which is intensely green in colour this does not occur if a little hydroxylammonium chloride solution is added. 

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. 

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. 

Hydrate Isomerism.—As its name implies, this depends on the position of water in the molecule, just as in the case of the acido compounds. If two or more molecules of water are present in a molecule of ammine, the water may be present within the co-ordination complex or outside of it. For instance, the compound Cr en2.(H20)2.Br3 exists in isomeric forms. It may have all the water within the complex, in which case the formula will be [Cr en2(H20)2]Br3. In solution the whole of the bromine is precipitated by silver nitrate. On the other hand, the compound may have one molecule of water in the complex and the other outside, in which case the formula is [Cr en2(IT20)Br]Br2.H20, and only two-thirds of the bromine are precipitated by silver nitrate. Another example of this kind occurs in the cobalt series chloro-aquo-tetrammino-cobaltic chloride, [Co(NTI3)4Cl.H20]Cl2, is violet in colour, and is isomeric with dichloro-tetrammino-cobaltie chloride monohydrate, [Co(N1I3)4CI2]C1.H20, which is green. 

Such a configuration should on replacement of one ammonia molecule by acidic or other monovalent radicle yield only one compound, and this is proved to be the case. On the other hand, if two acidic, or other groups replace two ammonia groups in the complex, isomerism should be possible, yielding two isomers of the formula [M(NII3)4R2 R. In the case of dinitro-tctrammino-cobaltic nitrate, [Co(NIi2)4(N02)2 N03, two isomeric forms are known to exist, one brown in colour, the other yellow. The two substances may be represented by the following formula —... 

The chromi-ammines are produced by the action of ammonia and ammonium salts on chromic salts, or by the action of ammonia in presence of ammonium salts on chromous salts and subsequent oxidation. The second method is analogous to that for the formation of cobalt-ammines. The chromi-ammines form a group of coloured substances, and comprise unstable and stable derivatives, some of which are very complex. 

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