Complex ions colour

The d orbital splitting depends on the oxidation state of a given ion hence twb complex ions with the same shape, ligands and coordination number can differ in colour, for example... 

In the pH range 7-11, in which the dye itself exhibits a blue colour, many metal ions form red complexes these colours are extremely sensitive, as is shown, for example, by the fact that 10 6 — 10 7 molar solutions of magnesium ion give a distinct red colour with the indicator. From the practical viewpoint, it is more convenient to define the apparent indicator constant K ln, which varies with pH, as ... 

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 ... 

If you look at the example of visible spectra on the opposite page, you will see that both the complex ions contain Co +, but the peaks of maximum absorbance are at different wavelengths. From the spectra suggest (1) the colours of these two complex ions and (11) why the two ions have different peaks of maximum absorbance and different colours. (Hint explain in terms of splitting of d orbitals.)... 

Also two salts are known of molecular formula Co(NH3)5Br(S04) one is reddish violet in colour, and a freshly prepared aqueous solution contains sulphate ions the other is red in colour, and a freshly prepared aqueous solution contains bromine ions but no sulphate ions. The former substance is bromo-pentammino-cobaltie sulphate, [Co(NH3)5Br]S04 the latter is sulphato-pentammino-cobaltie bromide, [Co(NH3)5S04]Br.2 It is interesting to note that in the second compound the sulphate radicle occupies one co-ordinate position, but it also requires two principal valencies, and thus the complex ion is monovalent. 

The chemistry of iron in aqueous solution is dominated by the 4 2 and 4 3 states, which are well characterized. The 4 3 state in acid solution is a good oxidizing agent the 4 2 state is the most stable. The [Fe(H20)6]3 + complex ion is a violet colour in the solid chlorate(VII) salt, but in solution it undergoes hydrolysis to give the familiar orange-red colour. The first stage of the hydrolysis may be written as ... 

The discharging of the colour by oxalates, tartrates, etc., appears to be caused by the formation of complex ions with the ferric ions of the ionised ferric thiocyanate, which causes further dissociation of the red non-ionised salt and consequent loss of colour. 

Potassium Vanadicyanide, K3[V(CN)6], is prepared by the addition of excess of concentrated potassium cyanide solution to a concentrated solution of vanadous chloride, VC18 precipitation in the cold with alcohol gives rise to small rhombohedral plates. The solution is not very stable and rapidly becomes turbid, while addition of an acid produces the green colour which is characteristic of the V ion. The complex ion [V(CN)8]" appears, therefore, to be unstable, unlike the corresponding [Fe(CN)e] ", [Cr(CN)J ", and [Co(CN)e] " complex ions. The solution reacts with salts of heavy metals to yield variously coloured precipitates of double cyanides.7... 

The work of Yaillant3 and Lewis 4 has shown that the colour changes cannot be quantitatively interpreted without considering that water plays a definite r61e in the reactions. It follows that if Donnan and Bassett s views on complex ion formation be correct, water is either produced or used up when cobalt chloride and chloride ion interact thus, for example, where the ion CoCl3 is assumed for simplicity ... 

Cobalt chloride dissolves in alcohol to a blue solution, which becomes violet and then red on the addition of water. The alcoholic solution becomes red when very diluted or when cooled much below 0° C. The blue colour in these solutions has been attributed to the formation of double compound by Engel, to complex ion formation by Donnan and Bassett, ana to both these causes by Kotschubei. 

The formation of complexes in qualitative inorganic analysis is often observed and is used for separation or identification. One of the most common phenomena occurring when complex ions are formed is a change of colour in the solution. Some examples are ... 

In the presence of a solution of tartaric acid or of citric acid, copper(II) hydroxide is not precipitated by solutions of caustic alkalis, but the solution is coloured an intense blue. If the alkaline solution is treated with certain reducing agents, such as hydroxylamine, hydrazine, glucose, and acetaldehyde, yellow copper(I) hydroxide is precipitated from the warm solution, which is converted into red copper(I) oxide Cu20 on boiling. The alkaline solution of copper(II) salt containing tartaric acid is usually known as Fehling s solution it contains the complex ion [Cu(COO.CHO)]2. ... 

Sodium acetate solution a reddish-brown colouration is obtained, attributed to the formation of a complex ion with the composition [Fe3(OH)2(CH3COO)6] +. The reaction... 

Molybdenum gives a similar reaction. If, however, a thiocyanate is added, the red complex ion [Mo(SCN)6]3 is formed, and upon the addition of concentrated hydrochloric acid the red colour disappears and the blue colour due to tungsten remains. 

The total concentration of the metal, in the solvated cations and the complex, [M], and the total concentration of the ligand, free and in the complex, [L], can be found by analysis. The method of determining the concentration of the complex, [ML ], depends upon the system. When either the free ligand or the complex is coloured, or has a convenient absorption elsewhere in the spectrum, optical densities (log intensity of transmitted light/intensity of incident light) at a specific wave length are measured. Sometimes the concentrations of the uncomplexed metal ions are obtained potentiometrically with a suitable electrode. Polarography and extraction... 

In the reaction of mercury(II) ions with thio-Michler s ketone (TMK, formula 46.2) the ions are reduced to Hg(I) and then complexed. This colour reaction, in various forms, has been recommended for selective and sensitive determination of Hg [40-42]. 

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