Colour origins

Charge transfer also occurs between ions in solution. The classical test for the Fe3+ ion in solution is to mix solutions of Fe3+ and thiocyanate ion SCN-, to form the [FeSCN]2"1" complex, and a deep blood-red colour forms. The colour originates from a charge-transfer transition between Fe3+ and SCN-. There was no red colour before mixing, confirming that the optical transition responsible for the colour did not originate from either constituent but from the new compound formed. 

Early versions of slow burning, sulfurless powders were brown in colour and known as Cocoa powder , the colour originating from an incompletely carbonised charcoal with a carbon level of about 50%. 

Notes. (1) Remove about 0.25 ml of the hydrolysate, cool it and basify it with 5 m sodium hydroxide solution. To a portion add a few drops of very dilute copper(u) sulphate solution, and note the absence of any colour change. As a control, prepare a specimen of biuret (NH2-CO-NH-CO-NH2) by heating about lOmg of urea just above its melting point for about 2 minutes. Add a little basified hydrolysate warm to dissolve, cool and add a trace of copper(u) sulphate. A deep pink colour superimposed upon the pale brown colour of the hydrolysate should be observed. If the hydrolysate gives a similar colour originally, it contains peptide material and hydrolysis should be continued until the biuret test is negative. 

The oxidation number of Cr changes from +6 to +3. The solution becomes light green, the colour originating from Cr3+ ions. Some important oxidations with dichromate are as follows ... 

Prussian blue is a mixed Fe(II)/Fe(III) complex polymeric species in which Fe(II) is octahedrally coordinated by C, and Fe(lll) is octahedrally coordinated by N, to give a structure containing Fe(II)-C-N-Fe(lll)-N-C-Fe(II)-linkages, in which the colour originates from electron transfer between the two metal oxidation states. It was discovered in 1704, and used in blueprints and also in the cyanotype photographic process developed by Herschel (see Chap. 11). The cyanotype process is made possible by the photochemical reduction of Fe(III) citrate (or oxalate) to Fe(II), which reacts with ferricyanide present in the coating formulation to give Prussian blue. A similar photoreduction of Fe(lII) oxalate to Fe(ll) is used in the ferrioxalate actinometer (see Chap. 14). 

The fact that many of the observed colours are of low intensity is consistent with the colour originating from electronic "d-d transitions. If we are dealing with an isolated gas-phase ion, such transitions are forbidden by the Laporte selection rule (eq. 19.4 where / is the orbital quantum number). The pale colours observed in complexes indicate that the probability of a transition occurring is low. Table 19.2 shows relationships between the wavelength of light absorbed and observed colours. 

Copper salt, (a) Add aqueous copper acetate solution to an aqueous solution of glycine. Note the formation of a blue colour which is considerably deeper than the colour of the original copper acetate solution. On heating the solution, blue needles of the copper salt usually separate. 

Reduction. Reduce 0 5 g. of the nitroaniline with HCl and tin, as described on p. 385. Note that after a few minutes the original yellow colour has entirely disappeared. Cool and add 20% aqueous NaOH solution a white precipitate is formed which redissolves to give... 

The enol form is thus temporarily removed from the solution and the ferric chloride colouration produced by the enol form consequently disappears and the solution becomes colourless. Some of the unchanged keto form of the ester then passes into the enol form in order to restore the original equilibrium and the ferric chloride colouration therefore reappears. 

Treat 0 1 g. of the quinone with dilute sodium hydroxide and zinc powder. Upon boding the mixture a red colour is produced this disappears when the solution is shaken owing to aerial oxidation to the original quinone. 

B. Mix 1 drop or several small crystals (ca. 0 05 g.) of the compound with 1 ml. of 0-5 V hydroxylamine hydrochloride in 95 per cent, ethanol and add 0-2 ml ot aqueous sodium hydroxide. Heat the mixture to boiling and, after the solution has cooled slightly, add 2 ml. of N hydrochloric acid. If the solution is cloudy, add 2 ml. of 95 per cent, ethyl alcohol. Observe the colour produced when I drop of 6 per cent, ferric chloride solution is added. If the resulting colour does not persist, continue to add the reagent dropwise until the observed colour pervades the entire solution. Usually only 1 drop of the ferric chloride solution is necessary. Compare the colour with that produced in test. 4. A positive test will be a distinct burgundy or magenta colour as compared with the yellow colour observed when the original compound is tested with ferric chloride solution in the presence of acid. 

Colour Index, and its Additions and Amendments, 3rd ed.. Society of Dyers and Colourists, London, and American Association of Textile Chemists and Colorists, Durham, N.C. It now consists of seven volumes. The Colourindex was originally written by and for the textile industry, but pigments are receiving increasing attention. 

By starch iodide paper (original colour white)... 

Today the principal outlets are knife handles, table-tennis balls and spectacle frames. The continued use in knife handles is due to the pleasant appearance and the ability of the material to after-shrink around the extension of the blade. Table-tennis balls continue to be made from celluloid since it has been difficult to match the bounce and handle of the celluloid ball, the type originally used, with balls fabricated from newer polymers. Even here celluloid is now meeting the challenge of synthetic polymers. Spectacle frames are still of interest because of the attractive colour. There are, however, restrictions to their use for this application in certain countries and cellulose acetate is often preferred. 

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