Self-colouring

Decorative self-coloured films can also be produced in sulphuric acid under conditions intermediate between normal and hard anodising. 

Particularly bright pinks require Lissamine Rhodamine G (C.I. acid red 50) or Lissamine Rhodamine B (C.I. acid red 52), and for extremely vivid yellows Lissamine Flavine FF (C.I. acid yellow 7) is used. Most of the dyes in this group can be used in combination with each other in shades where mixtures are necessary for matching. With the fast acid dyes it is not so easy to match shades with mixtures. Their tendency is towards uneven adsorption, and if the distribution of two or more components is not uniform, the differences in tone of the colour are much more apparent than equivalent differences in depth. The best results are, therefore, obtained by selecting a self-colour which matches the pattern or, if this is impossible, one which is so close that only a very small proportion of another dye is necessary. 

The development of this type of chemistry built into polyesters was part of an extensive effort undertaken by Eastman over a number of years to develop various grades of self-coloured, UV-brightened and UV-stabilised polyester fibres. Initial polyesters utilised reactive versions of the simplest cinnamate chemistry, such as ... 

A more promising new additive type is based on 4-hydroxyquinoline-3-carboxylic acid derivatives [138], which feature a very high extinction coefficient in the UV, but little or no self-colour. These are claimed to be highly efficient UV absorbers and thus particularly suitable for fibres and films. Similar additives may have been commercialised for use as UV barriers to protect the contents of plastics packaging. 

Self-coloured materials such as plastics or anodized aluminium, and adhesive plastics film also had their limitations. There is therefore every reason to assume that oil-based paints will be here in the year 2000. Since the general natures of the mechanisms of autoxidation and polymerization were realized about 30 years ago there have been few significant scientific papers and these subjects are not current research items. Sufficient is now known to ensure the most effective use of the fatty oils in commercial paint products. 

It proves advantageous that the reagent has a high molar coefficient of absorption at 600 nm. Self-colouring in natural waters does not interfere in this spectral range. A further favourable effect is that the oxidation products formed show only a very slow further reaction with excess ozone. For this reason, even local excess does not lead to uncontrolled consumption of oxidation agents. The stock solution is stored in a brown bottle and keeps for approximately 4 months. 

If the water displays any self-colour or turbidity, measurements should be made against the coloured or filtered water sample. This method of determination is easily reproducible, and, with appropriate variations, allows determination of sulphate in concentrations ranging from 1 mg/1 when using a 1 cm cuvette, and from 0.2 mg/1 when using a cm cuvette, up to 50 mg sulphate/1. The process has proved in practice to be a good, reliable and fast method of measuring sulphate. 

It has long been predicted that self coloured plastics, achieved by pigment addition during manufacture, would eliminate the need for plastic coating. Whilst this is true, new areas have developed, thus creating an overall increase in the volume of plastics being coated. 

B. Self-indicating reagents. This is well illustrated by potassium permanganate, one drop of which will impart a visible pink coloration to several hundred millilitres of solution, even in the presence of slightly coloured ions, such as iron(III). The colours of cerium(IV) sulphate and of iodine solutions have also been employed in the detection of end points, but the colour change is not so marked as for potassium permanganate here, however, sensitive internal... 

Potassium permanganate and iodine, which are important redox reagents, are both self-indicating, i.e. the colour of the reagent in each case is intense and will impart a perceptible colour to a solution when present in very small excess. One drop of a solution of potassium permanganate (0.02 mol dm 3) can be detected in a titrand solution of 100 cm3, and a similar amount of iodine by shaking the titrand with 5 cm3 of chloroform or carbon tetrachloride to produce an intense purple colour. Specific indicators react in a specific manner with one participant in the reaction. The best examples are starch, which produces an intense blue colour with iodine and potassium thiocyanate, which forms an intense red compound with iron(III). 

The molecular orbital (MO) approach developed more slowly but was ultimately much more successful than the other approaches. Dewar (1950) was able to predict absorption maxima for various types of cyanine dyes in excellent agreement with experiment. A major advance was made in 1953 when a self-consistent molecular orbital method specifically taking into account antisymmetrisation and electron repulsion effects was developed by Pariser, Parr and Pople. The PPP-MO method established itself over the next two decades as the most useful and versatile technique for colour prediction, especially when the microelectronics revolution provided facilities for overcoming the complexity of the necessary calculations. 

Ammonium ceric sulphate serves as a powerful oxidizing agent in an acidic medium. The salt has a bright yellow colour and so its solution. On reduction, the resulting cerous salt obtained is colourless in appearance and, therefore, strong solutions may be considered as self-indicating. In general practice, 0.05 N solutions are employed invariably for estimations. As this concentration is very dilute for observation of the respective end-point, hence the inclusion of an appropriate indicator becomes necessary. The oxidation reaction involved may be expressed as follows ... 

The various chemical processes influencing the colour formation in tea have been vigorously investigated. The self-association of black tea polyphenol theaflavin and its com-plexation with caffeine [175], and the role of epicatechin quinone in the synthesis and degradation of theaflavin [176] have been studied in detail. 

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