Colour bleaching

Halogens can colour / bleach vegetable dyes and kill bacteria. 

Tussur and other wild silks require special treatment. Tussur silk, in particular, is so difficult to bleach that it is commonly used in its natural fawn colour. Bleaching in a 0-6 per cent w/w solution at 70°, C (158°F) for 6 hours will give a fairly good white without excessive loss of tensile strength. The colour is improved by after-treatment in a liquor containing 2 g per litre of sodium hydrosulphite at 100°C. Silk can be bleached by stoving with sulphur dioxide, but the method has now virtually fallen into disuse. Treatment with sulphur dioxide after a peroxide bleach is sometimes used for exceptionally good whites. 

No colour reaction with bleaching powder cf. amines). 

The stabilised nitrate may then be bleached with sodium hypochlorite, centrifuged to remove much of the water in which the polymer has been slurried and dehydrated by displacement with alcohol while under pressure in a press. It is interesting to note that in these processes approximately 35 000 gallons (160000 litres) of water are used for every ton of cellulose nitrate produced. Control of purity of the water is important in particular the iron content should be as low as 0.03 parts per million since iron can adversely affect both the colour and heat stability of the polymer. 

Of these materials zein, the maize protein, has been used for plastics on a small scale. It can be cross-linked by formaldehyde but curing times are very long. Complicated bleaching processes have led to the production of almost colourless samples in the laboratory but the process cannot readily be extended to large-scale operation. The cured product has a greater water resistance than casein. Proteins from soya bean, castor bean and blood have also been converted into plastic masses but each have the attendant dark colour. 

With most woods, new surfaces first of all darken when exposed to light, particularly with teak and afrormosia, where any masking of portions of the surfaces of furniture can give blemishes. After full colour development, strong sunshine bleaches most woods to a common brown colour. 

Colour change Caused by exposure to light, the effect is greatest with hardwoods and is important in furniture, panelling and joinery any masked areas become blemishes. Strong sunshine bleaches most woods to a common brown colour. 

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Fig. 28. Valence band spectra (UPS) of an AIROF electrode in the coloured (1.25 V) and in the bleached (0.0 V) state. The electrode was rinsed after emersion with ultra pure H20. After [67],...

Dyeing cotton yarn with selected direct dyes and simultaneous bleaching with peroxide. It is claimed that the peroxide also increases the colour yield [324]... 

Coagulation of natural rubber latex by stages with the object of removing the yellow colouring matter in the first fraction. The latex in the second fraction produces a white crepe rubber. See Bleaching Agent. 

Anthocyanins usually give a purple red colour. Anthocyanins are water soluble and amphoteric. There are four major pH dependent forms, the most important being the red flavylium cation and the blue quinodial base. At pHs up to 3.8 commercial anthocyanin colours are ruby red as the pH becomes less acid the colour shifts to blue. The colour also becomes less intense and the anthocyanin becomes less stable. The usual recommendation is that anthocyanins should only be used where the pH of the product is below 4.2. As these colours would be considered for use in fruit flavoured confectionery this is not too much of a problem. Anthocyanins are sufficiently heat resistant that they do not have a problem in confectionery. Colour loss and browning would only be a problem if the product was held at elevated temperatures for a long while. Sulfur dioxide can bleach anthocyanins - the monomeric anthocyanins the most susceptible. Anthocyanins that are polymeric or condensed with other flavonoids are more resistant. The reaction with sulfur dioxide is reversible. 

The potentiometric method also surpasses the others for speed, simplicity, precision and accuracy as indicated in Table 12.1. Furthermore, it is particularly suited to the continuous monitoring of fluoride levels in drinking water. The spectrophotometric methods are lengthy because of the time required to develop a stable colour (up to 1 hour), the alizarin red-S complex being especially poor in this respect. It was noted, however, that for the three bleaching methods (1-3) the rate of change of absorbance by the blank closely followed that of solutions containing fluoride, i.e. the difference between the blank and a sample absorbance is nearly constant. 

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