Reductive bleaching

Polcin and Rapson published a comprehensive series of papers describing diffuse reflectance studies of the chromophores in spruce and hemlock groundwood pulps [89-91], and their response to some common bleaching agents [92,93]. Based on either reflectance spectra or differences in remission function, they concluded that 

Sodium dithionite, Na2S204, decreased the remission function at all wavelengths greater than 300 nm, but the largest decreases occurred in the visible region. Polcin and Rapson [93] attributed this to the reduction of simple quinones and coniferaldehyde. Kuys and Abbot [99] made similar observations in a spectroscopic study of dithionite bleaching of radiata pine TMR However, they suggested that coniferaldehyde was reduced by sodium bisulfite formed as a dithionite oxidation product, not by dithionite itself. 

Svensson et al. [100] compared dithionite bleaching of spruce TMP at pH 10 and pH 5, using principal components analysis to reduce reflectance spectra to contributing subspectra. The authors concluded that at pH 5 (the common pH for commercial dithionite bleaching) quinones were the major chromophores bleached. At pH 10, quinones were formed due to alkali darkening. In spite of the higher redox potential of dithionite at pH 10, bleaching was less efficient than at pH 5, and there was no evidence that any chromophores other than quinones were removed. 

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Reductions ofmtriles Reductive bleaching Reducymol Red wine Reed reaction... 

The most common chemical bleaching procedures are hypochlorite bleach for cotton hydrogen peroxide bleach for wool and cotton sodium chlorite bleach for cotton, polyamide, polyester, and polyacrylonitrile and reductive bleaching with dithionite for wool and polyamide. 

There are two types of bleachiag oxidative and reductive. Oxidative bleaching cleaves carbon—carbon double bonds thus destroyiag chromophores. Reductive bleaching reduces carbon—carbon double bonds to single bonds. The loss of extended conjugation ia the chromophore leads to loss of color. 

Ana.lytica.1 Methods. Various analytical methods involve titration with oxidants, eg, hexacyanoferrate (ferricyanide), which oxidize dithionites to sulfite. lodimetric titration to sulfate in the presence of formaldehyde enables dithionite to be distinguished from sulfite because aldehyde adducts of sulfite are not oxidized by iodine. Reductive bleaching of dyes can be used to determine dithionite, the extent of reduction being deterrnined photometrically. Methods for determining mixtures of dithionite, sulfite, and thiosulfates have been reviewed (365). Analysis of dithionite particularly for thiosulfate, a frequent and undesirable impurity, can be done easily by Hquid chromatography (366). 

For reductive bleaching of wool the two most popular chemicals are stabilized sodium dithionite (sodium hydrosulfite. Cl Reducing Agent 1) and thiourea dioxide (Cl Reducing agent 11). Most reductive bleaching of wool is carried out using stabilized dithionite (2—5 g/L) at pH 5.5—6 and 45—65°C for 1 h. Thiourea dioxide is more expensive than sodium dithionite, but is an effective bleach when appHed at the rate of 1—3 g/L at 80°C at pH 7 for an hour. 

Whiter fabrics are produced if an oxidative bleaching is followed by a reductive one this is often referred to as full bleaching. One such process involves a hydrogen peroxide oxidative bleaching, followed by addition of thiourea to the Hquor to generate thiourea dioxide for a reductive bleach in situ (115). 

When bleaching is requited, a reductive bleach with sodium hydrosulfite and sodium metabisulfite is used. Cotton blends may requite a hydrogen peroxide bleach at pH 9.0—9.5 prior to or iastead of the normal reductive bleach. Chlorine-type bleaches which damage elastomeric fibers are avoided. 

Table 10.30 Recommended conditions for the reductive bleaching of wool [11,259]...

Reductive bleaches are generally less costly than oxidative bleaches but tend to give a greenish white compared with the reddish white tones from peroxide. Combining the two in an oxidation/reduction sequence gives a more neutral white, this being known as a full... 

As an alternative to oxidative bleaching with sodium chlorite, acrylic fibres may be given a reductive bleach using sodium bisulphite in the presence of oxalic acid. This method is... 

Reductive amination, of carbonyl compounds, 16 573 Reductive bleaching, 21 440 Reductive dechlorination defined, 3 757t Reductive dehalogenation defined, 3 757t... 

There are two approaches to the bleaching of mechanical pulps. They may be either reductive or oxidative in nature. The reductive bleaching agents are usually bisulfite, dithionite or borohydr.de, and the oxidising agents are normally peroxide, hypochlorite, peracetic... 

Reductive bleaching -m paper recycling [RECYCLING - PAPER] (Vol 21)... 

Naturally white, but slightly yellowed fur skins from sheep and lambs are reduction bleached, most commonly with dithionite or/and in combination with hydrogen peroxide in a weakly alkaline medium. A true bleach in the sense of destroying the natural pigments in the hair can only be achieved by means of oxidation with, e g., hydrogen peroxide, catalyzed by iron(n) salts. This exothermic process is very difficult to control and must be monitored carefully. In addition, the skin must not be chrome tanned, as this could cause serious damage. 

Primarily azo disperse dyes are used to obtain the so-called snow-top effects. After dyeing at a pH of approximately 3.5 and a temperature of 50-60 °C for 1-2 h, the dyed hair top is given a reduction bleach with hydrosulfite. The bleaching solution is applied by brush or spray. 

Polyamides. Batch brightening usually takes place in combination with reductive bleaching (dithionite). Continuous processes (padding) are followed by steam, heat, or acid treatment (acid shock) [129], Anionic FWAs are absorbed similarly to acid dyes, or insoluble dispersion brighteners are suitable. Examples of products listed in order of increasing fastness are diaminostilbenes (14 and 19) pyra-zolines (50 and 54) distyrylbiphenyls (10) and bis(triazolyl)stilbenes (32). 

This dilution is for correcting slight overexposure or overdevelopment of fine-grain negatives and overall print reduction (bleaching). For print reduction follow the instructions given under Print Reducers, Farmer s Reducer. 

When, under identical conditions, ascorbic acid was used instead of mercaptoethanol, the reaction gave products with 3°/2° carbon reactivity of 0.28-0.42, suggestive of an autoxidation process (12). Furthermore, the kinetics of the reaction are biphasic for 2-mercaptoethanol and monophasic for ascorbic acid. These kinetics are consistent with the generation of a new catalytic system by the coordination of the thiol to the ferric center(s). For either reductant, bleaching of the complex was observed within minutes in the absence of substrate. 

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