Dithionite, bleaching

For satisfactory whiteness on wool, it is essential for the fibre to be well scoured and bleached, either oxidatively with hydrogen peroxide or by reduction using stabilised sodium dithionite. Brightener is usually applied together with the dithionite bleach. To achieve the highest possible whiteness, the wool should first be scoured to remove natural waxes and other contaminants, then bleached with peroxide and finally treated with FBA during a second bleach with dithionite. 

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. 

Sodium dithionite Bleaching textiles, pulp and paper removing rust stains... 

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. 

Uses. The dominant use of sulfur dioxide is as a captive intermediate for production of sulfuric acid. There is also substantial captive production in the pulp and paper industry for sulfite pulping, and it is used as an intermediate for on-site production of bleaches, eg, chlorine dioxide or sodium hydrosulfite (see Bleaching agents). There is a substantial merchant market for sulfur dioxide in the paper and pulp industry. Sulfur dioxide is used for the production of chlorine dioxide at the paper (qv) mill site by reduction of sodium chlorate in sulfuric acid solution and also for production of sodium dithionite by the reaction of sodium borohydride with sulfur dioxide (315). This last appHcation was growing rapidly in North America as of the late 1990s. 

Dithionite is a stronger reducing agent than sulfite. Many metal ions, eg, Cu", Ag", Pb ", Sb ", and Bi ", are reduced to the metal, whereas TiO " is reduced to (346). Dithionite readily reduces iodine, peroxides, ferric salts, and oxygen. Some of the decolorizing appHcations of dithionite, eg, in clay bleaching, are based on the reduction of ferric iron. 

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. 

Zinc hydrosulfite (zinc dithionite) is a powerhil reducing agent used in bleaching paper and textiles it is prepared from zinc dust and sulfur dioxide ... 

The reducing agents generally used in bleaching include sulfur dioxide, sulfurous acid, bisulfites, sulfites, hydrosulfites (dithionites), sodium sulfoxylate formaldehyde, and sodium borohydride. These materials are used mainly in pulp and textile bleaching (see Sulfur compounds Boron compounds). 

Wool with dark pigmented fibers is treated with ferrous sulfate, sodium dithionite, and formaldehyde before it is bleached with hydrogen peroxide. The ferrous ions are absorbed by the dark pigments where they increase the bleaching done by the peroxide. 

Wool may also be bleached with reducing agents, usually after bleaching with hydrogen peroxide. This is the normal practice with wool blends. In the reducing step, 0.2—0.5% sodium dithionite solutions are often used at pH 5.5—7 for 1—2 h at 45—65°C. Faster bleaching is obtained with 2inc hydroxymethane-sulfinate [24887-06-7] below pH 3 and above 80°C. 

Production of Sodium Borohydride. In the pulp and paper industry, sodium borohydride is used to generate sodium hydrosulfite (sodium dithionite), a bleaching agent, from sodium bisulfite. Methyl borate is used as an intermediate in the production of sodium borohydride (33). 

Sodium Tetrahydroborate, Na[BH ]. This air-stable white powder, commonly referred to as sodium borohydride, is the most widely commercialized boron hydride material. It is used in a variety of industrial processes including bleaching of paper pulp and clays, preparation and purification of organic chemicals and pharmaceuticals, textile dye reduction, recovery of valuable metals, wastewater treatment, and production of dithionite compounds. Sodium borohydride is produced in the United States by Morton International, Inc., the Alfa Division of Johnson Matthey, Inc., and Covan Limited, with Morton International supplying about 75% of market. More than six million pounds of this material suppHed as powder, pellets, and aqueous solution, were produced in 1990. 

Sulfur dioxide is uniquely reduced to dithionite (a process useful in bleaching paper pulp, p. 720). CO2 gives the formate ... 

A particularly interesting reaction (and one of considerable commercial value in the BOROL process for the in situ bleaching of wood pulp) is the production of dithionite, S204 , from SO2 ... 

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