DiThionite, sodium

Fieser s solution An aqueous alkaline solution of sodium anthraquinone -sulphonale (silver salt) reduced with sodium dithionite, Na2S204, and used as a scrubbing solution for partially removing O2 from, e.g., N2. 

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

Another method employed is the treatment of aqueous solutions of aminophenols with activated carbon (81,82). During this procedure, sodium sulfite, sodium dithionite, or disodium ethylenediaminotetraacetate (82) is added to increase the quaUty and stabiUty of the products and to chelate heavy-metal ions that would catalyze oxidation. Addition of sodium dithionite, hydrazine (82), or sodium hydrosulfite (83) also is recommended during precipitation or crystallization of aminophenols. 

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. 

Physical Properties. Sodium dithionite (sodium hydrosulfite, sodium sulfoxylate), Na2S204, is a colorless soHd and is soluble in water to the extent of 22 g/100 g of water at 20°C. 

Chemical Properties. Anhydrous sodium dithionite is combustible and can decompose exothermically if subjected to moisture. Sulfur dioxide is given off violentiy if the dry salt is heated above 190°C. At room temperature, in the absence of oxygen, alkaline (pH 9—12) aqueous solutions of dithionite decompose slowly over a matter of days. Increased temperature dramatically increases the decomposition rate. A representation of the decomposition chemistry is as follows ... 

The decomposition of dithionite in aqueous solution is accelerated by thiosulfate, polysulfide, and acids. The addition of mineral acid to a dithionite solution produces first a red color which turns yellow on standing subsequentiy, sulfur precipitates and evolution of sulfur dioxide takes place (346). Sodium dithionite is stabilized by sodium polyphosphate, sodium carbonate, and sodium salts of organic acids (347). 

Sodium dithionite is most stable and effective as a reducing agent in alkaline solutions, although with excess strong alkaH the following reaction... 

Addition of sodium dithionite to formaldehyde yields the sodium salt of hydroxymethanesulfinic acid [79-25-4] H0CH2S02Na, which retains the useful reducing character of the sodium dithionite although somewhat attenuated in reactivity. The most important organic chemistry of sodium dithionite involves its use in reducing dyes, eg, anthraquinone vat dyes, sulfur dyes, and indigo, to their soluble leuco forms (see Dyes, anthraquinone). Dithionite can reduce various chromophores that are not reduced by sulfite. Dithionite can be used for the reduction of aldehydes and ketones to alcohols (348). Quantitative studies have been made of the reduction potential of dithionite as a function of pH and the concentration of other salts (349,350). 

Sodium dithionite solution can be produced on-site utilizing a mixed sodium borohydride—sodium hydroxide solution to reduce sodium bisulfite. This process has developed, in part, because of the availabiHty of low cost sulfur dioxide or bisulfite at some paper mills. Improved yields, above 90% dithionite based on borohydride, can be obtained by the use of a specific mixing sequence and an optimized pH profile (360,361). Electrochemical technology is also being offered for on-site production of sodium hydrosulfite solution (362). 

Economic Aspects. U.S. capacity for production of merchant sodium dithionite (soHds basis) was estimated at 93,000 metric tons in 1994. There are three North American producers of sodium dithionite. Hoechst Celanese is the largest producer (68,000 tons capacity) with two formate production locations and one zinc process location. Olin (25,000 t capacity) produces solution product only at two locations using both the amalgam and electrochemical processes. In 1994, Vulcan started a small solution plant in Wisconsin using the Olin electrochemical process. In addition, it is estimated that 13,000 t/yr is produced at U.S. pulp mills using the Borol process from sulfur dioxide and sodium borohydride. Growth is estimated at 2—3%/yr. The... 

Gra.des. There are three primary commercial sodium dithionite products 88 min wt % anhydrous product, 70 wt % dry product (often blended with other stabilizers or additives), and 125 g/L stabilized solution. 

Sodium dithionite is considered only moderately toxic. The solution is reported to have an LD q (rat, oral) of about 5 g/kg. As with sulfites, fairly large doses of sodium dithionite can probably be tolerated because oxidation to sulfate occurs. However, irritation of the stomach by the Hberated sulfurous acid is expected. As a food additive, sodium dithionite is generally recognized as safe (GRAS) (367). 

Traditionally, these dyes are appHed from a dyebath containing sodium sulfide. However, development in dyeing techniques and manufacture has led to the use of sodium sulfhydrate, sodium polysulfide, sodium dithionite, thiourea dioxide, and glucose as reducing agents. In the reduced state, the dyes have affinity for cellulose (qv) and are subsequendy exhausted on the substrate with common salt or sodium sulfate and fixed by oxidation. 

Sulfurized V tDyes. These dyes occupy an intermediate position between the tme vat colors and sulfur dyes because, like vat dyes, they are dyed preferentially from a sodium dithionite—caustic soda bath. However, some dyes of this class can also be dyed from an alkaU sulfide bath or a combination of the two, depending on the dyeing method used and the nature of the substrate to be dyed. This has led to some confusion because Cl Vat Blue 42 and 43 are Hsted in the constitution section of the Colourindex under sulfur dyes. Although inferior to tme vat dyes in fastness properties, they offer the advantage of better fastness, especially to chlorine, than conventional sulfur dyes. 

Prereduced Powders. These are usually made from press cake paste to which a reducing agent has been added, such as sodium sulfide, sodium hydrosulfide, or sodium dithionite, which solubili2e the dye in water. Before drying, the dye paste may be mixed with dispersing and stahi1i2ing agents to aid appHcation. 

Later, a completely different and more convenient synthesis of riboflavin and analogues was developed (34). It consists of the nitrosative cyclization of 6-(A/-D-ribityl-3,4-xyhdino)uracil (18), obtained from the condensation of A/-D-ribityl-3,4-xyhdine (11) and 6-chlorouracil (19), with excess sodium nitrite in acetic acid, or the cyclization of (18) with potassium nitrate in acetic in the presence of sulfuric acid, to give riboflavin-5-oxide (20) in high yield. Reduction with sodium dithionite gives (1). In another synthesis, 5-nitro-6-(A/-D-ribityl-3,4-xyhdino) uracil (21), prepared in situ from the condensation of 6-chloro-5-nitrouracil (22) with A/-D-ribityl-3,4-xyhdine (11), was hydrogenated over palladium on charcoal in acetic acid. The filtrate included 5-amino-6-(A/-D-ribityl-3,4-xyhdino)uracil (23) and was maintained at room temperature to precipitate (1) by autoxidation (35). These two pathways are suitable for the preparation of riboflavin analogues possessing several substituents (Fig. 4). 

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. 

A large number of polymeric substances, (RAs) or (ArAs), are also known (113). They are usually prepared by the reduction of arsonic acids with hypophosphorous acid (100,114) or sodium dithionite (115). Most of these polymers have not been well characterized. An insoluble, purple material, poly(methylarsinidene) [26403-94-1], (CH As), prepared by the interaction of methylarsine and a dihalomethylarsine, however, has been shown by an x-ray investigation to have a ladderlike polymeric stmcture in which the inter-mng distances correspond to one-electron bonds (116) ... 

Dithionites. Although the free-dithionous acid, H2S2O4, has never been isolated, the salts of the acid, in particular zinc [7779-86-4] and sodium dithionite [7775-14-6] have been prepared and are widely used as industrial reducing agents. The dithionite salts can be prepared by the reaction of sodium formate with sodium hydroxide and sulfur dioxide or by the reduction of sulfites, bisulfites, and sulfur dioxide with metallic substances such as zinc, iron, or zinc or sodium amalgams, or by electrolytic reduction (147). 

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). 

In the benzene and naphthalene series there are few examples of quinone reductions other than that of hydroquinone itself. There are, however, many intermediate reaction sequences in the anthraquinone series that depend on the generation, usually by employing aqueous "hydros" (sodium dithionite) of the so-called leuco compound. The reaction with leuco quinizarin [122308-59-2] is shown because this provides the key route to the important 1,4-diaminoanthtaquinones. 

Diaminoanthraquinone and Related Compounds. Leuco-l,4-diaminoanthraquinone [81-63-0] (leucamine) (32) is an important precursor for 1,4 diaminoanthraquinone [128-95-0] (33) and is prepared by heating 1,4-dihydroxyanthraquinone (29) with sodium dithionite in aqueous ammonia under pressure. 

Most vat dyes are based on the quinone stmcture and are solubilized by reduction with alkaline reducing agents such as sodium dithionite. Conversion back to the insoluble pigment is achieved by oxidation. The dyes are appHed by either exhaust or continuous dyeing techniques. In both cases the process is comprised of five stages preparation of the dispersion, reduction, dye exhaustion, oxidation, and soaping. 

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