Sodium hydrosulfite,

SODIUM HYDROSULFITE Nci2S204,2H20, also N 2S2O4 

Fires resulting from contact with combustible materials are best extinguished by means of carbon dioxide, dry chemical, or sand. Avoid water unless flooding amounts are available.2 

Veiy soluble in water slightly soluble in alcohol.1 Addition of small amounts of water to the salt causes hazardous reaction.3 Oxidizes in air.1 

Contact with acids liberates toxic gas.3 

Oxidants. Violent decomposition can occur in the presence of oxidants.4 Sodium Chlorite. Mixture ignites.5 

---

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. 

The compound is odorless with a faintly acidic taste it is practically insoluble in water, ethanol and dilute acids but freely soluble in dilute aqueous alkaU with dissociation constants, pfC, 3.73, 7.9, 9.3. The compound is prepared by sodium hydrosulfite reduction of 3-nitro-4-hydroxyphenylarsonic acid [121 -19-7] and then acetylation in aqueous suspension with acetic anhydride at 50—55°C for 2 h (174,175). 

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. 

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). 

Reduction. Triaryknethane dyes are reduced readily to leuco bases with a variety of reagents, including sodium hydrosulfite, 2inc and acid (hydrochloric, acetic), 2inc dust and ammonia, and titanous chloride in concentrated hydrochloric acid. Reduction with titanium trichloride (Knecht method) is used for rapidly assaying triaryknethane dyes. The TiCl titration is carried out to a colorless end point which is usually very sharp (see Titanium COMPOUNDS, inorganic). 

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. 

Hydrosulfite may be purchased as a proprietary formulation containing buffering and stabilizing agents, usually as a dry powder, but sometimes as a solution. The powder is pyrophoric and should be kept dry to avoid the possibiHty of fires. Alternatively, solutions of sodium hydrosulfite may be generated on-site by using a purchased solution of sodium borohydride and sodium hydroxide to reduce sulfur dioxide. 

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). 

Vat Dyes. These water-iasoluble dyes ate appHed mainly to ceUulosic fibers as soluble leuco-salts after teductioa ia an alkaline bath, usuaUy with sodium hydrosulfite. FoUowiag exhaustion onto the fiber, the leuco forms ate reoxidized to the iasoluble keto forms and aftertreated, usuaUy by soapiag, to redevelop the crystal stmcture. The principal chemical classes of vat dyes ate anthraquiaone and iadigoid. 

In the batchwise process the temperature can be raised to 80°C to promote levelness providing dyes not sensitive to reductive breakdown are used. In the continuous appHcation method the vat dye is padded onto fabric and dried under conditions that avoid migration, passed through a solution of sodium hydrosulfite and caustic, through a pad mangle, and then steamed in saturated steam for up to 60 s. 

The majority of vat dyes used worldwide are appHed by continuous dyeing polyester—cotton blends are the most important substrate. The fabric is padded with vat dye dispersion, dried, padded with sodium hydrosulfite, caustic soda, and salt, steamed for 30—60 s at 102°C, rinsed, and dried. 

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. 

Afterscouring of polyester generally includes a reduction clearing with sodium hydrosulfite and alkaH at 60—80°C to remove any dye remaining on... 

Reduction. Many dyes, particularly azo dyestuffs, are susceptible to destmctive reduction. The reducing agents that can be used are sodium hydrosulfite, thiourea dioxide, sodium borohydtide, zinc sulfoxylate, and ferrous iron. 

Sodium hydrosulfite or sodium dithionate, Na2S204, under alkaline conditions are powerful reducing agents the oxidation potential is +1.12 V. The reduction of -phenylazobenzenesulfonic acid with sodium hydrosulfite in alkaline solutions is first order with respect to -phenylazobenzenesulfonate ion concentration and one-half order with respect to dithionate ion concentration (135). The SO 2 radical ion is a reaction intermediate for the reduction mechanisms. The reaction equation for this reduction is... 

Although it has been reported (138) that decolorization of wastewater containing reactive azo dyes with sodium hydrosulfite is possible only to a limited extent, others have demonstrated good reduction (decolorization). For example, using zinc hydrosulfite for the decolorization of dyed paper stock (139) resulted in color reduction of 98% for azo direct dyes (139). A Japanese patent (140) describes reducing an azo reactive dye such as Reactive Yellow 3 with sodium hydrosulfite into its respective aromatic amines which ate more readily adsorbable on carbon than the dye itself. This report has been confirmed with azo acid, direct, and reactive dyes (22). 

A. Reife, "Waste Treatment of Soluble Azo Acid, Direct and Reactive Dyes Using a Sodium Hydrosulfite Pretreatment Eodowed by Carbon Absorption," MATCC Book of Papers, 1990 International Conference and Exhibition, Philadelphia, Pa., Oct. 1—3,1990. 

Phenylenediamine [95-54-5] M 108.1, m 100-101 , pK O.67 (aq H2SO4), pKj 4.47 (4.85). Crystd from aqueous 1% sodium hydrosulfite (charcoal), washed with ice-water and dried in a vacuum desiccator, or sublimed in vacuo. It has been purified by recrystn from toluene and zone refined [Anson et al. J Am Chem Soc 108 6593 1986]. Purification by refluxing a CH2CI2 solution containing charcoal was also carried out followed by evaporation and recrystn [Koola and Kochi J Org Chem 52 4545 1987], protect from light. 

The only practical method of preparing 1,4-aminonaphthol is from a-naphthol through an azo dye, the nitroso compound not being readily available. The majority of investigators have reduced technical Orange I with stannous chloride Mi.is.is.ir.is by the procedures discussed above, and benzeneazo-a-naphthol has been reduced by the same reagent. In order to make possible the use of crude, technical a-naphthol a method has been developed for the preparation of the benzeneazo compound, its separation from the isomeric dye coming from the d-naphthol present as well as from any disazo compound by extraction with alkali, and the reduction of the azo compound in alkaline solution with sodium hydrosulfite. The process, however, is tedious and yields an impure product. 

---