This is a part Kirk-othmer encyclopedia.
Sodium Bisulfite. Sodium bisulfite, NaHSO3, exists in solution but is not a stable compound in the solid state. The anhydrous sodium bisulfite of commerce consists of sodium metabisulfite, Na2S2O5. Aqueous sodium bisulfite solution, having specific gravity 1.36 and containing the equivalent of 26-27 wt % SO2, is a commercial product.
Sodium Metabisulfite.
Physical Properties. Sodium metabisulfite (sodium pyrosulfite, sodium bisulfite (a misnomer)), Na2S2O5, is a white granular or powdered salt (specific gravity 1.48) and is storable when kept dry and protected from air. In the presence of traces of water it develops an odor of sulfur dioxide and in moist air it decomposes with loss of part of its SO2 content and by oxidation to sodium sulfate. Dry sodium metabisulfite is more stable to oxidation than dry sodium sulfite. At low temperatures, sodium metabisulfite forms hydrates with 6 and 7 moles of water. The solubility of sodium metabisulfite in water is 39.5 wt % at 20°C, 41.6 wt % at 40°C, and 44.6 wt % at 60°C (340). Sodium metabisulfite is fairly soluble in glycerol and slightly soluble in alcohol.
Chemical Properties. The chemistry of sodium metabisulfite is essentially that of the sulfite-bisulfite-metabisulfite-sulfurous acid system. The relative proportions of each species depend on the pH. The pH of a sodium bisulfite solution obtained by dissolving 10 wt % sodium metabisulfite in water at 20°C is 4.9; at 30 wt %, the pH is 4.4.
Manufacture. Aqueous sodium hydroxide, sodium bicarbonate, sodium carbonate, or sodium sulfite solution are treated with sulfur dioxide to produce sodium metabisulfite solution. In one operation, the mother liquor from the previous batch is reinforced with additional sodium carbonate, which need not be totally in solution, and then is treated with sulfur dioxide (341,342). In some plants, the reaction is conducted in a series of two or more stainless steel vessels or columns in which the sulfur dioxide is passed countercurrent to the alkali. The solution is cooled and the sodium metabisulfite is removed by centrifuging or filtration. Rapid drying, eg, in a stream-heated shelf dryer or a flash dryer, avoids excessive decomposition or oxidation to which moist sodium metabisulfite is susceptible.
Shipment and Storage. Sodium metabisulfite can be stored under air at ambient temperatures, but under humid conditions the product cakes and the available SO2 content decreases as a result of oxidation. Therefore, storage should be under cool, dry conditions. The product is shipped in 22.7- and 45.4-kg polyethylene-lined (moistureproof) bags and 45.4- and 181-kg fiber drums. Steel drums are used for export. The solution is shipped in tank cars and trucks. Dry sodium metabisulfite can be handled in iron or steel equipment. Sodium bisulfite solutions can be handled in 316 and 347 stainless steels, lead, rubber, wood, Haveg, glass-reinforced polyester, or cross-linked polyethylene.
Economic Aspects. U.S. production of sodium metabisulfite is estimated to be well in excess of 45,000 t, but statistics are confused by some commingling with sodium sulfite. The principal U.S. producers are Rhône-Poulenc and General Chemical. The price in mid-1995 was $0.63/kg for anhydrous sodium bisulfite.
Grades and Specifications. Sodium metabisulfite is available in photographic, food and NF, and technical grades (340). Typical analyses for the food and NF grade are 99 wt % Na2S2O5, 0.6 wt % Na2SO4; 0.4 wt % Na2SO3, 3 ppm iron, 0.2 ppm arsenic, 0.1 ppm lead, and 2 ppm selenium. Typical analyses for the technical grade are 98 wt % Na2S2O5, 1.1 wt % Na2SO4, 0.8 wt % Na2SO3, and 4 ppm iron. An aqueous solution typically assaying 20-42% NaHSO3 is sold in tank trucks.
Health and Safety Factors. Sodium metabisulfite is nonflammable, but when strongly heated it releases sulfur dioxide. The oral acute toxicity is slight and the LD50 (rat, oral) is 2 g/kg. Sodium bisulfite appears to be weakly mutagenic to some bacteria, in rodent embryos, and in a human lymphocyte test. There is inadequate evidence for carcinogenicity (183,343).
The solid product and its aqueous solutions are mildly acidic and irritate the skin, eyes, and mucous membranes. The solid material when moist generates the pungent, irritating odor of sulfur dioxide. Food-grade sodium metabisulfite is permitted in those foods that are not recognized as sources of vitamin B1, with which sulfur dioxide reacts (316) (see VITAMINS, THIAMINE).
Uses. Sodium metabisulfite is extensively used as a food preservative and bleach in the same applications as sulfur dioxide. Because sodium metabisulfite is most effective at low pH, the active agent is probably sulfur dioxide or sulfurous acid (340). Sodium bisulfite (sodium metabisulfite) is used in photography (qv), as a reductant, and as a preservative for thiosulfate fixing baths. Other reducing agent applications include reduction of chromate in plating effluents to less toxic chromium salts, which can then precipitate upon addition of lime (344,345). Sodium metabisulfite is also used to reduce chlorine in industrial process water and wastewater. In the textile industry, sodium bisulfite containing metabisulfite is used as a bleach, especially for wool (qv); as an antichlor after bleaching of nylon, for reducing vat dyes; and in rendering certain other dyes soluble. It is a less powerful reductant than sodium dithionite [7775-14-6].
In tanneries, sodium bisulfite is used to accelerate the unhairing action of lime. It is also used as a chemical reagent in the synthesis of surfactants (qv). Addition to alpha-olefins under radical catalyzed conditions yields sodium alkylsulfonates (wetting agents). The addition of sodium bisulfite under base-catalyzed conditions to dialkyl maleates yields the sulfosuccinates.
The reversible addition of sodium bisulfite to carbonyl groups is used in the purification of aldehydes. Sodium bisulfite also is employed in polymer and synthetic fiber manufacture in several ways. In free-radical polymerization of vinyl and diene monomers, sodium bisulfite or metabisulfite is frequently used as the reducing component of a so-called redox initiator (see INITIATORS). Sodium bisulfite is also used as a color preventative and is added as such during the coagulation of crepe rubber.
Ullmann's Encyclopedia of Industrial Chemistry
3.2. Sodium Disulfide
Sodium disulfide, Na2S2, is produced by adding
a higher sodium polysulfide and sodium, either
simultaneously or alternately, to a polysulfide
melt [39]. As the melting point of Na2S2
(474?C) is higher than the boiling point of sulfur,
it is preferable to use sodium tetrasulfide. Thus,
sodium disulfide free of water or solvent can be
produced in an easily controlled process. The
process has three variations [39]. The whole of
the polysulfide can be premelted, and the sodium
added to the melt until the desired stoichiometric
composition is obtained, or sodium and, for
example, Na2S4 can be added to the melt either
alternately or simultaneously [39].
There are many literature references to other
methods of producing sodium disulfide. For example,
sodium tetrasulfide can be dissolved in
alcohol and then reduced with sodium metal
to give sparingly soluble Na2S2 [21]. It has
not been possible to prove conclusively that
the product so obtained, which contains 4 –6%
alcohol, represents a defined alcoholate [12].
High-purity Na2S2 can be produced from the
elements in liquid ammonia [12].
Small amounts of sodium polysulfide for the
investigation of thermodynamic properties have
been prepared by melting together Na2S and sulfur
in a small sealed capsule [16].
Sodium disulfide is produced on an industrial
scale from sodium hydroxide solution and sulfur
[44], and is used for the production of polysulfide
polymers [36] (see Chapter 6).
