Sodium dithionite
- Sodium hydrosulfite
- Natriumhypodisulfit
- Unterdischwefligsaures sodium
- Blankit
- Na2S2O4 (anhydrous )
- Na2S2O4 · 2H2O (dihydrate )
- White, pungent powder (anhydrous )
- Yellowish crystals (dihydrate )
Solid fuel
- 2.38 g · cm -3 (anhydrous, 20 ° C)
- 1.58 g · cm -3 ( dihydrate, 20 ° C)
~ 80 ° C ( decomposition)
Risk
- 2500 mg · kg -1 ( LD50, rat, oral)
- 1500 mg · kg -1 ( LD50, mouse, oral)
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Sodium dithionite, (product name: Blankit ) is the sodium salt of the unstable in the free state hyposulfurous acid ( H2S2O4 ). Sodium dithionite is a strong reducing agent.
Representation and extraction
For the preparation of sodium dithionite there are various methods. About half of the world production is done with the Formiatverfahren. New plants are operated almost exclusively by this method. Here sodium is reacted in methanolic solution under pressure with sulfur dioxide.
Other methods, each with 10-20 % share of the world production of sodium tetrahydroborate process, zinc dust method and amalgam process. When Natriumborhydridverfahren sodium borohydride is converted in a strongly basic solution with sulfur dioxide.
The method is based on a zinc dust reduction of sulfur dioxide with zinc in an aqueous suspension, wherein at first zinc dithionite is formed. Followed by treatment with sodium hydroxide solution yields the target compound.
The amalgam process proceeds from sodium sulfite, which is reduced in an electrolytic cell by means of sodium amalgam.
The world production is about 600,000 tons per year. After the formate process produces in Germany, BASF, after the zinc dust method, the Belgian company Prayon.
Properties
Sodium occurs as a crystalline anhydrous anhydrous and dihydrate. The anhydrate is a white crystalline powder with a faint odor of sulfur dioxide. The dihydrate forms yellow prisms. Both crystal forms differ significantly in density. Sodium is highly soluble in water. Until the transition point at 72 ° C the dihydrate in water is somewhat more difficult to dissolve.
The solutions are only limited stability and are hydrolyzed by water to Natriumhydrogensulfoxylat and sodium. The hydrolysis proceeds more rapidly, the warmer the water is. In addition, the presence of air oxidation to sodium sulfite ( Na2SO3 ) and sodium sulfate occurs ( Na2SO4).
The dihydrate is very sensitive to atmospheric oxygen, particularly at smaller grain size. The oxidation heat thus occurring can lead to self-ignition. Upon heating of the anhydrate in air sodium sulphate is formed, thereby releasing sulfur dioxide from 80 ° C in an exothermic reaction. Above 150 ° C caused the exclusion of air in a violent reaction of sodium sulfite, sodium thiosulfate, sulfur dioxide and traces of sulfur.
The SS bond in Dithionitdianion is 238.9 pm unusually long and thus relatively weak. Are a small part in an equilibrium • SO2 - before radical anions, the equilibrium lies almost completely on the side of Dithionitdianions.
The substance irritates the eyes, skin and mucous membranes.
Use
Because of the reducing effect of sodium dithionite is used as a bleaching agent in stain salts, in the dye ( vat dyeing ), as well as for the bleaching of sugar, syrup, wood-containing paper and pulp, but also for the separation of silver from the fixer. In electroplating sodium dithionite is used in waste water treatment as a reducing agent.