Module: 4. Lecture: 20

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1 Module: 4 Lecture: 20 SODIUM THIOSULFATE INTRODUCTION Sodium thiosulfate, Na2S2O3 is a colourless crystalline compound which is more familiar as the pentahydrate, Na2S2O3.5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or hypo. Sodium thiosulfate is produced industrially from liquid waste products of sodium sulfide or during sulfur dye manufacture. An aqueous solution of sodium sulfite is heated with sulfur or by boiling aqueous sodium hydroxide and sulfur resulting into sodium thiosulfate on laboratory scale. John Herschel was developed the use of sodium thiosulfate as photographic fixer in John discovered the sodium thiosulfate to be a solvent for silver halides. Commercially sodium thiosulfate is produced using air oxidation of sulfides, polysulfides and hydrosulfides. It is also obtained as by-product in manufacturing of sulfur dyes and sulfur black. It is also produced using reaction of sodium sulfide with sulfur dioxide and soda ash or caustic soda. Aqueous solution of sodium thiosulfate is generally neutral. Decomposition of sodium thiosulfate produces sodium sulfite and sulfur at under neutral or at slightly acidic condition. Alkaline solution decomposes to sulfate and sulfide in the presence of air. MANUFACTURE 1. From soda ash sulfurdioxide Raw material Basis: 1000kg sodium thiosulfate from soda ash and sulfur dioxide Soda ash 385kg N P T E L 1

2 dioxide 235kg 118kg Reaction Manufacture process Soda ash solution of 26 0 Be was allowed to percolate downward through a series of absorption towers countercurrent to sulfur dioxide gas. The absorption towers were generally lead-lined, packed with wood slats, and supported by acid-proof brick. The solution leaving the bottom of the last tower was chiefly sodium bisulfite which contains 22% SO. Water Soda ash Soda ash Dissolving tank Steam out Steam in Iron pot Waste gases Water vapour Slurry + water Absorption tower dioxide Steam in Evaporator Steam out Crystallizer Centrifuge Sodium thiosulfate Mother liquor Figure: Manufacture of Sodium thiosulfate from soda ash and sulfur dioxide Block diagram of manufacturing process Diagram with process equipment Animation N P T E L 2

3 The solution was run into ceramic lined cast-iron pot fitted with an agitator. Sufficient soda ash was added to convert the bisulfite to neutral sodium sulfite. Then sulfur was added, and the contents of the kettle were heated to boiling. The resulting solution of sodium thiosulfate was pumped to a steam-heated evaporator, where it was concentrated to 51 0 Be. The hot liquor from evaporator was then cooled, crystallized, and centrifuged in the conventional manner. Sodium thiosulfate crystals are effloresent and must be packed at once in moisture-tight containers. 2. From sodium sulphite and sulfur Sodium thiosulfate is prepeared from Sodium sulfite and sulfur as the starting materials in many plants. The design and lay out the plant is same as soda ash and sulfur dioxide method. 3. From by-product sodium sulfide In the manufacture of sodium sulfide, a by-product sulfide-carbonate liquor is obtained which contains 8% sodium sulfide, 6% sodium carbonate. Which may be converted to sodium thiosulfate by reaction with sulfur dioxide. dioxide was reacted with excess of sodium sulfide to yield sodium sulfite, in such a case the sulfite may be converted to the thiosulfate by treatment with sulfur. Accordingly the plant used to produce sodium thiosulfate from by-product sodium sulfide is almost identical with that described using soda ash, sulfur dioxide, and sulfur. 4. From by-product of sulfur-dye manufacture The fusion product from the manufacture of sulfur dyes by fusion of sulfur, caustic soda, and certain organic compounds, is leached with water, and the leach liquor is filtered to remove the insoluble dye. The filtrate chiefly contains most of the original sulfur and caustic soda in the form of sodium thiosulfate. This filtrate may be concentrated and crystallized to yield sodium thiosulfate of good quality. Analytical and test methods Sodium thiosulfate in its aqueous solution was formed a white precipitate with HCl and evolution of SO2 gas was detected by its N P T E L 3

4 characteristic odour. The solution of sodium thiosulfate produces a dark violet colour on addition of ferric chloride solution. Health and safety factors The LD50 of anhydrous sodium thiosulfate for mice is 7.5±0.752g/kg. It has low toxicity, so it can be safely used in veterinary medicine. Under the Federal Food, Drug and Cosmetic Act, sodium pentahydrate is affirmed as a GRAS indirect and direct human food ingredient. Engineering aspects Newsprint are made with low-yield sulfite reinforcing pulp. Cooking the pulp could produced thiosulfate, which was then finds its way onto the machine. The more modern sulfonation processes might produce thiosulphate, but this has not been commonly observed. The Powell River Company was one of the earliest users of hydrosulphite as a brightening agent in the 1940s. Even zinc hydrosulfite were in newsprint from the darker coastal wood species. When inland mills started to use zinc hydrosulfite to prevent the adverse environmental affects of zinc in the effluent. So the industry then used sodium hydrosulfite which is significantly less stable than the zinc salt in the mildly acidic water then commonly used for newsprint manufacture. It has a tendency to break down so self-reduction reaction occurs at ph 4.5. One of the decomposition products is thiosulfate. So a change in brightening agent introduced a new anion into the machine white water, one whose corrosiveness was unexpected. PROPERTIES Molecular formula Molecular weight Appearance Odour Boiling point Melting point : Na2S2O3 :158.11gm/mole(anhydrous), gm/mole (pentahydrate) : White crystals : Odourless : C (pentahydrate) decomposition : 48 0 C (pentahydrate) when rapidly heated N P T E L 4

5 Density : 1.69gm/ml (pentahydrate) Refractive index : Solubility : Soluble in water, ammonia and slightly soluble in alcohols USES Sodium thiosulfate is used as antichlor in bleaching of paper pulp In manufacture of leather and for extraction of silver from ores As mordant in dyeing and printing textiles and reducer in chrome dyeing and bacteriological water assessment As bleaching bone, ivory and straw Important reagent in analytical chemistry For making chlorine level low in swimming pools As solute in supersaturation experiments In treatment of calciphylaxis in hemodialysis patients with end stage renal disease In extraction of Gold N P T E L 5