Thiosulfate, sodium

Large amounts of sodium thiosulfate are obtained as the byproducts of other industrial reactions, such as the production 

Sodium thiosulfate. Red atoms are oxygen yellow atoms are sulfur and turquoise atoms are sodium. Gray sticks indicate double bonds. 

A food additive for the purpose of maintaining the proper of acidity of a food product or sequestering 

An important, but seldom used, application of sodium thiosulfate is 

For certain medical purposes, such as the treatment of fungal infections of the skin in humans and, specifically, for the treatment of ringworm in animals  

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Silver chloride Aqueous ammonia or sodium thiosulfate. 

The following factors are the equivalent of 1 mL of normal sodium thiosulfate. Where the normality of the solution being used is other than normal, multiply the factors given in the table below by the normality of the solution employed. 

Sodium thiosulfate, O.IA. Weigh 24.818 g of fresh crystals of Na2S203 5H2O, dissolve in distilled water. Add 0.5 g of Na2C03 and 0.5 mL of chloroform as preservative. Dilute to 1 L. 

Many municipal water sources are chlorinated and contain sufficiently high levels of chlorine so as to be toxic to aquatic life. Chlorine can be removed by passing the water through activated charcoal filters or through the use of sodium thiosulfate metered into the incoming water. Municipal water is usually not used in aquaculture operations that utilize large quantities of water, either continuously or periodically, because of the initial high cost of the water and the cost of pretreatment to remove chlorine. 

The pH must be kept at 7.0—7.2 for this method to be quantitative and to give a stable end poiut. This condition is easily met by addition of soHd sodium bicarbonate to neutralize the HI formed. With starch as iudicator and an appropriate standardized iodine solution, this method is appHcable to both concentrated and dilute (to ca 50 ppm) hydraziue solutious. The iodiue solutiou is best standardized usiug mouohydraziuium sulfate or sodium thiosulfate. Using an iodide-selective electrode, low levels down to the ppb range are detectable (see Electro analytical techniques) (141,142). Potassium iodate (143,144), bromate (145), and permanganate (146) have also been employed as oxidants. 

Iodide ion, a moderately effective reducing agent, is used extensively for the deterrnination of oxidants. In such appHcations, the iodine Hberated by reaction between the analyte and the unmeasured excess of potassium iodide is ordinarily titrated with a standard solution of sodium thiosulfate. The reaction is as foHows ... 

The hberated iodine is measured spectrometricaHy or titrated with Standard sodium thiosulfate solution (I2 +28203 — 2 1 VS Og following acidification with sulfuric acid buffers are sometimes employed. The method requires measurement of the total gas volume used in the procedure. The presence of other oxidants, such as H2O2 and NO, can interfere with the analysis. The analysis is also technique-sensitive, since it can be affected by a number of variables, including temperature, time, pH, iodide concentration, sampling techniques, etc (140). A detailed procedure is given in Reference 141. 

Liquid phosgene is assayed by an iodometric method which iavolves the foUowiag reaction (52). The released iodine is titrated with sodium thiosulfate. 

Where free chlorine is present, eg, in drinking water, it is measured on-site, and a crystal (eg, 10 mg/40 mL) of sodium thiosulfate is added to the botde prior to sterilization to convert free chlorine to chloride. 

The hberated iodine is titrated with standard sodium thiosulfate solution. In the thiosulfate method, selenous acid is treated with an excess of standard sodium thiosulfate solution ... 

Iodized Salt. Iodized table salt has been used to provide supplemental iodine to the U.S. population since 1924, when producers, in cooperation with the Michigan State Medical Society (24), began a voluntary program of salt iodization in Michigan that ultimately led to the elimination of iodine deficiency in the United States. More than 50% of the table salt sold in the United States is iodized. Potassium iodide in table salt at levels of 0.006% to 0.01% KI is one of two sources of iodine for food-grade salt approved by the U.S. Food and Dmg Administration. The other, cuprous iodide, is not used by U.S. salt producers. Iodine may be added to a food so that the daily intake does not exceed 225 p.g for adults and children over four years of age. Potassium iodide is unstable under conditions of extreme moisture and temperature, particularly in an acid environment. Sodium carbonate or sodium bicarbonate is added to increase alkalinity, and sodium thiosulfate or dextrose is added to stabilize potassium iodide. Without a stabilizer, potassium iodide is oxidized to iodine and lost by volatilization from the product. Potassium iodate, far more stable than potassium iodide, is widely used in other parts of the world, but is not approved for use in the United States. 

A double end point, acid—base titration can be used to determine both sodium hydrosulfide and sodium sulfide content. Standardized hydrochloric acid is the titrant thymolphthalein and bromophenol blue are the indicators. Other bases having ionization constants in the ranges of the indicators used interfere with the analysis. Sodium thiosulfate and sodium thiocarbonate interfere quantitatively with the accuracy of the results. Detailed procedures to analyze sodium sulfide, sodium hydro sulfide, and sodium tetrasulfide are available (1). 

Sodium sulfite undergoes addition of sulfur to form sodium thiosulfate. At acidic pH, the chemistry of sodium sulfite is that of bisulfite, metabisulfite, and sulfur dioxide. 

CU2O, films can be deposited from mixing solutions of CuSO and sodium thiosulfate and NaOH. 

X-ray crystallographic analysis of the sodium thiosulfate pentahydrate [10102-17-7] crystal indicates a tetrahedral stmcture for the thiosulfate ion. The S—S bond distance is 197 pm the S—O bond distance is 148 pm (5). Neutron diffraction of a barium thiosulfate monohydrate [7787-40-8] crystal confirms the tetrahedral stmcture and bond distances for the thiosulfate ion (6). 

Thiosulfuric Acid. Thiosulfuiic acid [14921 -76-7] is relatively unstable and thus cannot be recovered from aqueous solutions. In laboratory preparation, a lead thiosulfate [26265-65-6] solution is treated with H2S to precipitate PbS, or a concentrated solution of sodium thiosulfate [7772-98-7] is treated with HCl and cooled to — 10°C to crystalline NaCl. Aqueous solutions of thiosulfuric acid spontaneously decompose to yield sulfur, SO2, and polythionic acids, H2S O. Thiosulfuric acid is a strong acid comparable to sulfuric acid. Dissociation constants, = 0.25, = 0.018, have been... 

Corrosion. Copper-base alloys are seriously corroded by sodium thiosulfate (22) and ammonium thiosulfate [7783-18-8] (23). Corrosion rates exceed 10 kg/(m yr) at 100°C. High siUcon cast iron has reasonable corrosion resistance to thiosulfates, with a corrosion rate <4.4 kg/(m yr)) at 100°C. The preferred material of constmction for pumps, piping, reactors, and storage tanks is austenitic stainless steels such as 304, 316, or Alloy 20. The corrosion rate for stainless steels is <440 g/(m yr) at 100°C (see also Corrosion and corrosion control). 

Sodium thiosulfate, either the anhydrous salt, Na2S202, or the crystalline pentahydrate, is commonly referred to as hypo or crystal hypo. When a concentrated sodium thiosulfate solution (50—60 wt %) is cooled to <48° C, the pentahydrate, containing 63.7% Na2S202, crystallines in monoclinic transparent prisms as shown in the equiUbrium phase diagram (Fig. 1). The monohydrate [55755-19-6] and the heptahydrate [36989-91-0] are also known. 

Although 16 different crystalline modifications have been identified (24,25), the a-pentahydrate is the stable form below 48°C. Solutions of sodium thiosulfate in the absence of seed crystals can be easily supercooled below their normal crystallisation temperatures. The dotted line extension of the dihydrate phase in Figure 1 is an indication that, if supercooling takes place below this line, solutions normally giving the pentahydrate may form the dihydrate [36989-90-9] s1ste2id. 

Selected physical properties of sodium thiosulfate pentahydrate are shown in Table 1. The crystals are relatively stable, efflorescing in warm, dry air and dehquescing slightly in moist air. They melt in their water of hydration at 48°C and can be completely dehydrated in a vacuum oven at this temperature, or at atmospheric pressure at 105°C. Anhydrous sodium thiosulfate can also be crystallised direcdy from a 72% solution above 75°C. It decomposes at 233°C ... 

Further heating to 440—500°C gives sodium sulfide and sulfur dioxide (33). Table 1. Physical Properties of Sodium Thiosulfate Pentahydrate... 

Ref. 26 gives a comprehensive summary of the properties of sodium thiosulfate and its aqueous solutions. 

Aqueous sodium thiosulfate solutions ate neutral. Under neutral or slightly acidic conditions, decomposition produces sulfite and sulfur. In the presence of air, alkaline solutions decompose to sulfate and sulfide. Dilute solutions can be stabilized by small amounts of sodium sulfite, sodium carbonate, or caustic, and by storage at low temperatures away from air and light. Oxidation is inhibited by Hgl2 (10 Ppm) amyl alcohol (1%), chloroform (0.1%), borax (0.05%), or sodium benzoate (0.1%). 

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