Thiosulfate solutions

Remark It is always possible to standardize an iodine solution with a thiosulfate solution provided the latter has recently been standardized (see Sect. 18.8). 

Thiosulfate solutions must be standardized before their use. Sodium thiosulfate Na2S203, 5H2O is readily obtainable in a state of high purity. It is efflorescent and its water content is always uncertain. Therefore, it is unsuitable as a primary standard. Its aqueous solutions, prepared with ordinary distillated water, are not stable. Under the influence of carbon dioxide, hydrogen sulfite ions and sulfur form according to the reaction 

Finally, sodium thiosulfate may be decomposed by microbiological action under the influence of thiobacillus thioparus. 

As iodine and tri-iodide ions solutions are not primary standard solutions, the standardization of thiosulfate solutions may be achieved with the help of iodine solutions extemporaneously prepared from primary standards. Iodine is usually prepared by the iodate/iodide reaction or by the oxidation of iodides with potassium dichromate, but other possibilities exist. 

Potassium iodate can be obtained in a state of at least 99.9% purity. It reacts with potassium iodide in acidic solution to liberate iodine  

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Iodine, Q.IN (0 to 1 —). Dissolve 12.690 g of resublimed iodine in 25 mL of a solution containing 15 g of KI which is free from iodate. After all the solid has dissolved, dilute to 1 L. If desired, check against a standard arsenite or standard thiosulfate solution. 

NaCl—HCl solution, add KI, and titrate I2 with thiosulfate solution. 

S H2S + F(excess) = S + 2 1 + 2 Back-titrate excess F with standard thiosulfate solution. 

H2S + 4 Br2 + 4 H2O = SO + 8 Br + 10 H" Use excess KBr and standard KBr03 solution. Fet stand until clear, add excess KI, and titrate with standard thiosulfate solution. 

Chlorine and bromine add vigorously, giving, with proper control, high yields of 1,2-dihaloethyl ethers (224). In the presence of an alcohol, halogens add as hypohaUtes, which give 2-haloacetals (225,226). With methanol and iodine this is used as a method of quantitative analysis, titrating unconsumed iodine with standard thiosulfate solution (227). 

The kinetics of this reaction, the conditions under which it goes to completion, and subsequent titration of the iodine with thiosulfate solutions have ah been investigated (53—55). 

As an emergency treatment, the washing of the contaminated body parts with a 5% thiosulfate solution is recommended. If swallowed, gastric lavage with 5% solution of thiosulfate, followed by saline catharsis should be accompHshed. If pulmonary signs are severe, oxygen should be suppHed with intermittent positive-pressure breathing apparatus. 

An alternative method for the analysis of permanganate is the use of conventional iodometric methods (177) where excess potassium iodide is added to a solution of permanganate under acidic conditions. The Hberated iodide is then titrated with standard thiosulfate solution using starch as an indicator. 

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. 

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

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 ... 

Silver Thiosulfate. Silver thiosulfate [23149-52-2], Ag 2 y is an insoluble precipitate formed when a soluble thiosulfate reacts with an excess of silver nitrate. In order to minimize the formation of silver sulfide, the silver ion can be complexed by haUdes before the addition of the thiosulfate solution. In the presence of excess thiosulfate, the very soluble Ag2(S203) 3 and Ag2(S203) 3 complexes form. These soluble thiosulfate complexes, which are very stable, are the basis of photographic fixers. Silver thiosulfate complexes are oxidized to form silver sulfide, sulfate, and elemental sulfur (see Thiosulfates). 

Thiosulfates. The ammonium, alkaU metal, and aLkaline-earth thiosulfates are soluble in water. Neutral or slightly alkaline solutions containing excess base or the corresponding sulfite are more stable than acid solutions. Thiosulfate solutions of other metal ions can be prepared, but their stabiUty depends on the presence of excess thiosulfate, the formation of complexes, and the prevention of insoluble sulfide precipitates. 

This equihbrium explains the stabilization of thiosulfate solutions using sulfite or bisulfite as one of the components of acid photographic fixing baths. 

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. 

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%). 

Sodium thiosulfate is a by-product of the manufacture of Sulfur Black and other sulfur dyes (qv), where organic nitro compounds are treated with a solution of sodium polysulfide to give thiosulfate. The dyes ate insoluble and ate recovered by fUtration. The fUtrate is treated with activated carbon and filteted to obtain a sodium thiosulfate solution. After concentration and crystallization, the final product assays ca 96% Na2S202 5H20 (34) (see Dyes AND... 

Cationic surface-active agents promote wetting of the sulfur and thereby increase the reaction rate (36). The quahty of the product is improved by using photographic-grade sodium sulfite or bisulfite. Excess sulfur is filtered before evaporation (qv) and crystallization (qv). Evaporation is energy-intensive thus it is important to produce the thiosulfate solution at the highest possible concentration. The purity of the product is typically >99% sulfite and sulfate ate the main impurities. 

Analytical and Test Methods. An aqueous solution of sodium thiosulfate forms a white precipitate with hydrochloric acid and evolves sulfur dioxide gas which is detected by its characteristic odor. The white precipitate turns yellow, iadicatiug the presence of sulfur. The addition of ferric chloride to sodium thiosulfate solutions produces a dark violet color which quickly disappears. 

The anhydrous monoclinic crystalline form has a density of 1.679 g/cm (59) no hydrates are known. SolubiUty in water is given in Table 4. Ammonium thiosulfate solutions decompose slowly below 50°C and more rapidly at higher temperatures. The anhydrous salt decomposes above 100°C to sulfite and sulfur (60) ... 

Table 5. American National Standard Specifications for Photographic-Grade Ammonium Thiosulfate Solution...

The principal use of photochemical-grade ammonium thiosulfate continues to be in photography, where is dissolves undeveloped silver haUdes from negatives and prints. It reacts considerably faster than sodium thiosulfate, and the fixing solutions can be used about twice as long as sodium thiosulfate solutions the washing period to remove residual thiosulfate is shorter. 

Gold thiosulfate complexes of the form Na2[Au(S202)2] 2H20 [19153-98-1] are prepared by addition of gold trichloride to concentrated sodium thiosulfate solution (89). The gold is completely reduced and some thiosulfate is oxidized to tetrathionate. This complex has been used in the treatment of rheumatoid arthritis. 

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

The mixture is cooled to room temperature, then filtered. The solvent is removed under reduced pressure, leaving the tribromide (47) as a foam. The foam is mixed with sodium iodide (9.55 g, 0.064 mole) and acetone (74 ml) and heated under reflux in a nitrogen atmosphere for 3.5 hr. The acetone is removed under reduced pressure and the residue is treated with chloroform and aqueous sodium thiosulfate solution. The chloroform layer is separated and washed with sodium thiosulfate solution until it is free from iodine, then dried over magnesium sulfate, filtered and evaporated to dryness under reduced pressure. The crude product (48) is obtained as a brown sohd (4.85 g) which is chromatographed over alumina (122 g, Merck acid-washed). The column is developed with hexane, benzene and ethyl acetate mixtures. The product (3.43 g) is eluted by benzene and benzene-ethyl acetate (10 1). Recrystallization from acetone yields purified 3jS-acetoxy-pregna-5,14,16-trien-20-one (48), 3.25 g, mp 158-159° 309 m/ (e 10,700). 

Photographers hypo (sodium thiosulfate solution) and ordinary hypo (sodium hypochlorite solution)... 

A mixture of 17 g of the methiodide and 32 ml of a 40 % aqueous potassium hydroxide solution is heated with stirring in a flask fitted with a condenser. The heating bath should be kept at 125-130°, and the heating should be continued for 5 hours. The cooled reaction mixture is then diluted with 30 ml of water and washed twice with 25-ml portions of ether. The aqueous layer is cautiously acidified in the cold with concentrated hydrochloric acid to a pH of about 2 and then extracted five times with 25-ml portions of ether. The combined extracts are washed twice with 10% sodium thiosulfate solution and are dried (magnesium sulfate). Removal of the solvent followed by distillation affords about 3 g of 4-cyclooctene-l-carboxylic acid, bp 125-12671-1 mm. The product may solidify and may be recrystallized by dissolution in a minimum amount of pentane followed by cooling in a Dry-Ice bath. After rapid filtration, the collected solid has mp 34-35°. 

The residue is dissolved in ether and the solution is washed with sodium chloride solution and then with a little sodium thiosulfate solution. The ethereal solution is dried over sodium sulfate and ether removed by distillation. A yield of 108 parts of 3,5,5-trimethyl-oxazolidine-2,4-dione is obtained having a melting point of 45° to 46°C with slight softening at 43°C. This represents a 75% theory yield on the ethyl o-hydroxy-iso-butyrate taken. The product may be further purified by dissolving the minimum quantity of dry ether and cooling to -10°C. The product so obtained melts sharply at 45.5° to 46.5°C, according to U.S. Patent 2,559,011. 

The benzoylperoxide used was analyzed by dissolving r g. in 25 cc. of dry ether and adding 2 cc. of 5 per cent sodium ethylate solution, keeping the temperature below — 50. The ether solution was extracted with exactly 100 cc. of cold water and an aliquot part of the aqueous extract taken. To this was added 2 cc. of 5 per cent potassium iodide and 2 cc. of dilute hydrochloric acid and the liberated iodine was titrated with 0.1 N sodium thiosulfate solution. The peroxide analyzed 90 per cent pure. 

A number of methods have been proposed for the detection of rancidity. The determination of active oxygen consists of dissolving the fat in a suitable medium such as chloroform and acetic acid, adding potassium iodide, and titrating the liberated iodine with a standard thiosulfate solution (16, 20). This is perhaps the most widely used method at the present time. Another procedure which has been proposed for the detection of peroxides employs ferrous ammonium sulfate and ammonium thiocyanate in acetone. The resulting red color of ferric thiocyanate is measured spectrophotometrically, and is said by the authors to yield more reproducible results than do the usual titration methods (21). 

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