Thiosulfate assay

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). 

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

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... 

To assay liquid biomine, an ampule of biomine is cmshed under tfie surface of an aqueous potassium iodide solution and the resultant iodine titrated with standard sodium thiosulfate. 

Assay of hydrogen cyanide can be done by specific gravity or silver nitrate titration. Sulfur dioxide in hydrogen cyanide can be deterrnined by infrared analysis or by reaction of excess standard iodine solution and titration, using standard sodium thiosulfate or by measurement of total acidity by... 

A 250-mL, two-necked, round-bottomed flask equipped with a magnetic stirbar, thermometer, and a reflux condenser fitted with a rubber septum and balloon of argon is charged with a solution of methyltrioxorhenium (MTO) (0.013 g, 0.05 mmol, 0.1% mol equiv) in 100 mL of methanol (Note 1). Urea hydrogen peroxide (UHP) (14.3 g, 152 mmol) is added (Notes 1, 2, 3, 4), the flask is cooled in an ice bath, and dibenzylamine (9.7 mL, 50.7 mmol) is then added dropwise via syringe over 10 min (Notes 1, 5). After completion of the addition, the ice bath is removed and the mixture is stirred at room temperature (Note 6). A white precipitate forms after approximately 5 min (Note 7) and the yellow color disappears within 20 min (Note 8). Another four portions of MTO (0.1% mol equiv, 0.013 g each) are added at 30-min intervals (2.5 hr total reaction time). After each addition, the reaction mixture develops a yellow color, which then disappears only after the last addition does the mixture remain pale yellow (Note 9). The reaction flask is cooled in an ice bath and solid sodium thiosulfate pentahydrate (12.6 g, 50.7 mmol) is added in portions over 20 min in order to destroy excess hydrogen peroxide (Note 10). The cooled solution is stirred for 1 hr further, at which point a KI paper assay indicates that the excess oxidant has been completely consumed. The solution is decanted into a 500-mL flask to remove small amounts of undissolved thiosulfate. The solid is washed with 50 mL of MeOH and the methanol extract is added to the reaction solution which is then concentrated under reduced pressure by rotary evaporation. Dichloromethane (250 mL) is added to the residue and the urea is removed by filtration through cotton and celite. Concentration of the filtrate affords 10.3 g (97%) of the nitrone as a yellow solid (Note 11). 

Table 15.1. Activities of enzymes involved in thiosulfate metabolism as assayed in cell-free extracts of T. thioparus.

The activities of thiosulfate reductase originally reported were low (-0.8nmol sulfide per minute per milligram protein), when assayed at pH... 

The original demonstration of APS reductase in T. thioparus used the methylviologen-dependent assay of APS cleavage to AMP and sulfite (Peck 1960), but in the oxidation of thiosulfate the reaction proceeds in the oxidative direction (Eq. 15.4), forming APS. This is the thermodynamically favorable direction of the reaction. Later work showed that APS formation by the reductase could be coupled to the reduction of ferricyanide or to cytochrome c (Peck et al. 1965 Lyric and Suzuki 1970), thereby showing the thermodynamic feasibility of APS as an intermediate in the oxidation pathways for sulhte and thiosulfate. 

The thiosulfate reductase/rhodanese/APS reductase system is thus supported by evidence from direct enzyme assay, whole-cell metabolism and energetics, and S-labehng experiments and provides a robust hypothesis to explain thionate oxidation and energy conservation in at least some chemolithotrophs. 

Hydroperoxides may be determined by measuring at 290 nm (e = 44100 M cm ) or 360 nm (e = 28000 cm ) the concentration of 13 formed in the presence of a large excess of ions. The reaction may be too slow for practical purposes, unless a catalyst is present. For example, an assay for lipid hydroperoxides conducted without a catalyst may require several measurements every 6 min until the absorbance reaches a maximum. Exclusion of air from the sample cuvette is important. The method is about 1000-fold more sensitive than thiosulfate titration The iodometric method with UVD at 360 was adopted for detecting the presence of hydroperoxides derived from protein, peptide or amino acid substrates subjected to y-radiation, after destroying the generated H2O2 with catalase. ... 

Spectrophotometric measurements of CD and CT values in oils or lipid extracts requires 45 min depending on the number of samples to be analyzed. The classical PV determination requires a bit of ti me for preparation of reagents and standardization of the sodium thiosulfate solution. The analysis itself is not very time consuming ( 30 min), but in all, about 3 to 3.5 hr should be anticipated. The FOX method has an assay time of < 10 min however, with sample preparation and time to construct the calibration curve, 2 to 3 hr should be expected. 

Assay Transfer about 1 g of sample, accurately weighed, into a 1000-mL flask containing 20 mL of water, and dilute to volume with water. Place 10 mL of this solution into a glass-stoppered flask, add 25 mL of sodium hydroxide TS, and allow the mixture to stand for 5 min. Add 25 mL of 0.1 A iodine, stopper the flask, allow the contents to stand in a cold, dark place for 10 min, and add 30 mL of 1 A sulfuric acid. Titrate the excess iodine with 0.1 A sodium thiosulfate, using starch TS as the indicator. Perform a blank determination (see General Provisions), and make any necessary correction. Each milliliter of 0.1 A iodine is equivalent to 0.9675 mg of... 

Assay Dissolve about 250 mg of sample, accurately weighed, in 15 mL of acetone contained in a 100-mL glass-stoppered bottle, and add 3 mL of a 1 2 solution of potassium iodide. Swirl for 1 min, then immediately titrate with 0.1 A sodium thiosulfate (without the addition of starch TS). Each milliliter of 0.1 A sodium thiosulfate is equivalent to 12.11 mg of C14H10O4. 

Assays of bromate and perbromate concentrations are required during the procedure. Bromate concentrations that are at least comparable to the perbromate may be determined iodometrically by reaction with sodium iodide in acid solution containing molybdate, followed by titration with standardized thiosulfate. After reduction of the bromate the solution should be ca. 0.1 M each in H+ and in free iodide ion. Perchloric, hydrochloric, or sulfuric acids may be used. The molybdenum(VI) concentration should be ca. 10 3 M. 

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