Standardization thiosulfate

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. 

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). 

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, 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). 

Ana.lysls. The available chlorine (av CI2) in hypochlorite solutions or soHds is deterrnined by reaction with aqueous KI, followed by acidification with either acetic or sulfutic acid and titration of the Hberated iodine with standard thiosulfate. The av CI2 in a hypochlorite is a measure of the oxidi2ing capacity expressed in terms of elemental chlorine one hypochlorite ion is equivalent to one CI2 molecule. Thus pure Ca(OCl)2 has an av CI2 of 2 x mol wt... 

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. 

Diacetoxyiodobenzene (iodobenzenediacetate) [3240-34-4] M 322.1, m 163-165 . Purity can be checked by treatment with H2S04then KI and the liberated I2 estimated with standard thiosulfate. It has been recrystd from 5M acetic acid and dried overnight in a vac desiccator over CaCl2. The surface of the crystals may become slightly yellow but this does not affect its usefulness. [Sharefkin and Saltzman Org Synth Coll Vol V 600 1973.]... 

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). 

Redox (reduction-oxidation) titrimetry is used primarily for nitrate detns. Five systems are in current use ferrous sulfate—dichromate, io dome trie, periodic acid oxidation (NaOH titrant), K permanganate, and titanous chloride-ferric ammonium sulfate. The ferrous sulfate— dichromate system is used for MNT DNT detns (Vol 2, C162-Lff Vol 6, F17-Rff Ref 17). In the iodometric procedure, the sample (ie, NG) is treated in a C02 atm with a satd soln of Mn chloride in coned HC1, the vol reaction products are bubbled thru a K iodide soln, and the liberated iodine is titrated with standard thiosulfate soln (Refs 1 17). The periodic... 

Iodine liberation is one of the oldest and most commonly used methods for assessing lipid substrate oxidation. In this method, hydroperoxides and peroxides oxidize aqueous iodide to iodine, which is then titrated with standard thiosulfate solution and starch as endpoint indicator. The peroxide value is calculated as milliequivalents of peroxide oxygen per kilogram of sample. 

In this work, halogens were added across the double bonds of unsaturated fatty acids (commercial available sunflower oil) to form addition compounds and the degree of uptake was measured. A measured excess of iodine monobrontide (IBr) was allowed to react with the oil in the dark. At the end of a timed period, an excess of K1 was added to convert the remaining IBr to The 1 formed was then titrated with standard thiosulfate. This was a back-titration method. A back-titration blank was also required, where the sample was omitted. The uptake of iodine was calculated... 

A sample of organolithium compound solution is added to an excess of standardized iodine solution in Et20 the excess iodine is extracted with aqueous KI and titrated with standard thiosulfate solution. Evaluation of the organometallic compound is made according to equation. A possible interference can be expected from the couphng reaction in equation 20. This was shown to be neghgible, as demonstrated spectrofluorometrically for phenyllithium , whereas a low titer was found for n-BuLi, attributable to this side... 

Although simple in conception and straightforward, the standard methods for POV determination require strict observance of analytical quality to avoid interference of artifacts. For example, according to the AOCS methods , the sample dissolved in AcOH-CHCI3 (method Cd 8-53) or AcOH-isooctane (method Cd 8b-90) solvent is treated with aqueous KI and the liberated iodine is titrated with standard thiosulfate solution, using starch indicator. Similar standard methods and modifications have been reported in various countries . 

Suppose that mass mA is analyzed in Experiment A and volume VA of standard thiosulfate is required for titration. Let the corresponding quantities in Experiment B be mB and VB. Let the average oxidation state of Cu in the superconductor be 2+p. Then p is given by (18)... 

When a reducing analyte is titrated directly with iodine (to produce 1 ), the method is called iodimetry. In iodometry, an oxidizing analyte is added to excess I to produce iodine, which is then titrated with standard thiosulfate solution. 

Freshly acidified iodate plus iodide can be used to standardize thiosulfate. The I3 must be used immediately or else it is oxidized by air. The disadvantage of KI03 is its low molecular mass relative to the number of electrons it accepts. This property leads to a larger-than-desirable relative weighing error in preparing solutions. 

Add S02 (or H2S03 or HS03 or SOj-) to excess standard If in dilute acid and back-titrate unreacted If with standard thiosulfate. 

Oxidizing agents can be treated with excess I to produce Iy (Table 16-5, Box 16-2). The iodometric analysis is completed by titrating the liberated Iy with standard thiosulfate. Starch is not added until just before the end point. 

The peroxide value (PV) of an oil or fat is defined as the quantity of peroxide oxygen present in the sample. This classical iodometric method is a volumetric analysis based on the titration of iodine released from potassium iodide by peroxides in a biphasic system using a standardized thiosulfate solution as the titrant and a starch solution as the indicator (see Background Information, discussion of peroxide value). This method will detect all substances that oxidize potassium iodide under the acidic conditions of the test, therefore the purity of the reagents is critical. 

Standardization of the thiosulfate solution is important. Various factors can affect its stability. Chief among these are pH, the presence of microorganisms and impurities, the presence of atmospheric oxygen, and exposure to sunlight. Proper attention to detail will yield standard thiosulfate solutions that need only occasional... 

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