Iodine-thiosulfate peroxide

Peroxide levels were determined by titration of a slurry of the particles with iodine-thiosulfate. 

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). 

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 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). 

The liberated iodine may be titrated using std thiosulfate soln, or, in trace analysis, detd by spectrophotometric methods. Other reducing agents commonly used in peroxide analysis are hydriodic acid, ferrous, titanous, stannous, and arsenious ions. Also (recently), triphenylphos-phine, which is oxidized to triphenyl phosphine oxide. The excess triphenyl phosphine may be detd gravimetric ally, tit rime trically, or spectro-photometrically... 

When unconsumed perbenzoate is present, a yellow ring is immediately formed between the two phases. If this test indicates the presence of peroxide, the extract should not be concentrated and distilled until it has been extracted first with a solution of potassium iodide in acetic acid to remove peroxide and then with aqueous sodium thiosulfate to remove iodine. 

A fresh sample of this 40% peracetic acid contains about 1.54 equivalents, or 0.77 mole, of peroxide per 100 ml. of solution, corresponding to 1.34 equivalents per 100 g. The concentration can be determined by treating the peroxide solution with potassium iodide and titrating the liberated iodine with standard sodium thiosulfate. The concentration of peroxide in peracetic acid decreases somewhat on long standing and should be checked before the peracetic acid is used. The yield of diacetate is lowered if the concentration of the peroxide is less than 1.0 equivalent per 100 g. of peracetic acid. The total amount of peroxide used should be 2.4 moles, or 4.8 equivalents, for each mole of iodo-benzene. 

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. 

The ethereal solution is usually dark but should not have a purple color. A purple color indicates the presence of iodine. Iodine can arise by light-catalyzed reaction in the latter stages of the reaction and during isolation of the product. For this reason the reaction should be shielded from strong light. In addition it is advisable for the ether employed in the reaction mixture to be peroxide-free. If iodine is present in the reaction product, it must be removed by extraction with aqueous sodium thiosulfate solution since an adequate separation is not obtained by distillation. 

A) Anon, IEC, NewEdn, 14, 305(1936)(Peroxide formation in ethers and a test for peroxides in ether) [Procedure a) Add 10ml ether (sample to test) to 150ml of 2N sulfuric acid soln, followed by 3 drops of 1% soln of Amm molybdate (a catalyst to favor the liberation of dine) and 15ml of 10% KI soln. Shake well aiiJ allow to stand for 15 mins b) Titrate the liberated iodine with 0.05N Na thiosulfate and shake well after each addn until near discoloration of soln c) Add a few cc of starch soln and continue titration until disappearance of blue color]... 

The determination of peroxide value (PV) by an iodometric titration is described in Basic Protocol 2. Iodine is liberated by hydroperoxides in the oil in the presence of excess iodide in a stoichiometric ratio. The amount of iodine present is determined by titration with a standard sodium thiosulfate solution using a starch indicator, thereby reflecting how much peroxide is present in the oil or lipid extract. 

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. 

Allow the sample solution to stand, agitating it occasionally, for exactly 1 min, and immediately add 30 mL of water. Titrate with 0.1 A sodium thiosulfate solution, adding the solution gradually while constantly agitating until the yellow iodine color has almost disappeared. Add 0.5 mL of a 10% sodium lauryl sulfate (SDS) solution, and then add approximately 0.5 mL of Starch Indicator Solution. Continue the titration while constantly agitating, especially near the endpoint to liberate all of the iodine from the solvent layer. Add 0.1 A thiosulfate solution dropwise until the blue color just disappears. If the titration is less than 0.5 mL using 0.1 A sodium thiosulfate, repeat the determination using 0.01 A sodium thiosulfate. Conduct a blank determination, and make any necessary correction. Calculate the peroxide value by the formula... 

The iodine clock experiment can be carried out using peroxide in place of iodate and sodium thiosulfate in place of arsenious acid. The rate law, by analogy to Eq. (3), is... 

Sodium thiosulfate reacts with alkyl halides to form salts of the type RSSOjNa (Bunte salts). Alkyl disulfides may be obtained from these salts by pyrolysis or reaction with iodine or hydrogen peroxide. The yields range from 47% to 6S>%. Cyano and carboxyl groups do not interfere. Benzoylation of sodium thiosulfate produces benzoyl disulfide in 58% yield. ... 

Methods based on iodine titration with thiosulfate Iodide, being a weak reduc-tant, can react with an enormous variety of oxidants to liberate an equivalent quantity of I2 that can be titrated with thiosulfate. Such oxidants include peroxides, peroxy compounds, peroxydisulfate, ozone, iron(III), chromate, selenium (as Se03 ), silver(II) oxide, xenon trioxide, iodate, and bromate. Bromide can be determined by oxidizing it to bromine, followed by extraction and determination of the bromine... 

A measured amount of fat is dissolved in acetic acid-chloroform solvent in the presence of potassium iodide. Peroxides in the fat liberate iodine, which is titrated with 0.10 N sodium thiosulfate. A blank run is similarly treated. Peroxide value (meq peroxide oxygen/kg oil = (B — 5) x 0.10 A thiosulfate x 1000/g oU where B = titer of blank in mL and S = titer of sample in mL. 

Iodide ion is a moderately effective reducing agent. In its applications, a standard solution of sodium thiosulfate is used to titrate the iodine liberated by reaction of the analyte with an unmeasured excess of potassium iodide. Some substances determined by using iodo-metric method are 104 , lOs", BrOs, ClOs, Br2, CI2, O2, O3, Cu " ", N02, and organic peroxide. 

Benzoyl disulfide has been obtained by the reaction of benzoyl chloride with hydrogen sulfide, hydrogen disulfide, hydrogen trisulfide, potassium sulfide, sodium disulfide, lead sulfide, sodium hydrosulfite, sodium thiosulfate, sulfhydrylmagnesium bromide, and thiobenzamide. It is also formed by reaction of benzoic anhydride with hydrogen sulfide. The better preparative methods involve the oxidation of thiobenzoic add by means of air,hydrogen peroxide or sulfur monochloride, or of the sodium or potassium salt by means of air, - chlorine, iodine, copper sulfate, - potassium ferricyanide, - or ferric chloride. - ... 

Peracetic acid (b), commercial 40% peracetic acid. Material available from the Buffalo Electro Chemical Co. (Becco) has the composition 40% peracetic acid, 5% hydrogen peroxide, 39% acetic acid, 1% sulfuric acid, and 15% water. The density is 1.15 g. per ml. A fresh sample contains 0.77 mole of peracetic acid per 100 ml. Since the peracid content decreases somewhat on standing, an old sample should be analyzed by treating an aliquot with potassium iodide and titrating the liberated iodine with standard sodium thiosulfate. For certain uses it is advisable to neutralize the sulfuric acid present by addition of sodium acetate. 

In a 300-cc. Erlenmeyer flask, 0,5 g. of benzoyl peroxide is dissolved in 15 cc. of chloroform. The solution is cooled to —5° and 25 cc. of o.i N ice-cold sodium methylate solution is added at once with cooling and shaking. After four to five minutes at — 5°, 100 cc. of iced water, 5 cc. of 10 per cent sidfuric acid, and 2 g. of potassium iodide in 20 cc. of 10 per cent sulfuric acid are added in the order mentioned with violent stirring. The liberated iodine is titrated with o.i N sodium thiosulfate solution. One cubic centimeter of 0.1 N sodium thiosulfate solution is equivalent to 0.0121 g. of benzoyl peroxide. 

The determination of peroxide number of aviation turbine fuel is important because of the adverse effects of peroxides on certain elastomers in the fuel system. In the test, the sample is dissolved (unlike ASTM D-6447) in l,l,2-trichloro-l,2,2-trifluoroethane and is contacted within an aqueous potassium iodide solution. The peroxides present are reduced by the potassium iodide, whereupon an equivalent amount of iodine is released that is titrated with standard sodium thiosulfate solution and a starch indicator. 

The peroxide number of petroleum wax (ASTM D-1832) is determined by dissolving a sample in carbon tetrachloride, acidifying with acetic acid, and adding a solution of potassium iodide any peroxides present will react with the potassium iodide to liberate iodine, which is then titrated with sodium thiosulfate. 

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