Sodium thiosulfate Iodometric methods

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

Chlorine gas may be identified readdy by its distinctive color and odor. Its odor is perceptible at 3 ppm concentration in air. Chlorine may be measured in water at low ppm by various titrimetry or colorimetric techniques (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington DC American Pubhc Health Association). In iodometric titrations aqueous samples are acidified with acetic acid followed by addition of potassium iodide. Dissolved chlorine liberates iodine which is titrated with a standard solution of sodium thiosulfate using starch indicator. At the endpoint of titration, the blue color of the starch solution disappears. Alternatively, a standardized solution of a reducing agent, such as thiosulfate or phenylarsine oxide, is added in excess to chlorinated water and the unreacted reductant is then back titrated against a standard solution of iodine or potassium iodate. In amperometric titration, which has a lower detection limit, the free chlorine is titrated against phenyl arsine oxide at a pH between 6.5 and 7.5. 

Iodometric Titration Method Iodometric titration assay, which is based on the oxidation of the iodide ion (1 ) by hydroperoxides (ROOH), is the basis of current standard methods for determination of PV (9). In this method, a samrated solution of potassium iodide is added to oil samples to react with hydroperoxides. The liberated iodine (I2) is then titrated with a standardized solution of sodium thiosulfate and starch as an endpoint indicator (7, 9, 20). The PV is obtained by calculation and reported as milliequivalents of oxygen per kilogram of sample (meq/kg). The official determination is described by lUPAC (21). Chemical reactions involved are given below ... 

Alicino l described an iodometric method for the determination of penicillins. Only after alkaline or penicillinase hydrolysis the penicillins consume iodine. The difference in consumption of iodine before and after hydrolysis is proportional to the quantity of the antibiotic. Russo-Alesi used the iodometric titration for estimation of ampicillin in formulations. Ampicillin solutions were hydrolyzed with sodium hydroxide for 30 minutes. After acidification the iodine solution was added. After an additional 30 minutes, the excess of iodine was titrated with thiosulfate solution. A blank consisted of unhydrolyzed ampicillin. The ampicillin standard was treated in exactly the same manner and used in calculations. The hydrolysis of ampicillin with penicillinase instead with sodium hydroxide makes the method more selective. 

In another popular chemical method, hexacyano-ferrate(ni) is reduced to hexacyanoferrate(II), and the unchanged Fe(III) can be quantitatively determined, e.g., by iodometric back-titration with sodium thiosulfate. Modifications have been proposed to allow direct photometric quantification. Other titri-metric methods have also been proposed, for example, using an iodometric method with vanadium(V) in perchloric acid as reagents. The simple reduction methods are regarded as nonspecific due to the interference of other similar compounds with reducing properties. 

This method is one of the oldest and most commonly used measurements of the extent of oxidation in oils. The standard iodometric procedures measure by titration, or colorimetric or electrometric methods, the iodine produced by potassium iodide added as a reducing agent to the oxidized sample dissolved in a chloroform-acetic acid mixture. The liberated iodine is titrated with standard sodium thiosulfate to a starch endpoint. The peroxide value (PV) is expressed as milliequivalents of iodine per kg of lipid (meq/kg), or as millimole of hydroperoxide per kg of lipid (referred to as peroxide). PV expressed as meq/ Kg = 2 X PV mmol/kg. 

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