Titrations with Other Reducing Agents

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). 

Chlorate Analysis. Chlorate ion concentration is determined by reaction with a reducing agent. Ferrous sulfate is preferred for quaHty control (111), but other reagents, such as arsenious acid, stannous chloride, and potassium iodide, have also been used (112). When ferrous sulfate is used, a measured excess of the reagent is added to a strong hydrochloric acid solution of the chlorate for reduction, after which the excess ferrous sulfate is titrated with an oxidant, usually potassium permanganate or potassium dichromate. 

The determination of ascorbic acid in foods is based, in part, on its ability to be oxidized or to act as a reducing agent. The most common method for determination of vitamin C in foods is the visual titration of the reduced form with 2,6-dichloroindophenol (DCIP) (4-7). Variations in this procedure include the use of a potentiometric titration (6), or a photometric adaptation (S) to reduce the diflSculty of visually determining the endpoint in a colored extract. The major criticisms of this technique are that only the reduced vitamin, and not the total vitamin C content of the food, is measured, and that there can be interference from other reducing agents, such as sulfhydryl compounds, reductones, and reduced metals (Fe, Sn, Cu), often present in foods. The DCIP assay can be modified to minimize the effects of the interfering basic substances, but the measurement is still only of the reduced form. Egberg et al. (9) adapted the photometric DCIP assay to an automated procedure for continuous analysis of vitamin C in food extracts. 

A sample of impure tin of mass 0.535 g is dissolved in strong acid to give a solution of Sn. The solution is then titrated with a 0.0448 M solution of NO3 , which is reduced to NO(g). The equivalence point is reached upon the addition of 0.0344 L of the NO3 solution. Find the percent by mass of tin in the original sample, assuming that it contains no other reducing agents. 

In its solutions in liquid NIL. cesium is like the other alkali metals, a powerful reducing agent, so that in such solutions, titrations of cesium poly sulfide with cesium are made by eleclrumeiric methods. The solubility of cesium salts in liquid NHi increases markedly with the radius of an anion (the chloride. CsCI. 0.0227 moles per kg. the bromide. CsBr, 0.215 moles per kg. and the iodide. Csl. 5.84 moles per kg), though the values are less than for the corresponding rubidium compounds. 

NaHCOg and titrated in KI using starch as indicator. This gives the hypochlorite the chlorite is obtd by difference. Chlorates do not interfere with these analyses but the presence of other oxidg reducing agents may(Ref 7)(See also Ref 2, pp 306 7 Refs 4a ... 

Chromiimi(lI) and titanium(II[I) are very powerful reducing agents, but they are readily air-oxidized and difficult to handle. The standard potential of the former is -0.41 V (Cr3+/Cr2+) and that of the latter is 0.04 V (TiO"+/Ti +). The oxidized forms of copper, iron, silver, gold, bismuth, uranium, tungsten, and other metals have been titrated with chromium(II). The principal use of Ti " " is in the titration of iron(in) as well as copper(II), tin(IV), chromate, vanadate, and chlorate. 

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