Titrations with Other Oxidizing Agents

Potassium permanganate is a widely used oxidizing titrant. It acts as a self-indicator for end-point detection and is a very strong oxidizing agent (JE° = 

The solution must be heated for rapid reaction. The reaction is catalyzed by the Mn product and it goes very slowly at first until some Mn is formed. Pure electrolytic iron metal can also be used as the primary standard. It is dissolved in acid and reduced to Fe + for titration (see Section 14.8). 

An added effect of complexing the iron(III) is that the phosphate complex is nearly colorless, while the chloro complex (normally present in chloride medium) is deep yellow. A sharper end-point color change results. 

The Z-R reagent prevents oxidation of Cl by Mn04 and sharpens the end point. 

Oxidation of chloride ion is not a problem with dichromate. However, the formal potential of the Cr207 /Cr + couple is reduced from 1.33 to 1.00 V in 1 M hydrochloric acid, and phosphoric acid must be added to reduce the potential of the Fe /Fe- couple. Such addition is also necessary because it decreases the equivalence point potential to near the standard potential for the diphenylamine sulfonate indicator (0.84 V). Otherwise, the end point would occur too soon. 

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The quantitative conversion of thiosulfate to tetrathionate is unique with iodine. Other oxidant agents tend to carry the oxidation further to sulfate ion or to a mixture of tetrathionate and sulfate ions. Thiosulfate titration of iodine is best performed in neutral or slightly acidic solutions. If strongly acidic solutions must be titrated, air oxidation of the excess of iodide must be prevented by blanketing the solution with an inert gas, such as carbon dioxide or... 

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 recovery of iodine from waste liquids.—E. Beilsteini2 recovered iodine from laboratory residues by evaporation to dryness with an excess of sodium carbonate and calcination until the organic matter is all oxidized. The mass is dissolved in sulphuric acid and treated with the nitrous fumes, obtained by treating starch with nitric acid, until all the iodine is precipitated. The iodine is washed in cold water, dried over sulphuric acid, and sublimed. Other oxidizing agents less unpleasant than the nitrous fumes employed by F. Beil stein—e.g. hydrogen peroxide—-were recommended by G. Torossian for the residues obtained in copper titrations. F. Beilstein s process is applicable to soluble but not to insoluble, oxidized forms of ioffine. F. D. Chattaway... 

The presence of other oxidizing agents such as manganese(IV) can interfere with the chlorine determination if the titration is performed at pH<3.5. Under this condition, the organic chloramines are often converted to either monochloramine or dichloramine, resulting in positive errors for these substances. Also, the volatilization of chlorine compounds can result in low recoveries. This can occur due to the violent agitation or aeration of some of the chloramines prior to the addition of iodide. 

Potassium iodate is a fairly strong oxidizing agent that may be used in the assay of a number of pharmaceutical substances, for instance benzalkonium chloride, cetrimide, hydralazine hydrochloride, potassium iodide, phenylhydrazine hydrochloride, semicarbazide hydrochloride and the like. Under appropriate experimental parameters the iodate reacts quantitatively with both iodides and iodine. It is, however, interesting to observe here that the iodate titrations may be carried out effectively in the presence of saturated organic acids, alcohol and a host of other organic substances. 

In many situations, accurate determination of an ion concentration by direct measurement of a cell potential is impossible due to the presence of other ions and a lack of information about activity coefficients. In such cases it is often possible to determine the ion indirectly by titration with some other ion. For example, the initial concentration of an ion such as Fe2+ can be found by titrating with a strong oxidizing agent such as Ce4+. The titration is carried out in one side of a cell whose other half is a reference electrode ... 

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. 

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