Iodine, REDOX reagents

Potassium permanganate and iodine, which are important redox reagents, are both self-indicating, i.e. the colour of the reagent in each case is intense and will impart a perceptible colour to a solution when present in very small excess. One drop of a solution of potassium permanganate (0.02 mol dm 3) can be detected in a titrand solution of 100 cm3, and a similar amount of iodine by shaking the titrand with 5 cm3 of chloroform or carbon tetrachloride to produce an intense purple colour. Specific indicators react in a specific manner with one participant in the reaction. The best examples are starch, which produces an intense blue colour with iodine and potassium thiocyanate, which forms an intense red compound with iron(III). 

Applications of iodine as a redox reagent are extensive because (7) its E° is intermediate and therefore it can act as either an oxidizing agent (as Ij) or a reducing agent (as I3 ), and (2) its is nearly independent of pH (at values less than about 8), a useful property when conditions for favorable conditional equilibrium constants are being selected. We include in this chapter a few examples to illustrate the range of application. 

Other REDOX reagents include iodine (I2), either by itself in a forward titration or in a back titration with sodium thiosulfate (Na2S2Os), and complex salts of the metal cerium (such as ammonium cerium sulfate,... 

Apart from acid-base titrations various addition-, substitution- and redox-reactions have been found to be of analytical interest. Iodine numbers of fats and essential oils may be determined i and bromine may be used to titrate organic compounds which can form bromoderivatives i. For the titration of phenol with bromine the addition of sodium acetate has been recommended. Redox reagents are in acetic acid chromium(VI) oxide, sodium permanganate, bromine, titanium(III) chloride and chromium(II) saltsi32,i33 xhe titrations are usually carried out in perchloric acid solutions and in an inert atmosphere but traces of water are tolerable. 

In this chapter, we begin our description of the principal titrimetric methods involving a redox reaction. Such methods are sometimes studied in the part of analytical chemistry named oxidoreductimetry. The first tidimetric methods we shall study are direct and indirect iodometries, two methods that are very closely related. They constitute an important part of the methods involving iodine and iodide ions as redox reagents. 

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

The concept of reduction potential is introduced in Chapter 6. When the reduction potentials of two species differ by 0.1 V or more, the resulting redox reaction will proceed rapidly and stoichiometrically so that it may be used as the basis for a titrimetric procedure. The end point of a redox titration may be observed by following the potential of the titrand with an indicator electrode or with a visual indicator. In two special cases, the reagent (potassium permanganate and iodine) is self-indicating (vide infra). 

Redox titrations are among the most important types of analyses performed in many areas of application, for example, in food analyses, industrial analyses, and pharmaceutical analyses. Titration of sulfite in wine using iodine is a common example. Alcohol can be deteirnined by reacting with potassium dichromate. Examples in clinical laboratories are rare since most analyses are for traces, but these titrations are still extremely useful for standardizing reagents. You should be familiar with some of the more commonly used titrants. 

Based on AA oxidation to dehydroascorbic acid in acidic medium using iodine-iodide solution as oxidizing reagent. The iodine amount consumed in the redox reaction was detected Electrocatalysis of AA on a glassy carbon electrode chemically modified with polyaniline films AA was determined at a vitreous C electrode modified with 3,4-dihydroxybenzaldehyde AA was determined with a chemically modified with methylene green (electron mediator) carbon paste electrode... 

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