Redox titration with visual indicator

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

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). 

Selection of a Visual Indicator for a Redox Titration Because of the relatively small number of indicators available and their pH dependence, selection is not as straightforward as in the case of acid-base titrations. For example, iron(ll) may be titrated with cerium(IV) or chromium(VI) (table 5.4), whilst equation (5.9) in conjunction with table 5.5... 

The end-point of titrations with cerium(IV) solutions can be detected visually (without or with use of a redox indicator) or potentiometrically. Whereas the intense purple color of a permanganate solution allows an easy visual detection of the end point, the yellow-orange color of cerium(IV) solutions is often not intense enough to act as an indicator. Only in a limited number of cases, for instance when oxalic acid or hydrogen peroxide is the analyte, can the titration be made without a redox indicator, provided that the concentrations of the analyte are not too low and that an appropriate blank correction is made. It is easier to detect the end point in hot solutions than in cold solutions, because of an intensification of the yellow color of the cerium(IV) ion with a rise in temperature. A large blank correction is required... 

A titration is the process of determining the quantity of a substance by adding measured increments of another substance, the titrant. The latter is almost always added as a standardised solution (or by electrolyte generation, as in a coulometric titration). The end-point of the titration, which should indicate the addition of an exact chemical equivalence, is recognized by a visual indicator or instrumentally. Titrations are based on acid-base reactions (for determination of acids or bases), redox reactions (for determining oxidants or reductants), chelating reactions (usually with EDTA-type compounds, for determination of metal ions) or precipitations (usually of halides or pseudohalides with silver ions). 

The visual detection of the sharp change in redox potential in the titration of an iron(III) salt with EDTA is readily made with variamine blue as indicator. 

The green colour due to the Cr3+ ions formed by the reduction of potassium dichromate makes it impossible to ascertain the end-point of a dichromate titration by simple visual inspection of the solution and so a redox indicator must be employed which gives a strong and unmistakable colour change this procedure has rendered obsolete the external indicator method which was formerly widely used. Suitable indicators for use with dichromate titrations include AT-phenylanthranilic acid (0.1 per cent solution in 0.005M NaOH) and sodium diphenylamine sulphonate (0.2 per cent aqueous solution) the latter must be used in presence of phosphoric) V) acid. 

Titrimetric luminescence methods record changes in the indicator emission of radiation during titration. This change is noted visually or by instruments normally used in luminescence analysis. Most luminescence indicators are complex organic compounds which are classified into fluorescent and chemiluminescent, compounds according to the type of emission of radiation. As in titrimetry with adsorption of colored indicators, luminescence titration makes use of acid-base, precipitation, redox, and complexation reactions. Unlike color reactions, luminescence indicators enable the determination of ions in turbid or colored media and permit the detection limit to be lowered by a factor of nearly one thousand. In comparison with direct luminescence determination, the luminescence titrimetric method is more precise. 

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