Cerium, REDOX titrations

Oxidation states higher than + 3 are exhibited by Ce, Pr, and Tb, but only Ce4+ is stable (kinetically) in water. It is a very strong oxidizing agent in aqueous solution ( ° = 1.74 V) and is used as a volumetric standard in redox titrations. Some of its salts [e.g., cerium(IV) ammonium nitrate, cerium(IV) sulfate] find application in... 

Cerium (IV) solution in 0.1 M H2S04 is an ideal oxidant for many redox titrations. The half-reaction is as follows ... 

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... 

Consider the redox titration of iron(II) with a standard solution of cerium(IV). This reaction is widely used for the determination of iron in various kinds of samples. The titration reaction is... 

The data in the third column of Table 19-2 are plotted as curve B in Figure 19-3 to compare the two titrations. The two curves are identical for volumes greater than 25.10 mL because the concentrati ons of the two cerium species are identical in this region. It is also interesting that the curve for iron(Il) is symmetric around the equivalence point, but the curve for uranium(IV) is not. In general, redox titration curves are symmetric when the analyte and titrant react in a 1 1 molar ratio. 

In analytical chemistry, a redox titration is based on an oxidation-reduction reaction between analyte and titrant. Common analytical oxidants include iodine (I2), permanganate (MnOJ), cerium(IV), and dichromate (Cr207 ). Titrations with reducing agents such as Fe " (ferrous ion) and Sn " (stannous ion) are less common because solutions of most reducing agents need protection from air to prevent reaction with O2. 

The readily occurring transition from colorless Ce + to bright yellow or orange Ce" + forms the basis for the use of cerium(IV) sulfate solutions in redox titrations ( cerimetric analysis). The ease of access to various tetravalent cerium compounds makes cerium(IV) most valuable in research as well as in various practical applications. Important fields of application for cerium(IV) compounds include organic syntheses, bioinorganic chemistry, materials science, and industrial catalysis (e.g., vehicle emissions control, oxygen storage). ... 

The standard redox potential is 1.14 volts the formal potential is 1.06 volts in 1M hydrochloric acid solution. The colour change, however, occurs at about 1.12 volts, because the colour of the reduced form (deep red) is so much more intense than that of the oxidised form (pale blue). The indicator is of great value in the titration of iron(II) salts and other substances with cerium(IV) sulphate solutions. It is prepared by dissolving 1,10-phenanthroline hydrate (relative molecular mass= 198.1) in the calculated quantity of 0.02M acid-free iron(II) sulphate, and is therefore l,10-phenanthroline-iron(II) complex sulphate (known as ferroin). One drop is usually sufficient in a titration this is equivalent to less than 0.01 mL of 0.05 M oxidising agent, and hence the indicator blank is negligible at this or higher concentrations. 

M cerium(IV) solution, and the equivalence point is at 1.10 volts. Ferroin changes from deep red to pale blue at a redox potential of 1.12 volts the indicator will therefore be present in the red form. After the addition of, say, a 0.1 per cent excess of cerium(IV) sulphate solution the potential rises to 1.27 volts, and the indicator is oxidised to the pale blue form. It is evident that the titration error is negligibly small. 

Ce4+ is yellow and Ce3+ is colorless, but the color change is not distinct enough for cerium to be its own indicator. Ferroin and other substituted phenanthroline redox indicators (Table 16-2) are well suited to titrations with Ce4+. 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

Whenever the system being titrated forms a reversible redox couple with its reaction product, the second electrode used in the generation reaction must be shielded from the bulk of the sample solution. For example, in the titration of iron(II) with anodically generated cerium(TV), the cathode is placed in a separate compartment to prevent the reduction of iron(III). In this example, iron(II) undergoes direct anodic oxidation during the bulk of the titration until the bulk concentration of iron(II) is so low that its rate of mass transfer can no longer sustain the applied current. At this point the intermediate oxidation of cerium(III) permits 100% current efficiency to be maintained to the end point. 

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,... 

As an example of redox reaction we chose the titration of cerium (IV) with tin (II). The titration is based on the following reaction ... 

In coulometric titrations, cerium(IV) is generated in situ by electrolysis. Pastor et al. (1982) studied the anodic generation of cerium(IV). The formal redox potential of the Ce" +/Ce + system decreases as concentration of potassium acetate in the solution increases. 

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... 

In some applications, the oxidation of the analyte by cerium(IV) is very slow. In this case, an alkali iodide can be added as a catalyst. Ce is able to oxidize 1 very fast to h, and iodine acts as the redox active species in the titration reaction. The iodide ions that are formed are regenerated by oxidization by Ce. A typical example is the iodide-catalyzed oxidation of arsenic(ni) by cerium(IV) (Yates and Thomas, 1956). 

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