process are unfavourable. EXAMPLE 5.3 Using the electrochemical series uples in
ID: 575826 • Letter: P
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process are unfavourable. EXAMPLE 5.3 Using the electrochemical series uples in Table 5.2 is the permanganate ion, MnO, the common analytical reagent used in redox titrations of iron. Which of the ions Fe", Cl-, and Ce can permanganate oxidize in acidic solution? Answer We need to note that a reagent that is capable of reducing MnO; ions must be the reduced of a redox couple having a more negative standard potential than the couple MnO/Mn . The standard potential of the couple Mno;/Mn2: in acidic solution i+1.5T Vs The standard potentials. of FeyFett GA/CI and CetYCe't are t07z +1.36, and +17 Mnoions are sufficiently strong oxidizing agents in acidic solution (pH 0) to o which have less positive standard potentials. Permanganate ions cannot oxidize Ce which has a mow positive standard potential. It should be noted that the presence of other ions in the solution can modify the potentials and the conclusions (Section 5.10); this variation with conditions is particularly important in the case of Ht ions, and the influence of pH is discussed in Section 5.6. The ability of Mno-io oxidize Cl means that HC)cannot be used to acidifty redox reactions involving permanganate but H.s0 6 V respectively. It follows that and Cl is used instead Self-test 5.3 Another common analytical oxidizing agent is an acidic solution of dichromate ions Cr.0 for which E+ (Cr,O', Cr = + 1.38 V. Isthe solution useful tor a redox titration of Fe2+ to Fe3+?Could I there be a side reaction when Cl- is present?Explanation / Answer
The reduction potential of Cr2O72- is +1.38 V.
The reduction potential of Fe3+ is +0.77 V.
As the reduction potential of Cr2O72- is more positive than that of Fe3+, this means that it would be able to oxidise Fe2+. More the reduction potential, more easily itself gets reduced and oxidises other specie. So, acidic solution of dichromate ions would be helpful in redox reaction of Fe2+ to Fe3+.
The reduction potential of Cl- is +1.36 V, which is still lesser than dichromate ions. So, it would be able to oxidise Cl- ions also. So, when Cl- ion is present, there will be a side reaction going on, although it is not of much difference.
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