Polarographic Reductions of Metal Ions

Many of metal ions (Mn+) are reduced at a DME to fonn metal amalgams [M(Hg)]  

If the reaction is reversible, the S-shaped current-potential curve in DC polarography is expressed by Eq. (8.1) (Section 5.3)  

We can determine from Eq. (8.2) the values of D for solvated metal ions. The value of D changes with changes in solvent or solvent composition. The viscosity (rj) of the solution also changes with solvent or solvent composition. However, the relation between D and tj can be expressed by the Stokes-Einstein relation  

Conversely, an increase or decrease in the value of Dt] reflects a decrease or increase in the radius of the solvated metal ion. This works as a criterion in determining the effect of solvents on metal ion solvation. 

If AGt°(M +, Si- S2) is the standard Gibbs energy of transfer of Mn+ from solvent Si to S2, there is a relationship  

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Polarographic reductions of metal ions are also influenced by the supporting electrolyte ... 

In polarographic reduction of metal ions Na" and <81MI 840-02) and, Co " ", and Ba <80M12), maximum waves in HMPA with n-Bu4N" CIO4 as supporting electrolyte were effectively suppressed by addition of salt (27 X = CIO4). 

The literature concerned with the polarographic reduction of metal ions on mercury in non-aqueous solvents is vast and has been reviewed by Mann and Barnes. Much less effort has been expended in the equally important field of the anodic behaviour of metal ions in such solvents. Redox couples, which may have very different standard potentials in non-aqueous media from those observed in aqueous solutions (e.g. Fe lFe ref. Q62), might form the basis of novel oxidative systems. 

So far, the reduction of metal ions into the metallic state was discussed involving a complete removal of the coordinated solvent molecules in the reduction process. We shall now consider such redox-systems in which both the oxidized and the reduced species are solvated. The polarographic reduction of Eu(III) to Eu(II) in different solvents occurs at such halt-wave potentials which are again related to the donicity of the solvent molecules118). In the Ei/j-DN plot a straight line is observed. Analogous results were obtained for the redox complexes Sm(III)-Sm(II) and Yb(III>Yb(II) 118> 120> (Fig. 27). 

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

The reduction of metal ions does not usually involve hydrogen ions and thus well formed waves can be obtained in unbuffered solutions. However the stability of many complexes formed between the metal and ligands in the solution is dependent on pH. Since changes in complexation will alter the polarographic behaviour of the metal ion, it is best to use a well buffered solution if complexing agents are present. 

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

Another example is provided by the polarographic reduction of 1,2-and 1,4-naphthoquinones (R) in various solvents in the presence of metal ions (21) ... 

Polarographic reductions have been studied for many kinds of metal ions and in a variety of lion-aqueous solvents. Large amounts of data on half-wave potentials are available and have been compiled in some books and reviews [18]. However, many of the old data were obtained using different reference electrodes or aqueous SCE, for which the problem of the liquid junction potential exists (Section 6.1.2). Gritzner [19] compiled half-wave potentials for metal ions as values referred to the BCr+/BCr system, which was recommended by IUPAC. Some of these are listed in Table 8.2. The potential of the BCr+/BCr system is not seriously affected either by the presence of water and other impurities or by differences in experimental conditions. Thus, although the determination of the half-wave poten-... 

Lineal log k. -DN relations have been observed for polarographic reductions of various metal ions [24],... 

Tab. 8.3 Rate constants and transfer coefficient for the polarographic reductions of alkali metal ions in DMF (25°C) [23b]...

Fig. 8.16 Influence of metal ions on the DC polarographic reduction wave of 1,2-naphthoquinone (0.5 mM) in AN-0.05 M Et4NCl04 [60 b]. Metal ions (5 mM) curve 1, none 2, l<+ ...

Olver and co-workers26,27 have studied the polarographic reduction of ZrCL and HfCL in acetonitrile with 0.1M [NEt4][C104] as the supporting electrolyte. Three plateaus were interpreted in terms of reduction to the III, II and metallic states. However, controlled potential electrolysis of ZrCL led to soluble [ZrCL]2- ions and poorly characterized precipitates, and no evidence for stable concentrations of Zr111 complexes could be found. Evidently, the Zr111 is rapidly oxidized by perchlorate, solvent or residual water. 

The electrochemistry of heteropolymolybdates parallels that of the tungstates but with the following differences the reduction potentials are more positive and the primed species (metal-metal bonded ) are much less stable. Scheme 7 applies for or-fSiMo O ]4-. Species in parentheses are detectable only by rapid scan cyclic voltammetry, and XVIII decomposes rapidly at 0°C. The reduced anions such as II and IV are easily obtained by controlled potential electrolysis or by careful chemical reduction, e.g. with ascorbate. The use of metal ion reductants generally leads to other reactions, (equation 7). The reduced anions slowly isomerize (equation 8). The isomerization can be followed polarographically (all S potentials are more positive) or by NMR spectroscopy. By this means / isomers of most Keggin and Dawson molybdates have been prepared. 

The oxidation-reduction potentials of metal ions differ in different solvents due chiefly to differences in the strength of coordination of the solvents to the metal ions. Thus, Schaap and coworkers,33 who measured reduction potentials polarographically in anhydrous ethylenediamine, found the order of half-wave potentials to be Cd2+ > Pb2+ > Cu2+ - Cu+ > Ti+, whereas, in aqueous solution, the order is Cd2+ > Ti+ > Pb2+ > Cu2+ -> Cu+. Oxidation—reduction potentials have been measured in a great variety of non-aqueous solvents, both protonic and non-protonic. Among the former are liquid ammonia and concentrated sulfuric acid.34 Among the latter are acetonitrile, cyanopropane, cyanobenzene, dimethyl sulfoxide, methylene chloride, acetone, tet-rahydrofuran, dimethylformamide and pyridine.34... 

Fig. 2, Energy cycle for the polarographic reduction of a solvated metal ion...

The applicability of donicities to cation-solvent interactions is most convincingly demonstrated by the polarographic reduction of various metal ions in solvents of different donicity. The observed variation of half-wave potentials with solvent donicity can be explained neither in terms of the Born equation nor by simple microscopic electrostatic models in view of the magnitude of the dipole moments of solvent molecules. The concept also provides the basis for an interpretation of complex formation reactions and the behaviour of electrolytes (ion pair equilibria) in a large number of EPD solvents. 

Similarly, more recent experiments on polarographic reduction of the metal ion failed to give any definite information on the valence state of copper in hemocyanin, either because the metal is very strongly linked to the protein or because it is hidden inaccessibly within the macromolecule (Klotz and Klotz, 1955). Despite numerous investigations the problem of the valence of copper in hemocyanin still remains unsolved. 

In the presence of many metal ions, diorthohydroxyazo dyes exhibit two polarographic reduction waves, the first due to free dye and the second to metal-dye complex. Highly sensitive analytical methods based on this principle have been developed for example, Ni or Fe may be determined in the presence of an excess of aluminum thank to thiazolylazo derivatives (563). 

In the application of the polarographic method of analysis to steel a serious difficulty arises owing to the reduction of iron(III) ions at or near zero potential in many base electrolytes. One method of surmounting the difficulty is to reduce iron(III) to iron(II) with hydrazinium chloride in a hydrochloric acid medium. The current near zero potential is eliminated, but that due to the reduction of iron(II) ions at about - 1.4 volts vs S.C.E. still occurs. Other metals (including copper and lead) which are reduced at potentials less negative than this can then be determined without interference from the iron. Alternatively, the Fe3 + to Fe2+ reduction step may be shifted to more negative potentials by complex ion formation. 

The ac polarograms of the mixture at varying frequencies (Fig. 19) shows four ac summit peaks corresponding to reduction of T1(I), In(III), Cd(II), and Zn(II). The summit peaks for In(III) and Cd(II) are very close and so their ac waves are not very sharp. The first summit peak corresponding to T1(I) appears to be due to the combined reduction of lead and thallium ions, as is evident from the summit peak height. Hence, ac polarographic analysis only enables the identification of four metal ions out of seven and... 

For an irreversible reduction the half-wave potential is determined not only by the standard electrode potential but also by the polarographic overvoltage. For a simple electrode process the metal ion-solvent interaction is mainly responsible for the polarographic overvoltage and hence E[ j of such irreversible reductions may also be considered as a function of the solvation 119f... 

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