Selected Polarographic Half-Wave Potentials

Source All values from Dean, J. A. Analytical Chemistry Handbook. McGraw-Hill New York, 1995. 

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The rather narrow electrochemical window of water, limited by the discharge of hydrogen and oxygen, has stimulated the use of nonaqueous solvents for electrochemical reactions. Procedures for measuring and reporting electrode potentials in nonaqueous solvents are presented in reference [128]. The solvent influence on the redox properties of cations and anions has been reviewed [172], as have applications of ion-selective electrodes in nonaqueous solvents [129] and the influence of nonaqueous solvents on the polarographic half-wave potentials of cations [173]. 

Figure 38. Polarographic half-wave potentials of selected elements measured in the following supporting electrolyte solutions A) 1 mol/L KCl B) 2 mol/L CH,COOH-r2 mol/L CH3COONH4 C) 1 mol/L NHj+ 1 mol/L NH4CI D) 1 mol/L HCl ...

The sensitivity and selectivity can be raised when recording as a function of potential not the current but its derivative with respect to potential. In this case a curve with maximum is obtained (Fig. 23.4) instead of the polarographic wave. The potential of the maximum corresponds to the half-wave potential in an ordinary polarographic curve, and the height of the maximum is proportional to the concentration of the substance being examined. A signal proportional to the derivative can be formed in polarographs with the aid of relatively simple electric circuitry. 

In the aqueous phase, Umland and Wallmeier [80UML/WAL] studied the reduction of selenite at the mercury electrode in the presence of Zn by polarography, see Appendix A. The solubility product of ZnSe(s) was obtained from the position of the half-wave potential of the second reduction step, HgSe(s) + Zn "" + 2e" ZnSe(s) + Hg(l), to be log (ZnSe, s, 298.15 K) = -(23.212.0). Combined with CODATA for Zn and the selected value for Se l it corresponds to AfG° (ZnSe, s, 298.15 K) = - (151.0 11.8) kJ mol. The value is in disagreement with the value obtained by extrapolation of high temperature data. Since the validity of the polarographic method has not been documented, the value of AfG°(ZnSe, a, 298.15 K) obtained from the extrapolation of the high temperature data has been selected. 

Badiello, Feroci, and Fini [96BAD/FER] made polarographic measurements of 1 X 10 M Cd in 0.15 M NaN03 containing selenate concentrations from 0 to 1 x 10 M. The half-wave potential, 1/2, did not change with selenate concentration and it was concluded that no complex formation occurred. The total expected change in 1/2 would be less than 3 mV if the value of the stability constant determined by Banks is correct. It is doubtful if the experimental accuracy of the polarographic work is sufficient to detect it. The value of the stability constant from the conductivity measurements will be selected ... 

Polarographic studies gave no evidence for the existence of the bivalent oxidation states of selected actinides in acetonitrile solution. Only one wave corresponding to reduction of americium(iii) or curium(iii) to the zero-valent state was observed and experiments with berkelium(iii) and einsteinium(iii) failed to give conclusive results because of rapid radiolysis of the acetonitrile solution. A study of the electrochemical reduction of americium, thulium, erbium, samarium, and europium showed that the elements did assume the bivalent state with the actinide bivalent cations having a smaller stability than the lanthanides. The half-wave potential of nobelium was found to be —1.6 V versus the standard hydrogen electrode for the reaction... 

The materials tested, and the half-wave potentials measured, are given in Table 4. If the final polarographic analysis of the pyrolysis products and the corresponding nitro-compounds is carried out in three different buffered solutions, this interesting method would gain additional selectivity. 

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