Potential half wave

An important point on the curve is that at which the diffusion current is equal to one-half of the total diffusion current the voltage at which this current is reached is the half-wave potential. The 

When this is the case, it can be shown that there is a relation between the potential, the current, and the diffusion current i l. which holds true throughout the polarographic wave  

This equation, in fact, may be practically used to test the reversibility (or Nernstian behavior) of an electroactive species. The graph of E versus log[(iL - i)/i] will be linear with a slope of 0.0591/n and intercept Ey. Determination of the slope enables us to determine n, the number of electrons involved in the process. Substitution of iJ2 in Equation 15.40 reveals that E = Ey when the surface concentrations [ox] and [red] are equal. 

however, that the half-wave potential Ey is usually similar but not exactly equivalent to the thermodynamic standard potential E°. First, the product of reduction may be stabilized by amalgam formation in metal-ion reductions second, there will always be a small liquid junction potential in electrochemical cells of this type that should be corrected for and finally, it can be shown that the potential Ey is the sum of two terms  

In most analytical work, the activity corrections are ignored and This is because the size 

---

Half-wave potential Ey2 In the same place ibid. 

Table 8.30 Half-Wave Potentials of Inorganic Materials 8.141...

Table 8.31 Half-Wave Potentials (vs. Saturated Calomel Electrode) of Organic...

Section 8 now combines all the material on electrolytes, electromotive force, and chemical equilibrium, some of which had formerly been included in the old Analytical Chemistry section of earlier editions. Material on the half-wave potentials of inorganic and organic materials has been thoroughly revised. The tabulation of the potentials of the elements and their compounds reflects recent lUPAC (1985) recommendations. 

Graphical determination of electrochemical reversibility, n, and half-wave potential in linear scan hydrodynamic voltammetry. 

In the absence of ligand the half-wave potential occurs when [R]x=o and [O] c=o are equal thus, from the Nernst equation we have... 

If the ligand is present in excess and the formation constant is sufficiently large, such that all of O is present as the complex, then [R]x=o and [OFp] c=o are equal at the half-wave potential, and equation 11.44 simplifies to... 

A voltammogram for the two-electron reduction of M has a half-wave potential of —0.226 V versus the SCE. In the presence of an excess of the ligand L, the following half-wave potentials are recorded... 

Selectivity Selectivity in voltammetry is determined by the difference between half-wave potentials or peak potentials, with minimum differences of+0.2-0.3 V required for a linear potential scan, and +0.04-0.05 V for differential pulse voltammetry. Selectivity can be improved by adjusting solution conditions. As we have seen, the presence of a complexing ligand can substantially shift the potential at which an analyte is oxidized or reduced. Other solution parameters, such as pH, also can be used to improve selectivity. 

This experiment describes the determination of the stability (cumulative formation) constant for the formation of Pb(OH)3 by measuring the shift in the half-wave potential for the reduction of Pb + as a function of the concentration of OH . The influence of ionic strength is also considered, and results are extrapolated to zero ionic strength to determine the thermodynamic formation constant. 

The limiting current was 5.67 tA. Verify that the reduction reaction is reversible, and determine values for n and 1/2. The half-wave potentials for the normal pulse polarograms of Pb + in the presence of several different concentrations of OH are shown in the following table. 

In some cases two polarographic waves are seen as the redox reaction occurs in steps. Half-wave potentials are given for each wave, with the corresponding change in oxidation states shown in parentheses. 

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

Polarographic studies on haloisoxazoles in anhydrous DMF containing R4N" were performed and the magnitude of the half-wave potentials were recorded. Cleavage of the C—X bond was faster in phenylhaloisoxazoles than in halobenzenes. Substitution patterns affected the reduction (79ZOB1322). 

---