Errors with Nemst equation

When considering potentiometric errors, it is necessary to appreciate how a liquid junction potential, Ej, arises, and appreciate how such potentials can lead to significant errors in a calculation. In addition, we saw how the IR drop can affect a potentiometric measurement (and described how to overcome this). Finally, we discussed the ways that potentiometric measurements are prone to errors caused by both current passage through the cell, and by the nature of the mathematical functions with which the Nemst equation is formulated. 

Compound I and Compound II) is required. The standard redox potential is then calculated from the Nemst equation. Errors in redox potentials estimated with this method may be in the order of 10 mV. This is the most commonly used method to determine redox potential of peroxidases [63, 66, 68, 69], and the redox agent is frequently K3hCl6 with E° = 0.93 V. However, it has been discussed that the species present in the equilibrium might not be equivalent to Compound I and Compound II formed in the presence of peroxide [100]. However, this drawback may be overcome in a recently reported modification of this method [64, 65]. By using stopped-flow spectrophotometry, it has been possible to utilize hydrogen peroxide as the redox agent, with errors of 3 mV or less. The redox potentials for mammalian peroxidases in Table 4.4 have been determined following this approach [65, 70, 71]. 

While the redox titration method is potentiometric, the spectroelectrochemistry method is potentiostatic [99]. In this method, the protein solution is introduced into an optically transparent thin layer electrochemical cell. The potential of the transparent electrode is held constant until the ratio of the oxidized to reduced forms of the protein attains equilibrium, according to the Nemst equation. The oxidation-reduction state of the protein is determined by directly measuring the spectra through the tranparent electrode. In this method, as in the redox titration method, the spectral characterization of redox species is required. A series of potentials are sequentially potentiostated so that different oxidized/reduced ratios are obtained. The data is then adjusted to the Nemst equation in order to calculate the standard redox potential of the proteic species. Errors in redox potentials estimated with this method may be in the order of 3 mV. 

The Nicolsky equation With common pH instruments, the Nemst equation can accurately predict the pH of an unknown solution, over the pH range 2 to 12. At more extreme values of pH, some response to other ionic species in solution begins to become apparent. This is the so-called salt error. For example, a vahnomydn-doped BLM for K" " suffers from some interference because of Na+. A better equation proposed by Nicolsky should be used. Accordingly,... 

Most ionophores form complexes not only with the analyte ion but also with the aqueous co-ions. Interference by co-ions is a major origin of experimental errors in the analysis. The Nemst equation (7.1.1), however, does not describe the contribution of the interfering ions to the measured potential. Traditionally, the influence of the interfering ions on the potentiometric responses has been described using the Nikolsky-Eisenman equation (38)... 

For impurity species present in dilute concentrations, some may find it more convenient to treat the species using dilute solution theory, which accounts only for interactions of the dilute species with the solvent Rigorously, Equation 12 was derived for a binary electrolyte with no impurity species in the solution. While it is not completely rigorous to treat one species with dilute solution theory while treating the main electrolyte with equations derived from concentrated solution theory in the absence of the impurity species, the error may be small. The flux of the dilute species is given by Equation 5. The mass balance for the main electrolyte remains unchanged. If 2 is defined by Equation 3, then Us must be defined as a function of the concentration of the impurity species in order to include the concentration overpotential of the impurity species in the kinetic expression. Equation 53. The Nemst equation. Us = Uf + /JTln( Ci cf). is often used to account for the concentration overpotential of dilute species i. If Og is defined by Equation 6, then Us should not be defined as a function of solution composition. 

These errors arising from asymmetric titration curves, as well as from interfering ion indications were reckoned with by Carr [219,220]. He started with the empirical form of the Nemst equation ... 

In comparing equation (16a) with experiment, we must remember that the assumptions made by Nernst in calculating the values of C must also be used in calculating Q. By doing so we compensate partially, at least, for the errors due to our ignorance of the specific heats at lower temperatures. In using Nemst s values of 0, we must therefore always assume that the specific heat of a gas (see p. 411) is given by = 3-5 4-w X 1-5 4-oT. 

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