Dissociation constant from electrode potential data

Using the spectral data of Fig. 22, and similar data obtained for the nitrophorins in the absence of NO and in the presence of histamine, imidazole, or 4-iodopyrazole, Nernst plots such as that shown in the insert of Fig. 22 were constructed, and the midpoint potentials of the nitrophorins and their NO and histamine complexes were calculated. The results are summarized in Table IV, where they are compared to those obtained earlier for NPl (49, 50, 55). All potentials are expressed vs NHE (+205 mV with respect to the Ag/AgCl electrode used in the spectroelectrochemical titrations and the Nernst plot shown in the insert of Fig. 22). It can be seen that the reduction potentials of all four nitrophorins in the absence of NO or histamine are within 20-40 mV of each other. The reduction potentials of their NO complexes, however, differ significantly from each other. For example, the reduction potential of NP4-NO is about 350 mV more positive than that of NP4 in the absence of NO, as compared to a 430 mV shift for NPl upon binding NO, and the positive shifts for NP2—NO and NP3—NO are somewhat smaller (318 and 336 mV, respectively, at pH 7.5) 49, 50). These differences relate to the ratios of the dissociation constants for the two oxidation states, as discussed later. 

We will use standard electrode potentials throughout the rest of this text to calculate cell potentials and equilibrium constants for redox reactions as well as to calculate data for redox titration curves. You should be aware that such calculations sometimes lead to results that are significantly different from those you would obtain in the laboratory. There are two main sources of these differences (1) the necessity of using concentrations in place of activities in the Nernst equation and (2) failure to take into account other equilibria such as dissociation, association, complex formation, and solvolysis. Measurement of electrode potentials can allow us to investigate these equilibria and determine their equilibrium constants, however. 

Abstract. The characterization of cyclodextrin(CD) systems by electrochemical methods, mainly by cyclic voltammetry is discussed. The addition of CD to the electrolyte solution causes a decrease in the peak current and also a shift in the apparent half-wave potential in cyclic voltammetry. Quantitative analysis in the both phenomena affords the formation constants of CD complexes. The formation or dissociation rate constants can be evaluated from the cyclic voltammetric data at high scan rates. Adsorption of CD on the electrode surface is also mentioned. 

---