Redox buffer strength

Equations 16-9 and 16-10 are analogous to the Henderson-Hasselbalch equation of acid-base buffers. Prior to the equivalence point, the redox titration is buffered to a potential near E+ = formal potential for Fc 1 Fe2+ by the presence of Fe 1 and Fe2+. After the equivalence point, the reaction is buffered to a potential near E+ = formal potential for Ce4+ Ce3+. [R. de Levie Redox Buffer Strength, J. Chem. Ed. 1999, 76, 574.]... 

The simultaneous presence of the oxidized and reduced form of a redox couple can stabilize the redox potential of a solution, just as the presence of an acid and its conjugate base can stabilize the pH. The formalism (R. de Levie, /. Chem. Educ. 76 (1999) 574) is quite similar to that of section 4.7, except that there are no terms for the oxidation or reduction of the solvent, because these are typically non-equilibrium processes which, moreover, are insignificant in the usual range of potentials considered. By analogy to (4.7 -1) we write, for the redox buffer strength B of a one-electron redox couple Ox+ e Red, such as Fe3++ e-—Fe2+orCe4++ e Ce3+,... 

The redoxbuffer strength serves the same role for the potential of a solution as the acid-base buffer strength serves for its pH. In both cases it is assumed that the corresponding equilibria are established quickly on the time scale of the experiment. With redox equilibria, which often involve bond breaking, this condition is less often met than with acid-base equilibria, where fast establishment of equilibrium is the norm. 

Fig. 5.10-2 The redox buffer strength of an aqueous vanadium solution of 0.01 M analytical concentration at pH = 0, calculated from (5.10-3) or by differentiation of the progress curve of Fig. 5.9-3. Again the two agree to within the computational accuracy of the differentiation algorithm used.

In fact, any type of titration can be carried out potentiometrically provided that an indicator electrode is applied whose potential changes markedly at the equivalence point. As the potential is a selective property of both reactants (titrand and titrant), notwithstanding an appreciable influence by the titration medium [aqueous or non-aqueous, with or without an ISA (ionic strength adjuster) or pH buffer, etc.] on that property, potentiometric titration is far more important than conductometric titration. Moreover, the potentiometric method has greater applicability because it is used not only for acid-base, precipitation, complex-formation and displacement titrations, but also for redox titrations. 

While ionic strengths as low as 1 mM have been used with the cell illustrated in Figure 1, most LCEC experiments are carried out with a minimum of 0.05 M buffer salts in the mobile phase. Postcolumn mobile phase changes (pH, ionic strength, solvent content) and post-column reactions (redox cross reactions, derivatiza-tions, enzyme catalyzed reactions) can expand the utility of electrochemical as well as other detectors. These subjects have recently been treated in some detail (9). Suffice it to say that direct detection, without post-column chemistry, is always preferable (more reliable, more sensitive, less expensive). 

In - voltammetry, the electrode redox reaction necessarily involves a change in ionic strength near the electrode (i.e., an ion is consumed, produced, or undergoes a change in charge number). Therefore, the activity coefficients of the electroactive species vary as a function of time and distance from the electrode. In this case, the presence of an excess of a - supporting electrolyte serves as an ionic strength buffer [iv],... 

The general chemistry of NAD(P)" and NAD(P)H will not be covered or further commented on in this chapter, except that NAD(P)H is relatively stable in aqueous solutions at pHs more alkaline than pH 7 and NAD(P)+ at pHs more acidic than pH 7. The stability is very much dependent on the buffer constituents and ionic strength. The pH where both redox forms together exhibit minimal destruction due to acid/base decomposition is found between pH 7 and 8, depending on whether the aqueous medium is unbuffered and when buffered, on the buffer constituents. In general, the stability of NADPH is less dependent on the buffer than is that of NADH. The reader is advised to refer to... 

Fig. 6 Cyclic voltammetric analysis of the kinetics of an electrode coated with antigen-antibody immobilized monomolecular layer of redox enzyme with a one-electron reversible cosubstrate in the solution, (a) Cyclic voltammetry at saturation coverage (2.6 x 10 mol cm ) of glucose oxidase with 0.1 M glucose and 0.1 mM ferrocenemethanol in a pH 8 phosphate buffer (0.1 M ionic strength). The dotted and dashed lines represent the cyclic voltammogram (0.04 V sec ) in the absence and presence of glucose (0.1 M), respectively. The full line represents the catalytic contribution to the current,/ cat (see text), (b) Primary plots obtained under the same conditions with, from top to bottom, 0.01, 0.02, 0.05, and 0.1 M glucose, (c) Secondary plot derived from the intercepts of the primary plots in (b).

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