General redox indicators

The most important class of redox indicators, however, are substances that do not participate in the redox titration, but whose oxidized and reduced forms differ in color. When added to a solution containing the analyte, the indicator imparts a color that depends on the solution s electrochemical potential. Since the indicator changes color in response to the electrochemical potential, and not to the presence or absence of a specific species, these compounds are called general redox indicators. 

A partial list of general redox indicators is shown in Table 9.18. Examples of appropriate and inappropriate indicators for the titration of Fe + with Ce + are shown in Figure 9.37. 

Two types of chemical indicators are used to obtain end points for oxidation/reduc-tion titrations general redox indicators and specific indicators. 

Color changes for general redox indicators depend only on the potential of the system. 

General redox indicators Indicators that respond to changes in... 

The sodium salt of these acids may be used to prepare aqueous solutions of indicators. Other examples of redox indicators include starch, potassium thiocyanate, methylene blue, and phenosafranine. Some selected general indicators in redox titrations are listed in Table 1.6.3. The properties of starch as an indicator in iodometric titration are discussed in the following section. 

Some redox indicators react specifically with one form of a redox couple to cause a visible color change. Two examples are starch as an indicator for iodine and thiocyanate for Fe(III). Such indicators, being limited in scope, are not amenable to general treatment. 

The driving force for metal deposition on a semiconductor electrode is determined by the difference between the Fermi level of the electrode and the reversible potential of the metal species in the solution. For the noble metals this difference at the OCP is generally negative indicating that the deposition process is spontaneous, which is the basis for electroless deposition. As shown in Fig. 6.8, the redox potentials of the... 

Because most redox indicators respond to changes in electrode potential, the vertical axis in oxidation/reduction titration curves is generally an electrode potential instead of the logarithmic p-functions that were used for complex formation and... 

General oxidation/reduction indicators are substances that change color on being oxidized or reduced. In contrast to specific indicators, the color changes of true redox indicators are largely independent of the chemical nature of the analyte and titrant and depend instead on the changes in the electrode potential of the system that occur as the titration progresses. 

For titrations of reductants with oxidants, a redox indicator is required to indicate when the potential of the solution has reached that at equivalence (B in Figure 1). For an observer to see that an indicator has fully changed color, it is generally taken that the ratio [Indred]/[Indox] needs to change from 1 10 to 10 1. Application of the Nernst equation shows that this involves a change of potential of 0.12/ i d V, that is 120 mV for a one-electron indicator or 60 mV for a two-electron indicator at room temperature. 

These are two-electron redox indicators with (, at 0.54-0.69V. Quoted E q values are generally extrapolations, as these compounds decompose in strong acid. They are especially useful in alkaline... 

Many diazo dyes have been examined as redox indicators. Although distinct color changes are observed for many on oxidation, the changes are nonreversible, and, in general, these compounds have no advantages over other classes of reversible indicators. 

Although a reagent e.g. permanganate may act as a self indicator because of its intense colour and the very faint colour of its reduced form Mn in acid solution, generally a redox indicator is necessary to find the end-point visually. A redox indicator is usually an organic molecule which has different colours in the oxidised and reduced forms i.e. In ,( and ln d. The ratio of their concentration is given by ... 

Redox Electrodes Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Ee + and Ee + in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry. 

The main by-products of the Ullmaim condensation are l-aniinoanthraquinone-2-sulfonic acid and l-amino-4-hydroxyanthraquinone-2-sulfonic acid. The choice of copper catalyst affects the selectivity of these by-products. Generally, metal copper powder or copper(I) salt catalyst has a greater reactivity than copper(Il) salts. However, they are likely to yield the reduced product (l-aniinoanthraquinone-2-sulfonic acid). The reaction mechanism has not been estabUshed. It is very difficult to clarify which oxidation state of copper functions as catalyst, since this reaction involves fast redox equiUbria where anthraquinone derivatives and copper compounds are concerned. Some evidence indicates that the catalyst is probably a copper(I) compound (28,29). 

Ox and Red are general symbols for oxidation and reduction media respectively, and n and (n-z) indicate their numerical charge (see Section 2.2.2). Where there is no electrochemical redox reaction [Eq. (2-9)], the corrosion rate according to Eq. (2-4) is zero because of Eq. (2-8). This is roughly the case with passive metals whose surface films are electrical insulators (e.g., A1 and Ti). Equation (2-8) does not take into account the possibility of electrons being diverted through a conductor. In this case the equilibrium... 

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

So far, electrochemical measurements have not provided any direct proof for the formation of a bipolaron state in oligbmers or polymers which is significantly more stable than the polaron state. In general, in terms of energy the redox potentials E° for bipolaron formation should be much lower than the potentials Ej for polaron formation (/E / < /E /). However, more recent electrochemical and ESR spectroscopic studies by Nechtschein et al. indicate that the bipolaron state is not much more stable than the polaron state... 

These measurements indicate that it is not possible to identify a single value of pe surrounding the O2/H2S interface in the environment. Redox couples do not respond to the pe of the environment with the same lability as hydrogen ion donors and acceptors. There is no clear electron buffer capacity other than the most general states of "oxygen containing" or "H2S containing." The reason for the vast differences in pec in the oxic waters is the slow oxidation kinetics of the reduced forms of the redox couples. The reduced species for which the kinetics of oxidation by O2 has been most widely studied is Mn. This oxidation reaction... 

The enzyme poised at well-defined redox potentials appears to be in rather homogenous IR states 65, 84). Curiously, in corresponding EPR-monitored experiments, the Ni signal generally corresponds to significantly less than one spin/mole, indicating that the sample is heterogeneous with respect to it (77). 

---