Oxidation/reduction indicators

This important oxidation - reduction indicator is readily prepared by a double Skraup reaction (compare Section V,l) upon o-phenylenediamine ... 

Table 8.26 Selected List of Oxidation-Reduction Indicators 8.122...

A. Internal oxidation-reduction indicators. As discussed in Sections 10.10-10.16, acid-base indicators are employed to mark the sudden change in pH during acid-base titrations. Similarly an oxidation-reduction indicator should mark the sudden change in the oxidation potential in the neighbourhood of the equivalence point in an oxidation-reduction titration. The ideal oxidation-reduction indicator will be one with an oxidation potential intermediate between... 

An oxidation-reduction indicator (redox indicator) is a compound which exhibits different colours in the oxidised and reduced forms ... 

One of the best oxidation-reduction indicators is the 1,10-phenanthroline-iron(II) complex. The base 1,10-phenanthroline combines readily in solution with iron(II) salts in the molecular ratio 3 base l iron(II) ion forming the intensely red l,10-phenanthroline-iron(II) complex ion with strong oxidising agents the iron(III) complex ion is formed, which has a pale blue colour. The colour change is a very striking one ... 

Oxidation number 851, (T) 853 rules for determining. 851 Oxidation number method 851 Oxidation-reduction cells 64 Oxidation-reduction curve 361 Oxidation-reduction indicators (T) 367 Oxidation-reduction reactions 67, 258, 360,... 

The principal use of 1,10-phenanthroline and several of its derivatives is, of course, as analytical reagents particularly for the determination of metals and in the form of their metal complexes as oxidation-reduction indicators. These uses of 1,10-phenanthrolines have been well... 

OXIDATION-REDUCTION INDICATOR. A substance that has a color in the oxidized form different from that of the reduced form and can be reversibly oxidized and reduced. Thus, if diphenylamine is present in a ferrous sulfate solution to which potassium dichromate is being added, a violet color appears with the first drop of excess dichromate. 

Ferroin is one of the most commonly used oxidation-reduction indicators. It is a 1,10-phenanthroline complex of iron having the following structure ... 

The electron activity (or intensity) at redox equilibrium may be measured by a potentiometer. A pH meter or a millivolt meter may be used for measuring the potential difference between a reference electrode (such as a calomel electrode) and an oxidation-reduction indicator electrode (such as platinum, gold, or a wax-impregnated graphite electrode). 

Methylene blue is a dye intermediate that is also used as an oxidation-reduction indicator. Its molecular formula in the trihydrate form is C16H18N3SC1 3 HzO. It is soluble in water, forming a deep blue solution. The structure of methylene blue is as follows ... 

Oxidation-Reduction Indicators.—A reversible oxidation-reduction indicator is a substance or, more correctly, an oxidation-reduction system, exhibiting different colors in the oxidized and reduced states, generally colored and colorless, respectively. Mixtures of the two states in different proportions, and hence corresponding to different oxidation-reduction potentials, will have different colors, or depths of color every color thus corresponds to a definite potential which depends on the standard potential of the system, and frequently on the hydrogen ion concentration of the solution. If a small amount of an indicator is placed in another oxidation-reduction system, the former, acting as a potential mediator, will come to an equilibrium in which its oxidation-reduction potential is the same as that of the system under examination. The potential of the given indicator can be estimated from its color in the solution, and hence the potential of the system under examination will have the same value. 

Since the eye, or even mechanical devices, are capable of detecting color variations within certain limits only, any given oxidation-reduction indicator can be effectively employed only in a certain range of potential. Consider, for example, the simple case of an indicator system for which n is unity the oxidation-reduction potential at constant hydrogen ion concentration is given approximately by... 

Suppose the limits within which color changes can be detected are 9 per cent of oxidized form, i.e., o/r is 9/91 1/10, at one extreme, to 91 per cent of oxidized form, i.e., o/r is 91/9 10 the corresponding potential limits at ordinary temperatures are then given by the foregoing equation as + 0.058, and iS — 0.058, respectively. If n for the indicator system had been 2, the limits of potential would have been JB - - 0.029 and B — 0.029. It is seen, therefore, that an oxidation-reduction indicator can be used for determining the potentials of unknown systems only if the values lie relatively close to the standard potential JB of the indi-... 

The standard potential of the indicator system is not known exactly, but experiments have shown that in not too strongly acid solutions the sharp color change from colorless to violet, with green as a possible intermediate, occurs at a potential of about — 0.75 volt. The standard potential of the ferrous-ferric system is 0.78 whereas that of the di-chromate-chromic ion system in an acid medium is approximately — 1.2 volt hence a suitable oxidation-reduction indicator might be expected to have a standard potential of about — 0.95 volt. It would thus appear that diphenylamine would not be satisfactory for the titration of ferrous ions by acid dichromate, and this is actually true if a simple ferrous salt is employed. In actual practice, for titration purposes, phosphoric acid or a fluoride is added to the solution these substances form complex ions with the ferric ions with the result that the effective standard potential of the ferrous-ferric system is lowered (numerically) to about — 0.5 volt. The change of potential at the end-point of the titration is thus from about — 0.6 to — 1.1 volt, and hence diphenylamine, changing color in the vicinity of — 0.75 volt, is a satisfactory indicator. 

If the solution contains a reversible oxidation-reduction indicator as well, its potential must be the same as that for the system ... 

A redox indicator is essentially a compound that has oxidized and reduced forms with different, and preferably intense, colors (Table 3). The half-reaction responsible for color change in a typical oxidation-reduction indicator can be written as ... 

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. 

The half-reaction responsible for color change in a typical general oxidation/ reduction indicator can be written as... 

Of all the oxidation/reduction indicators, ferroin approaches most closely the ideal substance. It reacts rapidly and reversibly, its color change is pronounced, and its solutions are stable and easily prepared. In contrast to many indicators, the oxidized form of ferroin is remarkably inert toward strong oxidizing agents. At temperatures above 60°C, ferroin decomposes. 

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