Indicator Redox Indicators

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

In this article, the physicochemical principles of redox reactions are outlined as a preliminary to an account of the function of redox indicators. Redox indicators are sensitive to the pH of a solution as well as to its redox potential, and the nature of this dependence is described. There then follows a discussion of the various types of redox indicators used, and Table 1 gives details of a range of these indicators. 

Oxidation-Redaction Indicators.—Redox Indicators.— Many dyestuffs and natural pigments are converted to colourless leuco compounds on reduction. The change is reversible, and the colour intensity of the compound is determined by proportions of the components present. 

Used as a O.lmM soln. in EtOH as an acid-base indicator redox indicator for titanometric detn. of Fe(//7),... 

Na salt Used as acid-base indicator redox indicator. Used as 0.02% aq. soln. Red cryst. Sol. H2O. 6.1. E° -bO.563 V. 

Thus, for example, an analysis using coloured solutions can be carried out, where an indicator cannot be used. Moreover, it is not easy to find a redox indicator which will change colour at the right point. Potentiometric methods can fairly readily be made automatic. 

A selected list of redox indicators will be found in Table 8.26. A redox indicator should be selected so that its if" is approximately equal to the electrode potential at the equivalent point, or so that the color change will occur at an appropriate part of the titration curve. If n is the number of electrons involved in the transition from the reduced to the oxidized form of the indicator, the range in which the color change occurs is approximately given by if" 0.06/n volt (V) for a two-color indicator whose forms are equally intensely colored. Since hydrogen ions are involved in the redox equilibria of many indicators, it must be recognized that the color change interval of such an indicator will vary with pH. 

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. 

The relationship between a redox indicator s change in color and the solution s electrochemical potential is easily derived by considering the half-reaction for the indicator... 

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. 

Redox doping Red oxide Redox indicators Redox polymers REDOX process Redox reactions Red PDC [80-22-8]... 

AH classes of azine dyes are vattable, ie, they are reduced to "colorless" forms, then oxidized back to the dye (2). They therefore offer good fastness to oxidation (3). Because of this property, many find uses as redox indicators in titrations. 

FIGURE 21.2 Experimental apparatus used to measure the standard reduction potential of the indicated redox couples (a) the acetaldehyde/ethanol couple, (b) the fumarate/succi-nate couple, (c) the Fe /Fe" couple. 

Low-spin, octahedral complexes are formed by ligands such as bipy, phen and CN , and their stability is presumably enhanced by the symmetrical configuration. [Fe(bipy)3] + and [Fe(phen)3] + are stable, intensely red complexes, the latter being employed as the redox indicator, ferroin , due to the sharp colour change which occurs when strong oxidizing agents are added to it ... 

Variamine blue (C.I. 37255). The end point in an EDTA titration may sometimes be detected by changes in redox potential, and hence by the use of appropriate redox indicators. An excellent example is variamine blue (4-methoxy-4 -aminodiphenylamine), which may be employed in the complexometric titration of iron(III). When a mixture of iron(II) and (III) is titrated with EDTA the latter disappears first. As soon as an amount of the complexing agent equivalent to the concentration of iron(III) has been added, pFe(III) increases abruptly and consequently there is a sudden decrease in the redox potential (compare Section 2.33) the end point can therefore be detected either potentiometrically or with a redox indicator (10.91). The stability constant of the iron(III) complex FeY- (EDTA = Na2H2Y) is about 1025 and that of the iron(II) complex FeY2 - is 1014 approximate calculations show that the change of redox potential is about 600 millivolts at pH = 2 and that this will be almost independent of the concentration of iron(II) present. The jump in redox potential will also be obtained if no iron(II) salt is actually added, since the extremely minute amount of iron(II) necessary is always present in any pure iron(III) salt. 

Procedure. Prepare the indicator solution by dissolving 1 g variamine blue in 100 mL de-ionised water as already pointed out (Section 10.48), variamine blue acts as a redox indicator. 

A list of selected redox indicators, together with their colour changes and reduction potentials in an acidic medium, is given in Table 10.9. 

C. Potentiometric methods. This is a procedure which depends upon measurement of the e.m.f. between a reference electrode and an indicator (redox) electrode at suitable intervals during the titration, i.e. a potentiometric titration is carried out. The procedure is discussed fully in Chapter 15 let it suffice at this stage to point out that the procedure is applicable not only to those cases where suitable indicators are available, but also to those cases, e.g. coloured or very dilute solutions, where the indicator method is inapplicable, or of limited accuracy. 

For the titration of colourless or slightly coloured solutions, the use of an indicator is unnecessary, since as little as 0.01 mL of 0.02 M potassium permanganate imparts a pale-pink colour to 100 mL of water. The intensity of the colour in dilute solutions may be enhanced, if desired, by the addition of a redox indicator (such as sodium diphenylamine sulphonate, AT-phenylanthranilic acid, or ferroin) just before the end point of the reaction this is usually not required, but is advantageous if more dilute solutions of permanganate are used. 

The green colour due to the Cr3+ ions formed by the reduction of potassium dichromate makes it impossible to ascertain the end-point of a dichromate titration by simple visual inspection of the solution and so a redox indicator must be employed which gives a strong and unmistakable colour change this procedure has rendered obsolete the external indicator method which was formerly widely used. Suitable indicators for use with dichromate titrations include AT-phenylanthranilic acid (0.1 per cent solution in 0.005M NaOH) and sodium diphenylamine sulphonate (0.2 per cent aqueous solution) the latter must be used in presence of phosphoric) V) acid. 

Method B Standardisation with sodium oxalate. Standardisation may also be carried out with sodium oxalate in this case, an indirect procedure must be used as the redox indicators are themselves oxidised at the elevated temperatures which are necessary. The procedure, therefore, is to add an excess of the cerium(IV) solution, and then, after cooling, the excess is determined by... 

The introduction of reversible redox indicators for the determination of arsenic(III) and antimony(III) has considerably simplified the procedure those at present available include 1-naphthoflavone, and p-ethoxychrysoidine. The addition of a little tartaric acid or potassium sodium tartrate is recommended when antimony(III) is titrated with bromate in the presence of the reversible... 

Reagents. Methylene blue solution. Dissolve 0.1 g of the solid (use redox indicator quality) in 100 mL distilled water. 

Potassium chloride (nitrate) bridge 583, 582 Potassium chromate as indicator, 343, 349 Potassium cyanoferrate(II) D. of, (ti) 384 Potassium cyanoferrate(III) D. of, (ti) 399 Potassium cyanonickelate(II) prepn., 328 Potassium dichromate solution analyses involving, 375 oxidising properties of, 375 internal indicators for, 377 preparation of, 0.02M, 375 redox indicators for, 377 standardisation of, by iron, (cm) 546, (ti) 376... 

Ferrocyanides stability, 6, 830 Ferrocytochrome c oxidation, 6, 621 Ferroin, 4,1203 redox indicator. 1,558 Ferrokinetics... 

FIGURE 6-14 DNA hybridization biosensors detection of DNA sequences from the E. coli pathogen. Chronopotentiometric response of the redox indicator upon increasing the target concentration in 1.0 pg/ml steps (a-c), in connection with a 2 min hybridization time. (Reproduced with permission from reference 46.)... 

A weighed amount of sample is dissolved in a mixture of propanone and ethanoic acid and titrated potentiometrically with standard lead nitrate solution, using glass and platinum electrodes in combination with a ferro-ferricyanide redox indicator system consisting of 1 mg lead ferrocyanide and 0.5 ml 10% potassium ferricyanide solution. The endpoint of the titration is located by graphical extrapolation of two branches of the titration plot. A standard solution of sodium sulfate is titrated in the same way and the sodium sulfate content is calculated from the amounts of titrant used for sample and standard. (d) Water. Two methods are currently available for the determination of water. 

The application of modified electrodes for the assay of antibodies in senun preparations using redox indicators encapsuled into antigene marked liposomes attached to an electrode surface was suggested First model studies towards this goal make use of ferricyanide ions entrapped in synthetic vesicles. 

The course of the reaction has not been elucidated. Probably redox reactions involving cerium(IV) and arsenic(III) are catalyzed by iodide ions and organic iodine compounds with methylene blue acting as a redox indicator. 

Cacotheline is a redox indicator which is yellow in the oxidized form and reddish-violet in the reduced form. 

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