Ferroin redox indicator

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 ... 

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. 

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

A redox indicator is a compound that changes color when it goes from its oxidized to its reduced state. The indicator ferroin changes from pale blue (almost colorless) to red. 

Ce4+ is yellow and Ce3+ is colorless, but the color change is not distinct enough for cerium to be its own indicator. Ferroin and other substituted phenanthroline redox indicators (Table 16-2) are well suited to titrations with Ce4+. 

Examples of redox indicators are Indigo Carmine (C.I. 73015 [ 860-22-0] 8), which is frequently employed, and ferroin, an iron(n) complex with three 1,10-phenanthroline ligands (14), which is widely used in water and wastewater analysis in the determination of chemical oxygen demand (COD) in an internationally standardized procedure. For the structure of the organic ligand, see p. 539. 

Dipyridyl and orthophenanthroline form particularly stable complexes with iron. The octahedral [Fe(phcn)3] + ion (Fig. 2C4) is blood-red but is oxidised to pale blue [Fe(phen)3] without any structural change. for the system = 1.14 V, making the compound, also known as ferroin, a most useful redox indicator for the oxidation of Fe + ion Fe +/Fe + = 0.77 V) by cerium(IV) ion (E Ce +/Ce + = 1.45 V). 

The addition of the redox indicator ferroin causes the classical oscillation system, which was first described in detail hy A. M. Zhabotinsky, to go through the various differently colored stationary states, which undergo interconversion at regular intervals. 

Ferroin A common name for the l,10-phenanthroline-iron(ll) complex, which is a versatile redox indicator. Its formula is (C 2HsN2)3Fe2+. 

The complex of 1,10-phenanthroline with Fe(II) is called ferroin and has been widely used in titrimetric analysis as a redox indicator. 

About 25 intermediates are formed in the reaction, while the final products are C02 and bromo derivatives of malonic acid. Ferroin, a redox indicator, contains the system Fe(II)/Fe(III). The oxidized form is blue, whereas the reduced form is red. 

Ferroin is a redox indicator visualizing changes in concentration of the Ce(IV)/Ce(III) system, rather stable under reaction conditions blue colour corresponds to an excess of the Fe(III) ions, red colour corresponds to an excess of the Fe(II) ions. Proportions of the ferroin components are given in Table 6.2. 

The excess of K2Cr207 is determined by back titration with a ferrous salt with a redox indicator, ferroin ... 

Ferroin , Fe(phen)3, has been used since 1933 as a redox indicator because with the first excess of oxidant Fe(phen)3 is immediately formed with a concomitant colour change from red to blue this redox reaction is reversible. By introducing substituents the redox potential is altered to give a range of indicators from which the correct one can be chosen for each situation, Values of (volts) in 1 M H2SO4 are as follows 5-nitro (1.25), 5-sulfono (1.20), ferroin itself (1.06), 5-methyl (1.02), 5,6-dimethyl (0.97), 4,7-dimethyl (0.88), and 3,4,7,8-tetramethyl (0.81). 2,3,4-Substituted phenanthroline complexes of Fe and Cu show a linear relationship between q and the dissociation constant of the ligand. 

The excess of potassium dichromate ions is back-titrated against a ferrous solution using Ferroin as redox indicator ... 

An indicator (In) can be used to detect the end point of a redox titration, just as an indicator can be used in an acid-base titration. A redox indicator changes color when it goes from its oxidized to its reduced state. One common indicator is ferroin, whose color change is from pale blue (almost colorless) to red. 

To understand what a redox indicator is and to determine if ferroin acts purely as an indicator or if it affects the reaction dynamics. 

After several periods of oscillations, add 2 mL of a 25 mM ferroin solution (available from Fisher). Ferroin (tris(l,10-phenanthroline)iron(II) sulfate) is a redox indicator. As the (Ce(IV)) increases, it can oxidize the iron in ferroin from iron(II) to iron(lll). The iron(II) complex is red and the iron(III) complex is blue consequently, the color changes as the potential changes. What effect does the ferroin have on the period and amplitude of the oscillations Is it really acting just as an indicator ... 

From a practical standpoint, we must choose an internal redox indicator whose color change is located in the vertical zone of the curve around E = itpe, where (p does not differ significantly from unity. Then the titration error is negligible (see Sect. 17.2). Ferroin and its derivatives are suitable. 

The reaction oscillates equally well if one replaces cerium by iron, or bromine by iodine. If the redox indicator ferroin is used, the solution switches continuously between red (Fe " ) and blue (Fe ), with an oscillation period between seconds and minutes, depending on the concentrations. In addition to the temporal oscillations shown in Figure 2.11, the reaction also exhibits spatial oscillations. Provided the reaction is carried out in a thin layer, there will be beautiful patterns of concentric rings travelling through the solution. 

Similar considerations apply to redox titrations, where the indicator dye should possess differently coloured oxidation states and a redox potential which is appropriate for the reaction to be studied. A large number of dyes are known which are suitable for this purpose. Ferroin (Scheme 7), a complex between iron(ll) and phenanthrolin with a deeply red colour, upon oxidation forms a pale blue iron(lll) complex. Another redox indicator is diphenylamine (Scheme 8), which is colourless in the reduced state. It is oxidized in acidic solution irreversibly to diphenylbenzidine (Scheme 9) which can be further oxidized reversibly to the intensely coloured diphenylbenzidine violet (Scheme 10). 

Hence manganese (III) sulphate solution can be standardised widi a standard solution of a reductant eg iron (II) in acid solution. The end-point can be found potentiometrically or by using a suitable redox indicator eg ferroin (iron (II) complex with 1,10-phenanthroline) which is red but turns blue when oxidised. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

The standard redox potential is 1.14 volts the formal potential is 1.06 volts in 1M hydrochloric acid solution. The colour change, however, occurs at about 1.12 volts, because the colour of the reduced form (deep red) is so much more intense than that of the oxidised form (pale blue). The indicator is of great value in the titration of iron(II) salts and other substances with cerium(IV) sulphate solutions. It is prepared by dissolving 1,10-phenanthroline hydrate (relative molecular mass= 198.1) in the calculated quantity of 0.02M acid-free iron(II) sulphate, and is therefore l,10-phenanthroline-iron(II) complex sulphate (known as ferroin). One drop is usually sufficient in a titration this is equivalent to less than 0.01 mL of 0.05 M oxidising agent, and hence the indicator blank is negligible at this or higher concentrations. 

M cerium(IV) solution, and the equivalence point is at 1.10 volts. Ferroin changes from deep red to pale blue at a redox potential of 1.12 volts the indicator will therefore be present in the red form. After the addition of, say, a 0.1 per cent excess of cerium(IV) sulphate solution the potential rises to 1.27 volts, and the indicator is oxidised to the pale blue form. It is evident that the titration error is negligibly small. 

One important group of colour indicators is derived from 1 10 phenantholine ortho-phenanthroline) which forms a 3 1 complex with iron(II). The complex known as ferroin undergoes a reversible redox reaction accompanied by a distinct colour change... 

Example 14.3. The Belousov-Zhabotinsky reaction [22,27-29], The reaction is an oxidation of malonic acid by bromate ion in sulfuric acid, catalyzed by a Ce(III)/Ce(IV) redox couple. Many variations with other organic acids and transition-metal ions are possible [22] (Belousov used citric acid, and manganese, ruthenium, or iron can replace cerium). The color of the solution alternates between clear [Ce(III)] and pale yellow [Ce(IV)], and more dramatically between red and blue if ferroin is added as indicator. 

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