Internal redox indicators

The internal redox indicators are dissolved in the titrand solution. They are organic dyes whose oxidized and reduced forms are different colors. They are derivatives for which their oxidation-reduction is achieved rapidly. Their half-redox equilibrium may be symbolized by... 

In order to conveniently locate the potential interval change in the conditions of the titration, it is more judicious to use the formal potentials of the internal redox indicators than their standard potentials. Therefore, Nernst s equation is written ... 

The most commonly used internal redox indicators are derivatives of 1,10-phenanthroline, diphenylamine, phenothiazine, and diphenylpyrazine. 

Some derivatives of diphenylamine are internal redox indicators that have been used very often. Diphenylamine dissolved in diluted acidic medium exhibits the formal potential = 0.76 V. In the presence of a strong oxidizing agent, it first undergoes an irreversible chemical oxidation to give the colorless diphenylbenzidine (Fig. 16.4). 

The class of diphenylpyrazine (phenazine) also provides internal redox indicators. These derivatives, as well as phenazine itself, exchange two electrons and one or two protons depending on the pH value (Fig. 16.7). 

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. 

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... 

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. 

Nonoxo, mixed-cage ligand complexes, [Mo(AsSe5)2], [Mo(CO)2(As3Se5)2] have been described, the first containing long intramolecular Se Se interactions indicative of partial internal redox behavior. 

CO)2(As3Se5)2], and (NBu4)2[Mo(AsSe5)2] have also been reported 778 the first of these contains a long intramolecular Se-,Se interaction indicative of partial internal redox. 

The reaction of cobalt(II) tetrasulfophthalocyanine, [Co(II)(tspc)] , with CH3, CH2CH2OH, CH(CH3)CH20H, CH(CH3)CH(CH3)0H, and CH2C(CH3)20H free radicals has been studied. Results indicate the initial formation of a [Co(II)(tspc-R)] species where the exact nature of the interaction of R and tspc is not clear. There follows a subsequent internal redox formation of [(tspc)Co(III)-R] via a first-order process. Subsequent decomposition produces methane for R or alkenes for ROH. Interaction of methyl radical with [Co(II)(nta)(H20)2]" [nta = N(CH2C02)r] yields [(nta)(H20)Co(III)-CH3] (Ki = 2.7 0.5 X 10 M and k i = 60 10 This reaction is followed by... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Any electrochemical device using a low molecular weight redox couple to shuttle electrons from the redox center of an enzyme to the surface of an indicator electrode, thereby increasing the effectiveness of amperometry in the detection of a substrate for the particular enzyme. The internal cavities of six-, seven-, and eight-membered cyclodextrins are trapezoids of revolution with larger open mouths dimensions (/. c., respective diameters of... 

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