Oxidation dyes

Tracer Type. A discrete quantity of a foreign substance is injected momentarily into the flow stream and the time interval for this substance to reach a detection point, or pass between detection points, is measured. From this time, the average velocity can be computed. Among the tracers that have historically been used are salt, anhydrous ammonia, nitrous oxide, dyes, and radioactive isotopes. The most common appHcation area for tracer methods is in gas pipelines where tracers are used to check existing metered sections and to spot-check unmetered sections. 

Oxidation H ir Colorant. Color-forming reactions are accompHshed by primary intermediates, secondary intermediates, and oxidants. Primary intermediates include the so-called para dyes, -phenylenediamine, -toluenediamine, -aminodiphenylamine, and p- am in oph en o1, which form a quinone monoimine or diimine upon oxidation. The secondary intermediates, also known as couplers or modifiers, couple with the quinone imines to produce dyes. Secondary intermediates include y -diamines, y -aminophenols, polyhydroxyphenols, and naphthols. Some of the more important oxidation dye colors are given in Figure 1. An extensive listing is available (24,28). 

The mechanism of oxidative dyeing involves a complex system of consecutive, competing, and autocatalytic reactions in which the final color depends on the efficiency with which the various couplers compete with one another for the available diimine. In addition, hydrolysis, oxidation, or polymerization of diimine may take place. Therefore, the color of a mixture caimot readily be predicted and involves trial and error. Though oxidation dyes produce fast colors, some off-shade fading does occur, particularly the development of a red tinge by the slow transformation of the blue indamine dye to a red phenazine dye. 

Also present but not essential in permanent hair colorants are nitro dyes which dye hair without oxidation. These dyes, nitro derivatives of aminophenols and benzenediamines, impart yellow, orange, or red tones. Although they have good tinctorial value, they are not as colorfast as the oxidative dyes. They also are used in semipermanent hair colorants. 

Attempts to broaden the range of materials available as dye precursors have been made (34,35). Oxidative dyes based on pyridine derivatives produce less sensitization than those based on benzene derivatives (36) however, they lack tinctorial power, lightfastness, and availabihty. Derivatives of tetra am in opyrim i din e are claimed to act as primary intermediates to give intense shades with good fastness and excellent toxicological properties (37). 

It is a bleaching agent that oxidizes dyes to make them colorless. 

Various methods have been used for the reoxidation of vat leuco dyeings atmospheric skying, hypochlorite, chlorite and acidified dichromate are now rarely employed. Atmospheric oxidation can be difficult to control and thus uneven with some dyes it is also too slow, particularly for continuous methods. Sodium hypochlorite is used only for those few black dyes that tend to become dark green when oxidised with peroxide obviously hypochlorite should be avoided with the many chlorine-sensitive dyes. Similarly sodium chlorite, acidified to below pH 5 with acetic acid, can only be used with certain dyes, although with these it certainly gives rapid oxidation. Dye selectivity is also a drawback with... 

Figure 11 Illustration of the interfacial CT processes in a nanocrystalline dye-sensitized solar cell. S / S+/S represent the sensitizer in the ground, oxidized and excited state, respectively. Visible light absorption by the sensitizer (1) leads to an excited state, followed by electron injection (2) onto the conduction band of Ti02. The oxidized sensitizer (3) is reduced by the I-/I3 redox couple (4) The injected electrons into the conduction band may react either with the oxidized redox couple (5) or with an oxidized dye molecule (6).

Apart from recapture of the injected electrons by the oxidized dye, there are additional loss channels in dye-sensitized solar cells, which involve reduction of triiodide ions in the electrolyte, resulting in dark currents. The Ti02 layer is an interconnected network of nanoparticles with a porous structure. The functionalized dyes penetrate through the porous network and adsorb over Ti02 the surface. However, if the pore size is too small for the dye to penetrate, that part of the surface may still be exposed to the redox mediator whose size is smaller than the dye. Under these circumstances, the redox mediator can collect the injected electron from the Ti02 conduction band, resulting in a dark current (Equation (6)), which can be measured from intensity-modulated experiments and the dark current of the photovoltaic cell. Such dark currents reduce the maximum cell voltage obtainable, and thereby the total efficiency. 

The interception of the oxidized dye by the electron donor in the electrolyte (i.e., iodide) occurs within 10ns. The rate of the reaction leading to the regeneration of the dye ground state was found to depend strongly on the nature and the concentration of the cations present in the solution. Small cations such as Li+, Mg2+, and La3+ can intercalate into the oxide surface, thereby favoring fast oxidation of iodide by the oxidized state of the sensitizer. 

Fig. 19B shows the la-catalyzed bleaching of Safranine O at different [la] for comparison. The experimental and calculated curves agree and prove that complete bleaching of the dye is achievable by just increasing the la concentration. The required amount is still very low, specifically, 10-6 M for the difficult-to-oxidize dye Safranine O. 

M. Neamtu, I. Siminiceanu, A. Yediler and A. Kettrup, Kinetics of decolorization and mineralization of reactive azo dyes in aqueous solution by the UV/H202 oxidation. Dyes Pigm., 53 (2002) 93-99. 

Lucas, MS Peres JA. Decolorization of the azo dye Reactive Black 5 by Fenton and photo-Fenton oxidation. Dyes and Pigments, 2006 71, 236-244. 

Additionally, Wang and Watt have shown that the FeMo protein alone can act as an uptake hvdrogenase(63). Specifically, H2 in the presence of [FeMo] causes the reduction of oxidizing dyes such as methylene blue or dichlorophenolindophenol in the absence of Fe protein. The hydrogen evolution and uptake behavior of nitrogenase proteins forces us to consider the ways in which hydrogen can interact with transition metal sulfur centers. This we discuss in the following section. 

Uses. Oxidative dye developing agent for photographic processes precursor for pharmaceuticals used in hair dyes... 

This cell involves the absorption of light by dye molecules spread on the surface of the semiconductor, which upon light absorption will inject electrons into the conduction band of the n-type semiconductor from their excited state. The photo-oxidized dye can be used to oxidize water and the complementary redox process can take place at the counter electrode [46,47]. Tandem cells such as these are discussed in Chapter 8. 

The electron energy term of the reduced dye may then be so energetic that direct electron injection is possible as indicated in Fig. 20a. The energy conditions for the inverse mechanism with hole injection by the oxidized dye (which reacts in the excited state primarily with an oxidizing supersensitizer) are shown in Fig. 20b. 

In the effort of improving the performance of such mediators, the possibility of using kinetically fast couples in conjunction with the best Co(II) mediators has been explored.62 Kinetically fast couples efficiently reduce the oxidized dye, but due to the fast recombination with injected electrons are totally unsuccessful as mediators in DSSC. However, when mixed with an excess of cobalt mediator, if their redox... 

Figure 17.25 Sequence of electron transfer events involving the oxidized dye (S +), comediator (D), and the cobalt complex (Co(II)).

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