Coloration — decoloration

Electrochemical Processes. The reductive cleavage of azo-group-containing dyes has been applied on a full scale for the decolorization of concentrates from batch dyeing. Depending on the color, decolorization of up to 80% of the initial absorbance can be obtained. Mixed processes consist of combinations of electrochemical treatment and precipitation by use of dissolving electrodes [43,49]. Such techniques have been described in the literature and have, in part, also been tested on a full scale. Anodic processes that form chlorine from oxidation of chloride have also been proposed to destroy dyes, but care has to be taken with regard to the chlorine and chlorinated products (AOX) formed [114,115]. 

Elemental composition Pb 86.62%, O 13.38%. The compound may be identified by its physical properties and characterized by x-ray crystallography. Lead may be analyzed in the acid extract of the oxide by AA or ICP spectroscopy. It also may be analyzed by its oxidative properties. It hberates iodine from an acidic solution of potassium iodide, and the liberated iodine may be titrated against a standard solution of sodium thiosulfate using starch indicator (blue color decolorizes at the end point). 

The standards are taken in separatory funnels. A few drops of phenolphthalein indicator followed by 1 A NaOH solution are added dropwise until a pink color forms. This is followed by the addition of 1 N H2S04 dropwise until the pink colors decolorize. 

Hereafter, a and b indicate the closed- and open-ring form isomers, respectively. The photocyclization and cycloreversion quantum yields were determined to be 0.46 and 0.015, respectively.1121 In the absence of oxygen, the coloration/decoloration cycle could be repeated more than 2000 times.[13] The basic performance of diaryl-ethenes is described below. 

Even if the side reaction quantum yield, 0S, is as small as 0.001 and B converts perfectly into A (0B— a= 1), 63 % of the initial concentration of A will decompose after 1000 coloration/decoloration cycles. Thus, if the cycle is to be repeated more than 10,000 times, the quantum yield of by-product formation has to be less than 0.0001. 

The coloration rate of 18a was determined to be 1.1 ps, while the decoloration rate of 19b was 2.1 ps. Both coloration/decoloration reactions take place in less than a few picoseconds in solution. 

A crucial point that must be addressed concerns the thermal stability and the fatigue phenomenon observed in the chromophores. It is a fact that many photo-chromic compounds are irreversibly degraded upon long exposure to light, thus limiting their use for various applications. Major advances in the preparation and performance of photochromic materials have been made in the past five years. Irie et al.11271 have recently developed new photochromic compounds, 1,2-diarylethenes, which display photochromic behavior with unchanged intensity even after 104 coloration decoloration cycles. 

The melted liquor is then purified by any of several combinations that include clarification and decolorization. In clarification, the liquor is treated chemically to remove suspended matter, colloids, and other impurities that contribute to turbidity. Clarified liquor is optically clear, but is still highly colored. Decolorization procedures include treating the clarified liquor with various filtration media, such as bone char, granular activated carbon, or ion exchange resin, either alone or in some combination. It is necessary to clarify the liquor before the filtration medium because the particulate impurities will quickly blind the filtration material and render it useless. 

The fatigue resistance was improved when the furyl group was replaced with indole or oxazole [29, 30]. The coloration/decoloration cycles of oxazolyl fulgide 26 could be repeated more than 500 times in toluene, even in the presence of oxygen. The fatigue resistance strongly depends on the aryl groups. 

Figure 7. Fatigue resistance of fulgide derivatives a) in toluene and b) in PMMA film. Aq and A were the absorbances of the first cycle and after n coloration/decoloration cycles, respectively.

The trans-Xo-ana and the reverse quantum yields of 40 were determined to be 0.60 and 0.10, respectively. The coloration/decoloration cycle could be repeated more than 500 times. The quantum yield of side product formation was also determined to be 5 X 10 which agrees with the fatigue resistance of this molecule [41]. Vari-... 

When we intend to apply organic molecular materials, especially photochromic dyes, to optical memory media, the indispensable condition is stability, both thermal and photochemical. The photogenerated isomers are required never to return to the initial isomers in the dark, even at elevated temperatures, e.g., 80 °C. In addition, the coloration/decoloration can be cycled many times while the photochromic performance is maintained, and the memory media are provided with nondestructive readout capability. Although several molecules which fulfill the former condition have been developed, some problems still remain to gain access to molecules and systems which fully satisfy the latter condition. 

Use Pigments, ceramic enamels, aniline colors, decolorizing agent in glass, insecticide, rodenticide, herbicide, sheep and cattle dip, hide preservative, wood preservative, preparation of other arsenic compounds. 

Photochromism is the phenomenon where the absorption speetrum of a molecule or crystal changes reversibly when the sample is irradiated by light of a eertain wavelength. When such a photoisomerization reaction is carried out in a polymer matrix instead of in solution, a strong decrease in the reaction rate (coloration = decoloration) is observed." The poly-... 

It is interesting to note, that iron (III) chloride paints the reactionary mix in dark brown color. Decolorizing of an obtained product neither processing on a centrifuge, nor adsorption of a solution of organosilicon oligomers on the activated coal was not possible. Therefore the further researches were carried out in the presence of catalytic amounts of aluminum chloride. 

FIGURE 4.14 Photograph of Rp—(DOBAA) —(ACMO)j,—Rp/fluorescein nanocomposites in 1,2-dichloroethane and dimethyl sulfoxide (DMSO) Rp—(DOBAA). —(ACMO)j,—Rp/ fluorescein nanocomposites can exhibit a coloring—decoloring behavior among protic solvents including protic-like solvents such as 1,2-dichloroethane and aprotic solvents such as DMSO, A,A-dimethylformamide, and tetrahydrofuran. 

FIGURE 4.15 Coloring—decoloring behavior of Rp—(AMPS) —Rp/acetone composite possessing a good repeatability. 

Diarylethenes with heterocyclic aryl groups are newcomers to the photochromic field. They belong to the thermally irreversible (P-type) photochromic compounds. The most striking feature of the compounds is their fatigue resistance. The coloration/decoloration cycle can be repeated more than 10 times while retaining the photochromic performance. Both properties, thermal stability of both isomers and fatigue resistance, are indispensable for application to optoelectronic devices, such as devices for memory and switches. In this chapter, recent research on diarylethene derivatives will be described. 

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