Azo dyes in solution

The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. 

The cis-trans isomerisation process in the photofading of azo dyes has been found to be a first-order process in polypropylene and independent of polymer crystallinity l. oye lightfastness on leather has however been found to depend upon the colour of the dye lS while alkyl and peroxy radicals have been observed in the photofading of Rhodamine and azo dyes in solution . More specifically basic triphenylmethane dyes have been found to give benzophenone products on photofading 20 structure of a series... 

B ) Coupling Orange T.—The procedure is the same as in (B) except that the alkaline solution of a-naphthol (Note 4) is cooled to 25 before adding the ice (Note 5). The azo dye in this case does not crystallize but forms a deep purple-red solution. 

Tang WZ, Zhang Z, An H, Quintana MO, Torres DF (1996) Ti02/UV photodegradation of azo dyes in aqueous solutions. Environ Technol 18 1-12... 

Kenji O, Kazuya I, Yoshihiro Y, Hiroshi B, Rokuro N, Yasuaki M (2005) Sonochemical degradation of azo dyes in aqueous solution a new heterogeneous kinetics model taking into account the local concentration of OH radicals and azo dyes. Ultrason Sonochem... 

Aleboyeh A, Olya MA, Aleboyeh H (2009) Oxidative treatment of azo dyes in aqueous solution by potassium permanganate. J Hazard Mater 162 1530-1535... 

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. 

An interesting variation of the latter technique finds application in enzyme chemistry. In this procedure a tissue section is exposed to a relatively colorless derivative of j8-naphthol, such as sodium j8-naphthyl acid phosphate. A phos-photase enzyme reacts with this reagent (often called an enzyme substrate ), leaving free j8-naphthol behind. Subsequent treatment with a solution of a diazonium salt produces highly colored spots in the tissue section. Thus not only can the presence of phosphotase enzyme be demonstrated, but also the location of the enzyme in the tissue can be determined. The intensity and chroma of the color produced and the solubility of the azo dye in the cell materials can be varied by judicious selection of the reagents. 

When colours are added to a food system, their characterisation is often more difficult due to interferences from other materials in the food or difficulty with their extraction from the food. This is particularly the case for high-protein foods, which bind colours very tightly and can make their quantitative analysis very difficult. However, analysis for azo-dyes in soft drinks is generally straightforward using modern methods. There is less interference than in other food systems and as the colours are already in solution, and not bound to other materials, this makes the analysis easier. In some cases, the colours can be analysed without prior concentration and in others they have to be concentrated by solid-phase extraction methods, for example, Ci8 cartridges followed by elution with a small volume of methanolic ammonia. 

Garcia Gutierrez [19] has described an azo coupling spectrophotometric method for the determination of nitrite and nitrate in soils. Nitrite is determined spectrophotometrically at 550 nm after treatment with sulfuric acid and N-1 -naphlhylclhylcnediamine to form an azo dye. In another portion of the sample, nitrate is reduced to nitrite by passing a pH 9.6 buffered solution through a cadmium reductor and proceeding as above. Soils were boiled with water and calcium carbonate, treated with freshly precipitated aluminium hydroxide and active carbon, and filtered prior to analysis by the above procedure. 

High performance hquid chromatography-mass spectrometry has been apphed to the determination of down to lmg L 1 of azo dyes in waste waters [37], Games et al. [38] used high performance hquid chromatography for solute focusing prior to superfluid chromatography-mass spectrometry in the analysis of waste streams. 

Figure 7-21 Comparison of SERRS spectra and RR spectra of 10 nM azo dye in different pH solutions (a) pH = 2 (b) pH = 12. (Reproduced with permission from Ref. 13.)...

Contrary to the general impression, phenol does not couple readily with diazo compounds. Diazo ethers are frequently formed and the reaction is mistaken for true coupling. The procedure given here, in which the acid diazo solution is mixed with the phenol (or cresol) and the mixture treated with soda without sodium hydroxide, gives the azo dye in much better yield. 

These can exist in t-isomeric modifications. The one primarily obtained is characterised by the feet that in alkaline solution it unites with phenols to form azo dyes while the second modification obtained by a longer action of the alkali, at higher temperature if necessary, does not possess this property at all, or only in a slight degree. If they (the latter) be treated with adds, they are converted back into the diazonium salts, and now have the property of combining with phenols to form azo dyes. In spite of numerous experiments the constitution of these substances is not wholly clear. 

Konstantinou I K, Albanis T A (2004), TiO,- assisted photocatalytic degradation of azo dyes in aqueous solution kinetic and mechanistic investigation. Appl Catal B Environ 49 1-14... 

Photodegradation of PR-MX5B, PY-HE4R, and PR-HE3B azo dyes in aqueous solution under UV light... 

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