Azo dyes, formation

Azo-dye formation. Dissolve 2-3 drops of aniline in 1 ml. of cone. HCl and add 3 ml. of water. Shaike to dissolve any hydrochloride which may have separated and cool in ice. Add a few drops of 20% sodium nitrite solution. Add this cold diazonium solution to a cold solution of the phenol in an excess of aqueous NaOH solution. Solutions or precipitates of azo-dyes ranging in colour from orange through scarlet to dark red, according to the phenol used, are obtained. Note in particular that i-naphthol gives a brownish-red, 2-naphthol a scarlet precipitate. Catechol decomposes. 

The introduction of hydroxy groups is important in dye chemistry because it opens the door to azo dye formation, using phenols and naphthols, and provides an important aux-ochrome. Hydroxylation methods include alkali fusion, replacement of labile groups,... 

Benzidine immobilized on cellulose and silica gel can be used for the determination of free chlorine (0.05-0.5 mg l ) in waters [1]. Free and combined chlorine in water and chlorine in air were determined using solid-phase extraction and azo dye formation [2]. Combined chlorine in water could be determined after masking the free chlorine with acetone. 

The first procedure was developed for the determination of several sulfonamides (sodium sulfacetamide, sulfadiazine, sulfaguanidine, sulfa-methizole, sulfamethoxazole, and sulfanilamide), in different pharmaceutical preparations (tablets, pills, capsules, drops, and suspensions), after azo dye formation. The retention of the azo dyes was excessive (analysis times longer than 40 min) wifti pure micellar SDS mobile phases. The retention times were still high (> 25 min) after addition of 1-propanol as modifier. Therefore, an alcohol giving a higher elution strength, such as 1-pentanol, was preferred. 

The salts associated with cured meats, nitrates and nitrites, are assayed by aqueous extraction and colorimetric determination. The ISO methods involve extraction in hot water containing borax solution and deproteination of a portion of the extract with Carrez reagents. Nitrite (ISO 2918 1975) is determined directly by formation of an azo dye with sulfanilamide and N-(l-naphthyl)ethylenediamine dihydrochloride which is measured at 538 mn. Nitrate (including nitrite) (ISO 3091 1975) is determined by cadmium reduction to nitrite and azo dye formation. Reduction of nitrate to nitrite may be carried out on a special column (as in the ISO method) or using spongy cadmium. Care must be taken to prevent interference from ascorbic acid if it is present in the sample at relatively high levels (>20pgml ). 

Azo-dye formation. Most phenols and keto-enolic compounds couple with diazotized primary aromatic amines to give highly coloured azo-dyes (see pages 66 et seq). 

With azo dyes derived from 4,5-dihydroxy-2,7-naphthalenedisulfonic acid [148-25-4] (chromotropic acid) as the coupling component, metal complex formation occurs with the perihydroxy groups without oxidation. 

Table 4-3. Binding Constants and Rate Constants for Complex Formation between a-Cyclodextrin and Azo Dyes ...

I-A1ON0-2-NAPHIH0L-4-SULE0NIC acid, 11, 72 16, 91 17, 91 Aminonaphtholsulfonic acids, coupling to form azo dyes, 16,16 p-Aminophenol, 16, 39 Aminopiperole, 16, 6 /3-Ahinopropionic acid, 16, 1 4-Aminoveratrole, 16, 4 Ammonium dichromate, 16, 74 Ammonium formate, 17, 77 Ammonium thiocyanate, 16, 74 Ammonium vanadate, 13, 1 to w-Amyl alcohol, IS, 17 hri.-Amyl alcohol, 13, 68 -Amylbenzene, 10, 4 -Amyl borate, 13, 17 -Amyl bromide, 16, 41 iso-Amyl iodide, 13, 62 n-Amyl iodide, 13, 62 n-Amybnagnesium bromide, 16, 41... 

In another investigation (Loewenschuss et al., 1976) dediazoniation was studied in TFE and in acetonitrile in the presence of pyridine. There is UV and NMR evidence for the formation of a diazopyridinium cation in addition, -CIDNP absorption and emission signals were observed. Systems containing diazonium salts and pyridine are important in industrial chemistry, as pyridine is used as a proton acceptor in the diazo coupling reaction (see Sec. 12.8) in a considerable number of syntheses of azo dyes. At the same time pyridine has an unfavorable effect on the yield because of the competing homolytic dediazoniation. 

Chromium, (ri6-benzene)tricarbonyl-stereochemistry nomenclature, 1,131 Chromium complexes, 3,699-948 acetylacetone complex formation, 2,386 exchange reactions, 2,380 amidines, 2,276 bridging ligands, 2,198 chelating ligands, 2,203 anionic oxo halides, 3,944 applications, 6,1014 azo dyes, 6,41 biological effects, 3,947 carbamic acid, 2,450 paddlewheel structure, 2, 451 carboxylic acids, 2,438 trinuclear, 2, 441 carcinogenicity, 3, 947 corroles, 2, 874 crystal structures, 3, 702 cyanides, 3, 703 1,4-diaza-1,3-butadiene, 2,209 1,3-diketones... 

One of the main concerns regarding azo dyes is related to the possibility of their reduction by azoreductases, with the formation of unsulfonated aromatic amines with potential carcinogenicity. 

Triphenylformazan behaves as a bidentate ligand forming 2 1 complexes (217) with divalent copper, nickel, and cobalt.377 Formazan metal complexes can be compared to complexes of azo dyes or beta diketones due to structural similarity.301,302 In general, formazan metal complexes have low stability toward acids. However, when electron-donating substituents are added to the aromatic ring, a considerable enhancement in stability is observed. Cationic complexes of type 218 are also known. The complexation of formazan with metal cation can be accompanied by oxidation to the tetrazolium salt and the formation of a complex... 

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