Color-forming reaction

Color-forming reactions Color generation Colorimeters Colorimetric methods... 

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). 

Analytical Methods. A method has been described for gas chromatographic analysis of trichloromethanesulfenyl chloride as well as of other volatile sulfur compounds (62). A method has been recommended for determining small amounts of trichloromethanesulfenyl chloride in air or water on the basis of a color-forming reaction with resorcinol (63). 

The driving force for this development followed the introduction of carbonless copying paper into the marketplace by the National Cash Register Company (NCR) in 1954. The principles of this system have long been described,3,4 and it suffices to demonstrate the color-forming reaction of CVL in Scheme 1. 

The CB sheet is coated on the back with microcapsules 5 to 10 pm in diameter, in which leuco dye solution in a nonvolatile solvent is contained. The CF sheet is coated on the front with an acidic coreactant such as reactive clay, zinc salt of salicylic acid derivatives, zinc-modified phenolic resin, etc. On the application of pressure, the microcapsule is ruptured in the area delineated by the pressure pattern, and the leuco dye solution is thereby transferred to the CB sheet to bring about a color-forming reaction with acidic coreactant resulting in a distinct image on the surface of the CF sheet. By inserting a middle sheet (or sheets) called the CFB sheet, which is coated on the front and back with acidic coreactant and microcapsules, respectively, multiple copies can be obtained. 

This section covers early indirect fiber optic chemical sensors (FOCS) for species that cannot be sensed directly but require the use of indicators, probes, labeled biomolecules, or color-forming reactions. 

The color-forming reaction is interesting because most a-amino acids give the same color irrespective of their structure.4 The sequence of steps that leads to the color is as follows ... 

In other imaging systems, ammonia or other amines are thermally released from the cobalt(III) complex upon reduction. This imagewise release can be monitored either as a pH change, causing a color-forming reaction, or by incorporating into the system a component such as o-phthalal-dehyde, which reacts with ammonia to form a black dye. 

Visualization of the spots was done by examination under UV-light or spraying with ninhydrin. The color-forming reaction with nitrobenzylpyridine (NBP) (see 5.2.1) has been used by dipping the chromatograms in solutions of NBP followed by heating at 90° and treating with triethylamine [68]. 

The Folin-Ciocalteau Assay of Protein Concentration. The Folin-Ciocalteau assay is one of the most sensitive and most commonly used assays to determine protein concentration (sensitive to about 10 /rg/m I protein). This procedure employs two color-forming reactions to assay protein concentration photometrically. In the first reaction (a biuret reaction), compounds with two or more peptide bonds form a dark blue-purple color in the presence of alkaline copper salts. In the second reaction, tryptophan and tyrosine side chains react with the Folin solution to produce cuprous ions. This reaction is most efficient under basic condi-... 

Historically, the first color-forming reaction to be discovered which involves electron transfer is probably the photoinitiated reaction of diphenylamine with carbon tetrabromide, which forms blue colors [42]. In fact, the major path for color formation is due to radical reactions, initiated by photolysis of C—Br bonds to produce Br3C and bromine atoms. An alternative mechanistic path, possible when the light is absorbed by the diphenylamine, would involve electron transfer. MacLachlan has shown that such processes do occur durin> photolysis of aminotriarylmethanes in the presence of electron acceptors such as CBr4 and CC14 [43]. Other electron deficient species (quinones, nitroaromatics) were also demonstrated to be effective. 

Study of Color Forming Reactions of Tryptophan, p-Dimethylamino-benzaldehyde, and Sodium Nitrite in Sulfuric Acid Solution. Analyt. Chem. 20, 30 (1948). 

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