Phenol Gibbs indophenol

The Gibbs indophenol test is useful for the identification of phenol in phenolic resins and in substances that split off phenol or phenol derivatives on heating. Polycarbonates or epoxy resins as well as some HT-thermoplastics are examples of this. Heat a small sample for a maximum of 1 min in a pyrolysis tube and cover the opening of the tube with a piece of prepared filter paper. To prepare the paper, drench it in a saturated ether solution of 2,6-dibromoquinone-4-chlorimide and then air-dry it. After the pyrolysis, hold the paper over ammonia vapor or moisten it with 1 - 2 drops of dilute ammonia. A blue color indicates phenol (cresol, xylenol). 

Epoxy resins g acrylonitrile ive a positive reaction for phenol according to the Gibbs indophenol test (see Section 6.1.3) due to the presence of bisphe-nol. In contrast to the phenolic resins, however, the formaldehyde test with chromotropic acid (see Section 6.1.4) is negative. 

Additional information can be obtained from certain special tests. Thus, the Gibbs indophenol test is positive with PEEK, PAR, and PEI (see Section 6.1.3). Using the color reaction with jo-dimethylamino benzaldehyde (Section 6.1.2), one can differentiate HT-polyamides, such as PA 6-3-T, from polymers that develop phenolic decomposition products during pyrolysis. While with polyamides, the red coloration obtained after the addition of concentrated hydrochloric acid remains, with polycarbonates it turns blue. 

Gibbs indophenol See test, phenol Gibbs indo-phenol. 

Free phenol n. The uncombined phenol existing in a phenolic resin after curing, the amount of which is indicative of the degree of cure. The presence of such free phenol can be detected by the Gibbs indophenol test. 

In these cases it is usually possible to explain why the color is formed. The reactions can be further classified, on the basis of the type of colored compound formed, into reactions leading to the formation of azo dyes, di- or triphenylmethane dyes, xanthene dyes, polymethine dyes, indophenols, etc. Azo dyes are formed, for example, in the reaction of diazonium salts and phenols (p. 192) or amines (p. 324), azomethines in the reaction of primary aromatic amines with aromatic aldehydes (p. 215), di- and triphenylmethane dyes in the reaction of aromatic aldehydes with aromatic hydrocarbons in concentrated sulfuric acid (p. 213), triphenylmethane dyes in the reaction of phenols with aromatic aldehydes or oxalic acid (p. 196), xanthene dyes in the reaction of anhydrides of dicarboxylic acids with resorcinol (p. 196), polymethine dyes are formed after the cleavage of the pyridine ring in the reaction of the glutaconaldehyde formed and barbituric acid (p. 378), indophenols on reaction of phenols with Gibbs reagent (p. 195), or 4-aminoantipyrine according to Emerson (p. 194), or on the Liebermann reaction (p. 195). 

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