Gibbs indophenol test

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). 

Almost all polycarbonates used as plastics contain bisphenol A. For positive identification, the color reaction with p-dimethylaminobenzaldehyde (see Section 6.1.2) or the Gibbs indophenol test (see Section 6.1.3) is used. 

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. 

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. 

Gibbs indophenol See test, phenol Gibbs indo-phenol. 

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