The ZO test is a compound formed by connecting an azo-N=N - group with two hydrocarbon groups, and its general formula is R-N=N-R. Azo compounds have both cis and trans geometric isomers; The trans isomer is more stable than the cis isomer, and the two isomers can be converted to each other under light or heating conditions.

Azo compounds are mainly synthesized through the coupling reaction of diazonium salts; Hydrogenated azo compounds and aromatic amines can be oxidized to corresponding azo compounds in the presence of oxidants such as NaOBr, CuCl2, MnO2, and Pb (OAc) 4; Oxidized azo compounds and nitro compounds can also be reduced to azo compounds in the presence of reducing agents such as (C6H5) 3P, LiAlH4, etc.

Azo group is a chromophore, and azo dyes are the most diverse and widely used type of synthetic dyes. Some azo compounds can be used as acid-base indicators and metal indicators in analytical chemistry. Some azo compounds can be used as initiators for polymerization reactions. The materials involving AZO azo compounds mainly include fabrics, leather, paint, plastics, rubber, colorants, etc.

The hazards and limitations of AZO azo, many azo compounds have carcinogenic effects, such as cream yellow, which has been used for coloring margarine and can induce liver cancer, which is prohibited; Methyl red, as an indicator, can cause bladder and breast tumors. Some azo compounds, although not carcinogenic, have toxicity similar to nitro compounds and aromatic amines. To protect human health and provide consumer safety, the Netherlands, Austria, and Germany have successively adopted mandatory rules to prohibit the use of azo containing colorants in consumer goods. On September 11, 2002 and January 6, 2003, the European Parliament and the European Commission also issued Directives 2002/61/EC and 2003/3/EC, restricting the use of carcinogenic azo colorants in certain textiles and leather products, and prohibiting the sale of goods colored with restricted azo colorants.

At present, AZO FREE azo control has become one of the most important quality monitoring projects in international textile and clothing trade, and also one of the most basic quality indicators for ecological textiles. The German government issued a decree in 1994 requiring all leather and textiles entering Germany to undergo AZO testing, which was followed by some EU countries.