Disazo indicators

Disazo indicators. In this group of indicators are included Congo red, benzopurpurin B, and benzopurpurin 4B, none of which are recommended for use as indicators for the determination of pH. Though the red alkaline form is soluble in water, the blue or violet acid form is insoluble. 

Affinity values of the symmetrical disazo dyes Cl Direct Yellow 12 (3.6), Red 2 (3.8 X = CH3, Y = H) and Blue 1 (3.2) on cellophane have been measured recently under hydrostatic pressures up to 600 Mpa [120]. The affinity of Yellow 12 increased slightly but values for the other two dyes decreased considerably with increasing hydrostatic pressure (Table 3.23). The sulpho groups on the central stilbene nuclei of the Yellow 12 molecule tend to inhibit aggregation, whereas Red 2 and Blue 1 aggregate much more readily. The small increase in the affinity observed with Yellow 12 may indicate that isomerisation from the cis to the more stable Linns form may occur as the hydrostatic pressure is increased. 

The mono and disazo resorcinol dyes can act as indicators and change color very markedly, depending on pH. Monoazo and disazo varieties are often found as byproducts with higher molecular weight resorcinol dyes. This property makes them less obtrusive on leather. Because of their strong intermolecular forces of attraction, resorcinol dyes are very well fixed on leather, despite their low to medium molecular weight. Their indicator properties are scarcely noticeable, in contrast to the case with textiles. This explains why these dyes are not necessarily suitable for textiles, particularly polyamides. 

Solvent Red 27 is a disazo dye used to color gasoline. Its formula indicates that it is made by diazotizing aminoazoxylene and coupling to beta-naphthol. 

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