Derivatives of 8-hydroxyquinoline are also used in spectrophotometric analysis. Chloro-oxine (5,7-dichloro-8-hydroxyquinoline) and bromo-oxine (5,7-dibromo-8-hydroxyquinoline, formula 4.43) react similarly to oxine. The absorption maxima of the complex in chloroform shifted to longer wavelengths, and the sensitivity of the reaction was higher than in the case of oxine.

8-Hydroxyquinoline derivatives containing sulfonic acid groups, such as ferrous [iron (IN) reagents, form water-soluble complexes with metal ions.

8-Mereaptoquinoline (thioxine, formula 4.44) is a sulfur analogue of 8-hydroxyquinoline that forms sparingly soluble chelates with metal ions (Fe, Cu, Mn, Cd, Co, Mo, W, and Ni)[ 110,111]. Thioxin compounds are colored and are extractable into chloroform. The molar absorbance ranges from 5.103 to 1.104. 8-Mercaptoquinolates are extracted from more acidic solutions than oxinates, but are readily oxidized.

In general, the spectra of quinoline and its derivatives are very similar to those of the corresponding naphthalene, as can be seen by comparing the curves of β-naphthol and 6-hydroxyquinoline (Fig. 5.1). The curve for 2-hydroxyquinoline is also shown in the same figure. This has greater intensity in the 250 mμ region due to quinolone tautomerism.

The 8-hydroxyquinoline method is used to determine aluminum in plant materials, and some 4-hydroxyquinolin-2-one derivatives have significant antiviral effects. 109 Compounds 88 and 89 are potent inhibitors of HCV replication. 110,111 (Figure 23) The effects of compounds 88 and 89 were due to viral RNA polymerase inhibition.