To characterize "crowned" 4-(2, 4-dinitrophenylazo)phenol (CDPA), the acid dissociation constant (pK_a) was measured together with those of related compounds. The complexation behavior of CDPA was examined in various nonaqueous solvent systems, and it was found that in the CHCl₃-MeOH-triethylamine (TEA) system, the ionic radii of the alkali and alkaline earth metal ions are linearly correlated both with the wave numbers of the absorption maxima and with the molar absorptivities of the resulting complexes except for Be(II). The molar ratios of metal ions to CDPA in the complexes were determined by means of the continuous variation method. Next, the complexation of CDPA with Li(I) was fully examined in another solvent system (CHCl₃-dimethyl sulfoxide-TEA) and a sensitive colorimetric assay method for Li(I) was established in the range of 25-250 ppb of Li(I). This method was applied to the analysis of a commercial pharmaceutical preparation, lithium carbonate (tablet).