From the color reaction of hydroxyfluorans and its halogen derivatives with metal ions, 3′, 4′, 5′, 6′-and 2′, 3′, 6′, 7′-tetrahydroxyfluoran, and 2′-, 4′-halo-and 2′, 4′-dihalo-3′, 6′, 7′-and 3′, 5′, 6′-trihydroxyfluorans were found to undergo sensitive coloration. Examinations were made on the color reaction of 3′, 5′, 6′-trihydroxyfluoran, 3′, 4′, 5′, 6′-tetrahydroxyfluoran, amino acetic acid and iminodiacetic acid complexones of hydroxyfluorans, and their derivatives substituted in 2′-, 4′-, or 2′, 4′-positions with chlorine, bromine, and/or iodine with metal ions (Hg
2+, Th
3+, Bi
3+, Zr
4+, In
3+, Ce
3+, La
3+, Cd
2+, Zn
2+, Cu
2+, Co
2+, Ni
2+, Ba
2+, Sr
2+, Ca
2+, Mg
2+). 3′, 4′, 5′, 6′-Tetrahydroxyfluoran, hydroxyfluoran complexones, and bromo and chloro derivatives of 3′, 5′, 6′-trihydroxyfluoran were found to form colored complex in high sensitivity, and the tone of color was deep, especially in the case of 2′, 4′-disubstituted 3′, 5′, 6′-trihydroxyfluoran, and change of color in the chelate titration was also very sensitive. Coloration of iodine-substituted 3′, 5′, 6′-trihydroxyfluoran with a metal ion was hypsochromic and color change was also indistinct. Good indicator quality was found in Bi
3+, Hg
2+, Th
3+, Cd
2+, Co
2+, Zn
2+, Cu
2+, Ni
2+, La
3+, Pb
2+, and rare earth metal ions.
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