ALC is utilized for colorimetric detection of fluoride ion. A red solution of the La(III) or Ce(III) complex turns blue in the presence of fluoride ions. The maximum wavelength of the fluoride complex is 620 nm at pH 4-5. As little as 0.1 to 1 ppm of fluoride ion can be determined using this method. The structure of this complex has been reported as La2L2F2, (La5L4F2)n or Ce5L4F4 (L: ALC). ALC is insoluble in alcohol and ether, slightly soluble in water, and easily soluble in alkaline water. The fluoride complex can be extracted with iso-amylalcohol. Since ALC is less toxic than Alizarin red, it is used to mark and trace young fish. The aqueous solution of ALC is yellow at pH < 6, red at pH 6 – 10, and blue violet at pH > 11. ALC can also be used to detect aluminum ions. pKa1(COOH )=2.40, pKa2(OH)=5.54, pKa3(NH+)=10.07, pKa4(OH)=11.98 (m=0.1).
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