Fluorometric determination of aluminum using 2,2′-dihydroxyazobenzene after removal of an excess reagent

The removal method of an excess reagent was applied to the fluorometric determination of aluminum using 2,2'-dihydroxyazobenzene (DRAB). The detection limit of the analyte was affected by the fluorescence intensity of an excess reagent, that is, reagent blank. Therefore, the decrease in the fluorescence intensity of an excess reagent has been required for the development of highly sensitive fluorometry. At first in this study, the removal of DRAB was examined using solvent extraction with a small amount of chloroform. Aluminum reacted with DRAB at pH 8 to form an Al-DHAB complex (ratio of composition, Al to DRAB, 1 : 2). Excess DRAB was removed from the aqueous phase by chloroform-extraction at pH 3 before measuring of the fluorescence on the Al-DHAB complex in the aqueous phase. The ratio of the composition of the Al-DHAB complex varied from 1 : 2 to 1 : 1 by changing the pH in aqueous solution. The fluorescence from the Al-DHAB complex was measured by a flow method at an excitation wavelength of 477 nm, and at an emission wavelength of 603 nm. As a result, aluminum was determined by measuring a 1 : 1 complex of Al-DHAB over a concentration range of from 0.12 to 8 ppb. Moreover, a removal method without extraction solvent was examined by the filtering aqueous phase using a filter phase separator. The DRAB was deposited in the aqueous solution at pH 3 because of the insolubility of the free DRAB. On the other hand, the Al-DHAB complex was stable in the aqueous phase, that is, the filtrate after filtration by the filter phase separator. Excess DRAB was removed at 74% from the aqueous phase. Therefore, the fluorometric intensity of the reagent blank decreased to 1/4. As a result, aluminum was determined over the range from 0.12 to 300 ppb. The present method was applied to the determination of aluminum in a tap-water sample.