Solvent extraction and fluorometric determination of fluoride ion at ppb level in the presence of large excess of aluminum(III) and iron(III) by using an expanded porphyrin, sapphyrin Original Research Article

Solvent extraction of fluoride as low as 10−6 M into chloroform using [2,23-diethyl-8,17-bis(2-ethoxycarbonylethyl)-3,7,12,13,18,22-hexamethylsapphyrin (H3sap), sapphyrin] have been studied in the presence of 0.1 M sodium nitrate by spectrophotometric and fluorometric methods at pH 3–13 and 25°C. Sapphyrin is protonated at pH<8 and exists as the ion-pair complexes with nitrate ion, such as H4sap+·NO3− and H5sap2+·2NO3− in chloroform. The equilibrium constants, K1 for [H4sap+·NO3−]org[H3sap]org−1[H+]aq−1[NO3−]aq−1 and K2 for [H5sap2+·2NO3−]org[H4sap+·NO3−]org−1[H+]aq−1[NO3−]aq−1 were found to be 9.90±0.08 and 7.20±0.07 in the logarithmic form, where the subscripts of org and aq denote the chemical species in organic and aqueous phases, respectively. Fluoride ion was extracted from chloroform with the positively charged sapphyrin that gave a new absorption maximum wavelength at 448 nm. The extraction constant, log (KFP), for the reaction of (F−)aq+(H5sap2+·2NO3−)org⇌(H5sap2+·F−·NO3−)org+(NO3−)aq was 3.66±0.01. These results were applied to the determination of fluoride in natural waters containing Al(III) and Fe(III) by the measurement of fluorescence intensity at 684 nm under excitation at 448 nm. Al(III) of 10−4 M and Fe(III) of 10−3 M were quantitatively masked by DCTA (trans-1,2-aminocyclohexane-N,N,N′,N′-tetraacetic acid).