Separation and Determination of Fluoride Ion at ppb Level in the Presence of Large Excess of Al(III) and Fe(III) by Using Sapphyrin
Fluoride ion as low as 10 -6 mol dm -3 was determined by using [2,23-diethyl-8,17-bis(2-ethoxycarbonyl)-3,7,12,13,18,22hexamethylsappyrin (H 3 sap, sapphyrin)] in the presence of large excess of aluminum(III) and iron(III), and the fluoride-sapphyrin complexes were also extracted into chloroform with nitrate ion. The formation of the ion-pair complex of fluoride ion with the sapphyrin is given as H 5 sap 2+ + F -= (H 5 sap)F + at pH 1-4.3 and the equilibrium constant is 4.84±0.05 in logarithms.
... 0.05 in logarithms. The extraction equilibrium of the fluoride sapphyrin complex is written as (F -) a + (H 5 sap 3+ ·2NO 3 -) o = (H 5 sap 3+ ·F -·NO 3 -) o + (NO 3 -) a and the extraction constant is 3.66±0.01 in logarithms at 25 ºC and I = 0.1 mol dm -3 (NaNO 3 ), where the subscripts a and o denote the chemical species of aqueous and organic phases, respectively. The fluoride-sapphyrin complex gives enlarged fluorescence intensity at 680 nm on excitation 450 nm. The present method is free from the interference of a 200-fold amount of aluminum(III) and a 2000-fold amount of iron(III) for the determination and separation of fluoride ion as low as 5x10 -7 mol dm -3 in the presence of 1,2-diaminocyclehexane-N,N,N',N'-tetraacetic acid. The present method was applied to determination of fluoride ion in various water samples (tap water, river water, rain water, underground water and hot spring water).