The extraction of aluminium(III) with 2-methyl-8-quinolinol derivatives (HA), such as 2-methyl-8-quinolinol (HMQ), 2-methyl-5-methoxymethyl-8-quinolinol (HMO
1Q), and 2-methyl-5-octyloxymethyl-8-quinolinol (HMO
8Q), from a weakly acidic solution into heptane was studied in both the absence and presence of 3,5-dichlorophenol (Hdcp) as the synergist. In the absence of Hdcp, aluminium(III) was not extracted with HMQ and HMO
1Q due to a steric hindrance of the methyl group at the 2-position at all, but was slightly extracted with HMO
8Q. The unusual extraction behavior of aluminium(III) with HMO
8Q should be ascribed to the formation of a hydrophobic aluminium(III)-HMO
8Q complex. Aluminium(III) was quantitatively extracted with HA into heptane upon the addition of Hdcp. The large synergism of Hdcp was ascribed to the formation of an inner-sphere complex, Al(dcp)(A)
2, in an aqueous phase and an outer-sphere complex, Al(dcp)(A)
2·Hdcp, in the organic phase. The compositions of extracted aluminium(III)-HA-Hdcp were assigned to be Al(dcp)(A)
2·Hdcp. The synergistic extraction constants (
Kex,s = [Al(dcp)(A)
2·Hdcp]
org[H
+]
3[Al
3+]
-1[HA]
org-2[Hdcp]
org-2) for Al(dcp)(A)
2·Hdcp are 10
-5.75±0.12 (HMQ), 10
-4.74±0.05 (HMO
1Q), and 10
-3.98±0.19 (HMO
8Q), respectively, at
I = 0.1 M (H, Na) ClO
4 (1 M = 1 mol dm
-3) and 25°C.
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