The complexation reactions between some rare earth metal cations (Ln; Y3+, La3+ and Ce3+) with 18-crown-6 (18C6), dicyclohexyl-18-crown-6 (DC18C6),benzo-18-crown-6 (B18C6) and decyl-18-crown-6(Dec18C6), have been studied in methanol–acetonitrile(MeOH–AN) and methanol–water (MeOH–H2O) binary mixtures using a competitive spectrophotometric method.2-(2-thiazolylazo)-4-methyl phenol (TAC or L) was used as colorimetric complexant. It was found that the selectivity order of TAC for Ln cations is highly changed with changing the composition of the mixed solvents. Moreover,as the concentration of acetonitrile increases in MeOH–ANbinary mixture, the stability of Ln–TAC complexes increases and passes through a maximum at a certain mole fraction of acetonitrile. In addition, the stability of Ln–crown ether complexes increases with increasing the concentration of methanol in MeOH–H2O and acetonitrile in MeOH–AN binary solutions. A non linear behaviour was observed for variation of stability constants of all complexes versus the composition of the mixed solvents. The results show that 18C6 generally forms more stable complexes with La3+ and Ce3+ cations than DC18C6 inmethanol and MeOH–H2O binary mixtures, while thissequence is reversed in the methanol-acetonitrile binary mixtures which are rich with respect to acetonitrile.

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