A COMPETITIVE POLAROGRAPHIC STUDY OF ALKALINE-EARTH COMPLEXES WITH SOME CROWN ETHERS USING THE TL(I)/TL(HG) COUPLE AS AN ELECTROCHEMICAL PROBE

The complex formation of alkaline earth cations with macrocyclic polyethers benzo-15-crown-5, 18-crown-6, dicyclohexyl-18-crown-6, dibenzo-18-crown-6 and dibenzo-30-crown-10 in acetonitrile solution was studied by a competitive polarographic method using the Tl+/Tl (Hg) couple as a sensitive electrochemical probe. The formation constants of the resulting 1:1 complexes were determined and found to decrease in the order Ba2+ > Sr2+ > Ca2+ > Mg2+. Among the 18-crowns used, and in the case of all alkaline earth cations studied, the stabilities of the complexes varied in the order dicyclohexyl-18-crown-6 > 18-crown-6 > dibenzo-18-crown-6. The influence of a number of members in the macrocycle and of substituents in the polyether ring on the stability of the complexes is discussed.