Highly selective methylammonium-selective membrane electrode made by the modification of the upper rim of calix[6]arene derivatives

摘要

The upper rim of calix[6]arene-hexaacetic acid hexaethyl esters was modified with various alkyl substituents to develop ionophores for constructing organic ammonium ion-selective membrane electrodes. We found that the substitution of bulkier alkyl groups enabled a more effective discrimination of simple aliphatic ammonium ions such as methyl ammonium and ethylammonium. In particular, an electrode based on p-1,1,3,3-tetramethylbutylcalix[6]arene-hexaacetic acid hexaethyl ester showed the highest response to methylammonium among various organic ammonium ions and inorganic cations, giving the following selectivity coefficients; log k_(CH_3NH_3~+,X)~(Pot) : C_2H_5NH_3~+, -0.2; C_3H_7NH_3~+, -1.4; C_4H_9NH_3~+, -1.9; C_6H_(13)NH_3~+, -1.6; phenethylammonium, -1.9; Na~+, -2.8; K~+, -1.8; Rb~+, -1.6; Cs~+, -0.2; NH_4~+, -2.8. The electrode exhibited a near Nernstian response to methylammonium in the concentration range of 1 x 10~(-5) to 1 x 10~(-2) M with a slope of 58.5 mV per concentration decade in 0.1 M MgCl_2. The limit of detection was 3 x 10~(-6) M. The electrode could be used over a pH range of 2-10. Formation of a 1:1 complex between p-1,1,3,3-tetramethylbutylcalix[6]arene-hexaacetic acid hexaethyl ester and methylammonium was confirmed by electrospray ionization mass spectrometry. The response characteristics of the methylammonium-selective electrode were compared with those of NH_4~+-selective electrodes so far developed.