CHROMOGENIC CRYPTAHEMISPHERAPLEXES FOR THE DETERMINATION OF POTASSIUM IN AQUEOUS-SOLUTIONS

A series of novel chromogenic cryptahemispheraplexes whose core structures are hybrids between those of cryptands and spherands were designed and synthesized for the selective complexation of potassium in aqueous solutions. Two different cavity sizes obtained by combining diaza-18-corand-6, [2.2], and diaza-21-corand-7, [3.2], with the half spherand were explored. These compounds bear various picrylamine-type chromophoric groups for generating a spectrophotometric response. The resulting compounds having the [2.2] cavity are 2,4-dinitro-6-(trifluoromethyl)anilino (2), 2,6-dinitro-4-(trifluoromethyl)anilino (3), 4-cyano-2,6-dinitroanilino (4) and 6-(N,N-diethylsulfonamide)-2,4-dinitroanilino (5) and having the [3.2] cavity 2,4,6-trinitroanilino (6). A decrease in the pK(a) of the chromogenic compound is observed with complexation of potassium ion, which results in an increase in absorbance observed at the characteristic wavelength of the ionized form of the chromophoric group. The pK(a) values for the NaBr complexes (the original form of the chromogenic cryptahemispheraplexes) ranged from 7.93 to 9.27 with DELTA-pK(a) [pK(a)(KL) - pK(a)(NaL)] values of -0.39 (6), -0.59 (3), -0.99 (4), -1.30 (5), and -1.69 (2). The best responses to potassium were obtained with compounds 2, 5 and 6, which also had the best molar absorptivities, 1.9 x 10(4)-2.0 x 10(4) 1 mol-1 cm-1. Compound 6, owing to the larger cavity size, was less sensitive towards potassium and suitable for use with potassium solutions of higher concentration.