Electronic absorption spectroscopic investigation of the solution binding of Hg(II) and Pd(II) to the upper rim substituted chromogenic calix[4]arene: 5,11-bis-(2-allylthiophenylazo)-25,26,27,28-tetrtahydroxy calix[4]arene and 5,11,17,23-tetrakis-(2-cetylthiophenylazo)-25,26,27,28-tetrahydroxycalix[4]arene
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