Ion binding properties of 5,11,17,23-tetra-tert-butyl-25,27-bis(diethylcarbamoylmethoxy)-26,28-bis(diphenylphosphinoylmethoxy)calix[4]arene towards alkaline-earth cations

The ability of 5,11,17,23-tetra-tert-butyl-25,27-bis(dietliylcarbamoylmethoxy)-26,28-bis(diphenylphosphinoylmethoxy)calix[4]arene(calix-1) to complex alkaline-earth ions was evaluated by determination of stability constant of respective complexes in methanol and mixed methanol-X solvents (X = acetonitrile or tetrahydrofuran) at 20 degrees C by UV-VIS spectroscopy. The analysis of the obtained spectra has shown a 1:2 (calix-1/metal) stoichiometric ratio for Ca2+ and Sr2+ complexes, and 1:1 stoichiometry for Ba2+ in all studied solvents. An addition of either acetonitrile or tetrahydrofuran to methanol increased stability of M-calix-1 (M-an alkaline-earth ion) complexes. The results have shown a significantly greater affinity of calix-1 receptor towards calcium ion than to other alkaline-earth ions. To demonstrate analytical applications of the studied calixarene, it has been used as an ionophore in a new PVC coated graphite membrane of Ca2+-selective electrode. The membrane comprised 6% calix-1, 61% solvent mediator (ortho-nitrophenyloctyl ether), 30% PVC, and 3% anion excluder (sodium tetraphenylborate). The electrode exhibited a near-Nernstian response of 30.7 +/- 0.2 mV/decade over a concentration range of calcium ion of 1 x 10(-5) - 1 x 10(-3) mol L-1, detection limit was 3.6 x 10(-6) mol L-1. The working pH range of the proposed electrode was 4-7. The electrode was successfully used as a potentiometric sensor in complexometric titration of calcium ion in various solutions.