Construction and Characterization of Indium Liquid Ion Selective Electrodes Based on Crown Ethers in a PVC Matrix Membrane


Abstract: Several indium ion selective electrodes were constructed based on 15-crown-5 (15C5), dicyclohexano-18-crown-6 (DCH18C6) and dibenzo-24-crown-8 (DB24C8) as sensors and di-octyl phenyl phosphonate (DOPP), di-butyl phthalate (DBPH), tri-butyl phosphate (DBP), tri-n-amyl phosphate (TAP) and bis (2-ethyl hexyl) phosphate (BEHP) as plasticizers in a PVC matrix membrane. Electrode parameters, pH effect and interferences were also studied. The indium electrode based on 15C5 using DOPP as a plasticizer gave the best response for low levels of In^{3+} ions. The linear range for this electrode ranged from 3.8 x 10^{-5} to 5.0 x 10^{-2} M (with correlation coefficient r = 0.9997), the slope was 20.1 mV/decade with a detection limit of 1.2 x 10^{-5} M, and the working pH range was 3.8--5.9. Interferences of Na^+ , K^+ , NH_4^+ , Ca^{2+} , Ba^{2+} , Cd^{2+} , La^{3+} , Nd^{3+} and Eu^{3+} on the electrode response were also investigated using a mixed solution method. Mono- and divalent cations were found to interfere less than trivalent cations. The concentrations of indium ions in the prepared standard solutions were determined by direct, standard addition (SA), multiple standard addition (MSA) and titration methods. The results were compared with those obtained by atomic absorption spectrophotometry. The precision and accuracy of each method were calculated and expressed in terms of the relative standard deviation and the relative error. Optical measurements were also studied by measuring the absorbance for each membrane after steady intervals of immersion in a standard solution of its ions using UV/Visible spectrophotometry. Each membrane showed a rapid decrease in absorbance when it reached the end of its life due to leaching of the membrane contents to the external solution.

Keywords: Indium electrodes, crown compounds, selectivity measurements, optode measurements

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