Ion Selective Polymeric Membranes as Chemically Selective Coulometric Electrodes

Abstract

Potentiometric and optical ion sensors use highly selective lipophilic ionophores as a special additive in a plasticized PVC membrane have been thoroughly studied for application as a coulometric electrode. This dissertation presents theory, experimental evidence and analysis of membranes optimized to suit the needs of a coulometric electrode. These electrodes function on the basis of ion transport from the viscous polymeric phase into the sample by means of current control. Selective coulometric release of non-electroactive ionic reagent is carried out using an ion-selective polymeric membrane. This is an elegant method for establishing calibration-free titration techniques. The ion selective membrane has been applied as a coulometric electrode for several well-known and useful complexometric and precipitation titrations. A coulometric technique involving constant current pulses and ramping current pulses were utilized with either potentiometric or pulse galvanostatic detection in conjunction. A small volume of 1mL was used as a sample. The inner membrane phase boundary is polarizable unlike the metal-based coulometric electrodes. Experiments were carried out with PVC membranes containing a lipophilic salt in a high concentration of 10wt%. Chronopotentiometry has been utilized to characterize the ion selective membranes for their polarizability under constant current coulometry. Anodic electrolysis current across the membrane results in coulometric release of cations. The high selectivity as a result of lipophilic ionophore in the membrane results in good coulometric efficiency, even in the presence of interfering ions in the background. This process completely obeys Faraday’s law until before the region of concentration polarization. This polarization behaviour depends on the nature and the activity of sample anions and cations and on the magnitude of current applied with the duration of applications taken into consideration. Knowledge of this polarization behaviour is essential for design and applications of coulometric electrodes. A simple model is put forward based on the chronopotentiometric experiments. This coulometric technique is utilized to release calcium from the calcium selective membrane electrode to titrate EDTA in the complexometric direct and back titrations. This is especially useful in the estimation of nickel concentration. The coulometric protamine selective membrane has been utilized to release protamine and titrated against anticoagulant heparin in a clinically relevant concentration range as an example of coulometric non-electroactive polyion release The pulse galvanostatic PVC/DOS based membranes were also developed for whole blood protamine sensing in view of this matrix being better for in vivo applications.