Selective coulometric release of ions from ion selective polymeric membranes for calibration-free titrations

Abstract

Coulometry belongs to one of the few known calibration-free techniques and is therefore highly attractive for chemical analysis. Titrations performed by the coulometric generation of reactants is a well-known approach in electrochemistry, but suffers from limited selectivity and is therefore not generally suited for samples of varying or unknown composition. Here, the selective coulometric release of ionic reagents from ion-selective polymeric membrane materials ordinarily used for the fabrication of ion-selective electrodes is described. The selectivity of such membranes can be tuned to a significant extent by the type and concentration of ionophore and lipophilic ion-exchanger and is today well understood. An anodic current of fixed magnitude and duration may be imposed across such a membrane to release a defined quantity of ions with high selectivity and precision. Since the applied current relates to a defined ion flux, a variety of non-redox active ions may be accurately released with this technique. In this work, the released titrant's activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faraday's law of electrolysis. Initial examples of coulometric titrations explored here include the release of calcium ions for complexometric titrations, including back titrations, and the release of barium ions to determine sulfate.

Potentiometric flow injection sensing system for determination of heparin based on current-controlled release of protamine

“…Recently, it has been found that the ion fluxes across the membrane can be modulated and controlled precisely by applying an external current [23,[30][31][32]. Previous reports have shown that ions across the polymeric membrane ion-selective electrode can be selectively and precisely delivered to the sample solution for calibration-free titrations [33]. Recently, Crespo et al developed a technique based on a thin layer Coulometry and revealed that protamine can be selectively transferred from a thin layer to the outer solution at a fixed applied potential [34].…”

Beyond potentiometry: Robust electrochemical ion sensor concepts in view of remote chemical sensing

“…If spontaneous zero current ion fluxes are negligibly small, an imposed current pulse will deliver an defined quantity of material into the sample without any need for titrimetric standards or calibration procedures, with a very high degree of selectivity. This concept has just recently been demonstrated with a polymeric ion-selective membranes for the purpose of accurately delivering reagents [22]. Figure 4 demonstrates the coulometric delivery of calcium for the titration of EDTA.…”

“…Figure 4 demonstrates the coulometric delivery of calcium for the titration of EDTA. Here, one ion-selective membrane was electrochemically controlled to deliver calcium ions, and a second ion-selective membrane of similar composition was used as the detector electrode [22]. The simple relationship between applied number of coulombs and expelled amount of ions (Faraday's law) coupled with the high ion selectivity of such membranes makes this a very promising technique to deliver reagents without the need for maintaining accurate standard solutions.…”

“…Chemically selective controlled potential coulometry with ion-selective membranes [22]. An imposed current translates into the release of a defined quantity of ions from the membrane into the sample.…”

Potentiometric sensing of butyrylcholinesterase based on in situ generation and detection of substrates

“…In addition, the ion flux of primary ions in the direction of the outer solution could be modulated by an external voltage or a current. 19 BuchE, as a target enzyme for pesticide determination and drug screening, can catalyze the hydrolysis of Buch. The ISE membrane potential generated by the surface Buch may therefore be sensitive to BuchE.…”