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.
For about one hundred years, potentiometry with ion-selective electrodes has been one of the dominating electroanalytical techniques. While great advances in terms of selective chemistries and materials have been achieved in recent years, the basic manner in which ion-selective membranes are used has not fundamentally changed. The potential readings are directly co-dependent on the potential at the reference electrode, which requires maintenance and for which very few accepted alternatives have been proposed. Fouling or clogging of the exposed electrode surfaces will lead to changes in the observed potential. At the same time, the Nernst equation predicts quite small potential changes, on the order of millivolts for concentration changes on the order of a factor two, making frequent recalibration, accurate temperature control and electrode maintenance key requirements of routine analytical measurements. While the relatively advanced selective materials developed for ion-selective sensors would be highly attractive for low power remote sensing application, one should consider solutions beyond classical potentiometry to make this technology practically feasible. This paper evaluates some recent examples that may be attractive solutions to the stated problems that face potentiometric measurements. These include high amplitude sensing approaches, with sensitivities that are order of magnitude larger than predicted on the Nernst equation; backside calibration potentiometry, where knowledge of the magnitude of the potential is irrelevant and the system is evaluated from the backside of the membrane; controlled current coulometry with ionselective membranes, an attractive technique for calibration free reagent delivery without the need for standards or volumetry; localized electrochemical titrations at ion-selective membranes, making it possible to design sensors that directly monitor parameters such as total acidity for which volumetric techniques were traditionally used; and controlled potential coulometry, where all ions of interest are selectively transferred into the ion-selective organic phase, forming a calibration free technique that would be exquisitely suitable for remote sensing applications. KeywordsIon-selective electrodes; chemical sensors; reference electrode; calibration free; coulometry; remote sensing Direct Potentiometry: The State of the Art and Promise for Remote SensingIon-selective electrodes are among the oldest known types of chemical sensors [1]. In most cases, an ion-selective membrane separates the sample from an inner solution, and the electromotive force between reference electrodes in the aqueous sample and the inner solution Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the p...
Plasticized polymeric membrane electrodes containing selective chemical receptors (ionophores) that are used in potentiometric and optical sensors have recently been shown to be attractive in controlled current coulometry for calibration free reagent delivery. For this purpose, a galvanostatic pulse of fixed duration is applied across an ionselective membrane containing added ion-exchanger sites, resulting in the release of a calculated amount of ions from the membrane into the sample phase. This paper evaluates the operational limits of such coulometric actuators with chronopotentiometry, using silver-selective membranes as model systems. Diffusion theory predicts the depletion of ionophore and ion-exchanger at one of the two interfaces in a matter of minutes, owing to the relatively small diffusion coefficients in such membranes. Chronopotentiometry on membranes containing lipophilic cations at either side of the membrane, rather than silver ions, confirms that the ionophore depletes at the inner side of the membrane. At the sample side, ionophore is expected to increase in concentration and does not result in a loss of selectivity. Chronopotentiometric responses show a drastic transition at long times, typically 30 min, which cannot plausibly be explained by the depletion of added lipophilic ion-exchanger at the sample side since the diffusion coefficients are similar to that of ionophore. It is postulated that the intrinsic anion-exchanger sites of the PVC matrix are relatively immobile and do not easily concentration polarize upon application of a transmembrane current pulse, in agreement with Bucks earlier work on fixed site membranes. Indeed, silver and calcium-selective membranes fabricated with increased concentrations of such fixed sites, by using carboxylated PVC, exhibited chronopotentiometric breakdown times larger than 60 min and no loss in coulometric efficiency. The results obtained here will help in designing coulometric actuators with improved characteristics on the basis of hydrophobic polymeric ion-selective membranes.
Comprehensive 2D gas chromatography has been utilized for analyzing complex mixtures of hydrocarbons of diesel feeds. Here, we evaluated 19 diesel feeds for their paraffinic, naphthenic, and aromatic group compositions dictating their flammability properties. Compositional ranges of feeds were as follows: paraffins: 9.6–57.8%, naphthenes: 7.9–38.5%, and aromatics: 10.5–82.3%. Diesel's flammability performance is estimated by thermodynamic conditions and rates of radical formation of hydrocarbon type in actual engine condition, limiting cetane number. However, limitations are overcome by understanding the relative compositional variations of feeds by simple ranking of feeds based on C15‐16 compositions. Due to the multidimensional variability of feeds, a principal component analysis was adopted later for its distinguishing capability. Paraffinic, naphthenic, and aromatic group's principal component analysis clustered up feeds based on the higher concentration of individual hydrocarbon group. We explored hierarchical cluster analysis to organize feeds into classes of mixed C9 to C26 paraffin's composition in the diesel range. Further, for discriminating C15–C16 enriched and depleted feeds in total paraffin composition, a row dendrogram with heat map was drawn. The above multivariate methods have led to a fair distinction of nonadditive feed compositions influencing flammability properties by radical formation rate.
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