Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes

An electrochemical sensing protocol based on supported liquid ion-selective membranes for the direct detection of total alkalinity of a sample that contains a weak base such as Tris (pKa = 8.2) is presented here for the first time. Alkalinity is determined by imposing a defined flux of hydrogen ions from the membrane to the sample with an applied current. The transition time at which the base species at the membrane–sample interface depletes owing to diffusion limitation is related to sample alkalinity in this chronopotentiometric detection mode. The same membrane is shown to detect pH (by zero current potentiometry) and acidity and alkalinity (by chronopotentiometry at different current polarity). This principle may become a welcome tool for the in situ determination of these characteristics in complex samples such as natural waters

Thin-Layer Chemical Modulations by a Combined Selective Proton Pump and pH Probe for Direct Alkalinity Detection

We report a general concept based on a selective electrochemical ion pump used for creating concentration perturbations in thin layer samples (~ 40 mL). As a first example, hydrogen ions are released from a selective polymeric membrane (proton pump) and the resulting pH is assessed potentiometrically with a second membrane placed directly opposite. By applying a constant potential modulation for 30 s, an induced proton concentration of up to 350 mm may be realized. This concept may become an attractive tool for in situ titrations without the need for sampling, because the thin layer eventually re-equilibrates with the contacting bulk sample. Acid–base titrations of NaOH and Na2CO3 are demonstrated. The determination of total alkalinity in a river water sample is carried out, giving levels (23.1 mm) comparable to that obtained by standard methods (23.6 mm). The concept may be easily extended to other ions (cations, anions, polyions) and may become attractive for environmental and clinical applications

Direct Ion Speciation Analysis with Ion-Selective Membranes Operated in a Sequential Potentiometric/Time Resolved Chronopotentiometric Sensing Mode

Ion-selective membranes based on porous polypropylene membranes doped with an ionophore and a lipophilic cation-exchanger are used here in a new tandem measurement mode that combines dynamic electrochemistry and zero current potentiometry into a single protocol. Open circuit potential measurements yield near-Nernstian response slopes in complete analogy to established ion-selective electrode methodology. Such measurements are well established to give direct information on the so-called free ion concentration (strictly, activity) in the sample. The same membrane is here also operated in a constant current mode, in which the localized ion depletion at a transition time is visualized by chronopotentiometry. This dynamic electrochemistry methodology gives information on the labile ion concentration in the sample. The sequential protocol is established on potassium and calcium ion-selective membranes. An increase of the ionophore concentration in the membrane to 180 mM makes it possible to determine calcium concentrations as high as 3 mM by chronopotentiometry, thereby making it possible to directly detect total calcium in undiluted blood samples. Recovery times after current perturbation depend on the current amplitude but can be kept to below 1 min for the polypropylene based ion-selective membranes studied here. Plasticized PVC as membrane material is less suited for this protocol, especially when the measurement at elevated concentrations is desired. An analysis of current amplitudes, transition times, and concentrations shows that the data are described by the Sand equation and that migration effects are insignificant. A numerical model describes the experimental findings with good agreement and gives guidance on the required selectivity in order to observe a well-resolved transition time and on the expected errors due to insufficient selectivity. The simulations suggest that the methodology compares well to that of open circuit potentiometry, despite giving complementary information about the sample. The tandem methodology is demonstrated in a titration of calcium with nitrilotriacetic acid (NTA) and in the direct detection of calcium in undiluted heparinized and citrated blood

Coulometric Calcium Pump for Thin Layer Sample Titrations

A selective electrochemical calcium pump based on a fast diffusive calcium ionophore-based membrane is reported. An initially nonpolarized ionophore-based membrane allows one to establish a net calcium flux by applying a potential step function (i.e., 250 mV for 30 s). The resulting calcium flux is released into a microliter scale thin layer reservoir, and the resulting ion perturbation is monitored by either a potentiometric or a coulometric readout. This chemical perturbation in the thin layer thus acts as a titration agent that is precisely controlled by coulometry. A linear correlation between released and detected calcium is confirmed by the two different readout modes. Having demonstrated the efficiency of the calcium pump in background electrolyte solutions, a complexometric titration with known concentrations of EDTA in the thin layer sample was performed. With the potentiometric readout, titrations in the range of 0.25−0.75 mM gave a precision of 3%, whereas the coulometric readout gave a range of 0.02−0.12 mM and a precision of 2%. Improved precision is expected by better control of the thin layer geometry by microfabrication. The significance of this work is that the coupling of a selective calcium pump with a thin layer element can give rise to rapid and complete sample concentration changes and result in a promising platform for titrations either on the laboratory bench or for in situ measurements in environmental or diagnostic settings

Counter electrode based on an ion-exchanger Donnan exclusion membrane for bioelectroanalysis

Ion-exchanger based Donnan exclusion membranes (IEDEM) are studied here as separators for counter and pseudo-reference electrodes in bioelectroanalysis. Since the potential across the membrane remains indifferent for a wide range of current densities in contact with electrolyte solutions, IEDEM behave as ideally non-polarizable membranes. Consequently, such membranes may be suitable with counter or reference electrode, depending on the adopted cell configuration (three- or two-electrode system). Four configurations were characterized in order to establish the limitations of commercial anion-exchanging membranes, using chronopotentiometry as readout protocol. Three- and two-electrode configurations with and without membrane exhibited similar characteristics in terms of drift and reproducibility (observed drift and RSD were 0.0007 s1/2 per scan number and 1.71%, respectively). Several currents amplitudes were applied to evaluate the upper current limits for the membranes, which was found at about 10 mA [42.8 mA cm−2]. This value is significantly above those typically used in chronopotentiometric experiments, which involve hundreds of μA. Three different analytes were measured in human whole blood using an IEDEM as a counter electrode. A divalent cation (calcium), a polyion (protamine), and an anion (chloride) were successfully determined in blood and compared to reference methods. Finally, the obtained results suggest that such membranes may be used in bioelectrochemical sensing approaches to replace expensive but less appropriate electrode materials for the measurement in matrices that contain lipids and proteins

Chronopotentiometry of pure electrolytes with anion-exchange donnan exclusion membranes

We explore here the chronopotentiometric responses of pure electrolytes with anion-exchange Donnan exclusion membranes (IEDEM). As these electrolytes are locally depleted at the membrane surface in the absence of background electrolyte, electrical migration cannot be neglected. Yet, a linear relationship between the signal readout (square root of the transition time, τ0.5) and the electrolyte concentration is observed, albeit with much larger apparent diffusion coefficients than expected. We develop here a simplified migration–diffusion model based on the Nernst–Planck equation to explain the experimental data. As the flux of the counterion at the membrane surface must be zero for a permselective membrane, electrical migration is understood to precisely counteract the contribution of diffusional mass transport. This results in a simplified understanding of the mass transport processes at such membranes. Numerical simulations are performed to compare the predicted and experimental data. Based on these mechanistic and practical insights, permselective anion-exchange membranes are shown to respond to a range of ions in a similar fashion in the concentration range of 0.1–10 mM. The membranes are able to sustain significantly high current densities of 0.4 mA mm−2 and may become useful as ion detectors or counter electrode separation materials

Reversible Sensing of the Anticoagulant Heparin with Protamine Permselective Membranes

A permselective membrane electrode allows the rapid and operationally reversible detection of the polycationic polypeptide protamine in physiological samples. Anticoagulant levels of heparin can be measured in undiluted whole blood by adding a known excess of its antidote protamine to discrete blood samples

Thin layer coulometry based on ion-exchanger membranes for heparin detection in undiluted human blood

We explore here for the first time a potentially calibration-free methodology for the detection of protamine (and, by titration, heparin) in undiluted human blood in the therapeutic concentration range from 20 to 120 mg L-1. The use of a thin layer sample (5.8 μL) confined between a tubular protamine selective membrane (inner diameter, 600 μm) and a Ag/AgCl wire (diameter 400 μm) achieves an exhaustive depletion from the sample. Coulometry detection was chosen for the interrogation of the thin layer, employing a double pulse technique with 120 s for each pulse. Protamine calibration curves were recorded at physiological concentrations and in undiluted human blood. A linear relationship was obtained in both cases, but a diminished sensitivity was observed in contact with blood, which is explained with a partial passivation of the inner Ag/AgCl element. Heparin-protamine titrations were performed in undiluted human blood samples, mimicking the final application with patients undergoing critical care. The observed values correlate satisfactorily with those of an alternative technique, so-called flash-chronopotentiometry on planar membranes

Direct Alkalinity Detection with Ion-Selective Chronopotentiometry

We explore the possibility to directly measure pH and alkalinity in the sample with the same sensor by imposing an outward flux of hydrogen ions from an ion-selective membrane to the sample solution by an applied current. The membrane consists of a polypropylene-supported liquid membrane doped with a hydrogen ionophore (chromoionophore I), ion exchanger (KTFBP), and lipophilic electrolyte (ETH 500). While the sample pH is measured at zero current, alkalinity is assessed by chronopotentiometry at anodic current. Hydrogen ions expelled from the membrane undergo acid–base solution chemistry and protonate available base in the diffusion layer. With time, base species start to be depleted owing to the constant imposed hydrogen ion flux from the membrane, and a local pH change occurs at a transition time. This pH change (potential readout) is correlated to the concentration of the base in solution. As in traditional chronopotentiometry, the observed square root of transition time (τ) was found to be linear in the concentration range of 0.1 mM to 1 mM, using the bases tris(hydroxymethyl)aminomethane, ammonia, carbonate, hydroxide, hydrogen phosphate, and borate. Numerical simulations were used to predict the concentration profiles and the chronopotentiograms, allowing the discussion of possible limitations of the proposed method and its comparison with volumetric titrations of alkalinity. Finally, the P-alkalinity level is measured in a river sample to demonstrate the analytical usefulness of the proposed method. As a result of these preliminary results, we believe that this approach may become useful for the in situ determination of P-alkalinity in a range of matrixes

A low-cost thin layer coulometric microfluidic device based on an ion-selective membrane for calcium determination

A prototype of a low-cost and easy-to-use thin layer coulometric microfluidic device based on an ion-selective membrane for calcium detection is described. The microfluidic device was fabricated and consequently assembled with inexpensive materials without using sophisticated and centralized fabrication laboratory facilities. The linear range of the device is found to be 10–100 μM for a 60 s current integration time. Preliminary validations showed that the microfluidic device is suitable for the quantification of calcium in mineral water