Potentiometric Mode

The relative changes of the oxygen concentration in the liquid metal are determined from the solution of the diffusion equation in the endless cylinder  

For Fq 0.1 with accuracy 1.5%, the first member of the row n = 1 would be sufficient. Then, 

The oxygen flow through the solid electrolyte membrane can be found by combining Equation (4.110) with Equations (4.46) and (4.48)  

The purification speed is found as v (t) = j (t) S. Consequently, the purification speed at 1 after an appropriate substitution is given as 

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Potentiometric mode There is no essentially different principle involved from that on which the fuel cell is based. The distinction is that in the case of the fuel cell the required output is power whereas with the sensor it is either a small voltage or small current that monitors some chemical characteristic of the ambient. 

Most scanning electrochemical microscopy (SECM) experiments are conducted in the amperometric mode, yet microelectrodes have for many years been used as potentiometric devices. Not surprisingly, several SECM articles have described how the tip operated in the potentiometric mode. In this chapter we aim to present the background necessary to understand the differences between amperometric and potentiometric SECM applications. Since many aspects of SECM are covered elsewhere in this monograph, we have focused on the progress made in the held of potentiometric microelectrodes and presented it in the context of SECM experiments. Starting with an historical perspective, the key discoveries that facilitated the development and applications of micro potentiometric probes are highlighted. Fabrication techniques and recipes are reviewed. Basic theoretical principles are covered as well as properties and technical operational details. In the second half of the chapter, SECM potentiometric applications are discussed. There the differences between the conventional amperometric mode are developed and emphasized. 

The earliest forms of scanning electrochemical microscopic experiments were carried out in the potentiometric mode. Evans (65) reported the use of... 

In the potentiometric mode the single sensor tip is only able to image the distribution of a specific ion. A complex multibarrel tip is needed to follow different species simultaneously. This is in contrast to the amperometric mode, where a redox mediator is selected by adjusting the tip potential. Thus, by having more than one redox mediator in solution, it is possible to produce several amperometric images simply by changing the tip potential each time. To our knowledge no one has made use of this idea, but in principle it is feasible. 

One should not see the potentiometric mode as an alternative but as a complement to the amperometric mode. The potentiometric mode offers the possibility of performing measurements on non-redox active species, e.g., not detectable amperometrically in aqueous solutions. Another advantage is the increased selectivity of the potentiometric response compared to the Faradaic response. The logarithmic response is also useful when the species of interest is in low concentrations. Since the tip is passive, only substrate generation-tip detection experiments are possible. However, the tip does not alter the concentration profile of the chemically or electrochemically generated species. The potentiometric micropipette tips have very small dimensions, typically 1 /xm or less, and the shielding is minimal. 

However, one should not forget that the potentiometric mode has several drawbacks. The fabrication of most potentiometric tips is much more involved than that of the conventional amperometric tips. It is necessary to have the electrode made by a very skilled technician. Despite this, even when following a proven recipe the success rate is relatively low. The response of potentiometric tips is not always Nemstian and a calibration is required before and after performing the experiment. The behavior typically varies from one microelectrode to another. Potentiometric tips cannot rely on positive and negative feedback diffusion, thus it is difficult to assess the tip-substrate distance from the tip response. Several approaches are available, but most are cumbersome. In the potentiometric mode the response... 

Ion-selective electrodes with tip diameters in the range of 0.5 to 10 pm have been developed for ions such as potassium, calcium, and chloride, and these have been used to study the distribution of these ions in both the extra- and intra-cellular fluid. These electrodes are used in the potentiometric mode, and the specificity is established by using a selective membrane that is only permeable to the ion of interest. Voltammetric techniques have also been useful for in vivo measurements the most widely used is the oxygen electrode, which incorporates a polymer film that is only permeable to oxygen. 

Electrodes are used as sensors in either a potentiometric mode or an amperometric mode. As the names imply, potentiometric electrodes measure electrochemical activity by relating it to a potential (voltage). Amperometric electrodes measure electrochemical activity by relating it to a quantity of current (amperes). Both modes have found wide application. Ion-selective electrodes generally operate in the potentiometric mode. Amperometric sensors, conversely, generally use nonselective electrodes which can be made selective by electrochemical and nonelectrochemical modification. Potentiometric electrodes operate via a number of presently ill-defined mechanisms. However, regardless of the mechanism, the measured potential is due to an interfacial chemical equilibrium that does not involve a bulk transfer of material. Amperometric electrodes, on the other hand, do involve the bulk transfer of material. 

However, the lambda sensors in the potentiometric mode, due to their log dependence on oxygen partial pressure, cannot be used in lean-bum conditions, as deviations of a few mV are sufficient to produce imacceptable errors in the control of air/fuel ratio. This has led to the development of amperometric sensors, in which current is measured which has a linear dependence upon oxygen concentration (Kaneko et al. 1987, Maskell and Steele 1988). 

Serrapede, M., Denuault, G., Sosna, M. et al (2013) Scanning electrochemical microscopy using the potentiometric mode of SECM to study the mixed potential arising from two independent redox processes. Analytical Chemistry, 85, 8341-8346. 

Enzyme activity has been the subject of numerous studies by SECM [39—42]. The enzymes were usually attached raito a uOTicOTiductive surface, and either the feedback or the generation-collection modes were used (Fig. 4). Yet, there are a few studies that used a potentiometric mode in which the enzymatic activity affected local changes in pH that were measured with a pH microelectrode. DNA, proteins, and antibodies have also been targeted... 

The SECM potentiometric mode makes use of ion-selective micropipettes. These devices increase the range of detectable species by SECM but are technically very challenging to make and must be cahbrated before and after experiments. The electronic requirements to acquire good data are very specific and must include a means to adequately evaluate the tip-substrate distance. Potentiometric detection remains underexploited in SECM studies and has great promise in terms of biological applications. 

Chemical sensing and biosensing were performed using an ammonium-selective PVC-membrane and a urease-membrane, additionally [4] Both membranes were successively coated onto the sensor array The membranes were completely covered with pure buffer solution, whereas the double liquid lunction was filled with 1 0 M ammonium chloride or urea in the same buffer solution Thus, after the contact of the double liquid junction and the buffer solution on the chip ammonium chloride or urea, respectively, diffused into the pure buffer solution on the chip, finally resulting in a homogeneous distribution all over the sensor array The distribution of ammonium chloride as well as urea was imaged by successive scans in potentiometric mode The scan time for a complete potentiometric scan was alwavs 2 5 min... 

A Standard one-electrode configuration for potentiometric measuranents consists of the potentio-metric electrode connected to a voltage follower and the reference electrode connected to electrical ground. To improve noise level and drift, a differential potentiometric mode can be employed in a three-electrode configuration consisting of the SECM potentiometric probe and two independent external references. For example, the potential difference between the potentiometric probe and a large inert Pt electrode is compared to the potential difference between a large reference electrode of the same nature as the potentiometric probe and the Pt electrode The Pt wire... 

The earliest forms of SECM experiments were carried out in the potentiometric mode. Evans reported how he used a traveling reference electrode to map equipotential surfaces in solution and calculate corrosion rates on a water pipeline. Isaacs and coworkers revived the idea in 1972 and coined the acronym SRET for scanning reference electrode technique. Instruments used to be homemade, often with rudimentary traveling devices such as the arm of an X-Y recorder and the spatial resolution was at best submillimeter. A computer-controlled instrument was commercially produced by Uniscan Instruments with a spatial resolution around 20 pm. The SRET has now been superseded by the scanning vibrating electrode technique, known as SVET, which achieves higher spatial resolution and improved sensitivity. A SVET manufactured by Uniscan Instruments is now commercially available from Uniscan Instruments and Princeton Applied Research. 

Experimentally, the tip position is precisely known but only relative to a reference location in the solution. In the amperometric mode, the tip acts as a kind of electrochemical radar that produces/ consumes redox species and the resulting concentration gradients in the tip-substrate gap yield a unique tip current-distance dependence. In the potentiometric mode, however, the tip is passive and does not interfere with the concentration profiles except by shielding the sample surface from the bulk solution. Several procedures have therefore been reported to assess the tip-substrate distance in these conditions. The simplest consists in approaching the tip under visual observation with a microscope until it is seen to barely touch the substrate surface. This position is then defined as the zero on the tip-substrate distance scale. The method is adequate when the user does not require an accurate estimate of the tip-substrate distance. More sophisticated procedures have been reported and are now described in the following sections. 

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