High-resistance instruments

FIG. 1. A potentiometric cell with an ion-selective electrode. Not shown is the high-resistance instrument connecting the two electrodes. 

Voltmeters and potentiometers The instruments described here are generally referred to as corrosion voltmeters. As mentioned previously, the current flowing through any potential-measurement circuit must be small to avoid errors due to polarisation. Moreover, if the current flow is too large, errors will be introduced owing to the voltage drop caused by the contact resistance between the reference electrode and the electrolyte. It is thus clear that the prime requirement of a potential measurement circuit is high resistance. 

Valve voltmeters were widely used in the past, but have been replaced by transistor voltmeters. With instruments of this type it is possible to achieve an input resistance of 50 MQ or more, the current required to operate the instrument being of the order of 10" A. The early instruments had a tendency to zero drift on the lower ranges, but this has been overcome in the modern transistor types. Such instruments are most often used to make potential readings in extremely high-resistance electrolytes. The accuracy of such instruments is of the order of 2% full-scale deflection. It is necessary to ensure that both types are so designed that they do not respond to alternating currents. 

As their name suggests, these instruments are capable of carrying out a variety of measurements, e.g. structure/electrolyte potentials, current, resistivity and voltage. Most instruments of this type contain two meters in one case, one being a low-resistance millivolt/voltmeter and milliamp/ammeter, and the second a high-resistance voltmeter. 

Owing to the high resistance of the glass membrane, a simple potentiometer cannot be employed for measuring the cell e.m.f. and specialised instrumentation (Section 15.14) must be used. The e.m.f. of the cell may be expressed by the equation ... 

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

The sensitivity of instruments using low resistance circuits is determined primarily by the sensitivity of the galvanometer (Figure 4.5). Electrode systems that have a high resistance, e.g. glass electrodes, require a high impedance voltmeter, which converts the potential generated into current which can be amplified and measured. Such instruments are commonly known as pH meters but may be used for many potentiometric measurements other than pH. 

The synthesis of AIN described by Janes et al. (2003) may be mentioned as an example. The applications of this compound, mainly as a management material for silicon-based electronics, have been summarized together with its relevant properties (low coefficient of thermal expansion, close to that of Si, high thermal conductivity, high resistivity and low dielectric constant). Different preparation methods, often involving complex instruments, have been mentioned ion beam evaporation,... 

As electrode systems involving ISEs usually have a high resistance, up to several Gf2, the meter should have the highest possible input impedance. To obtain a measuring error of 0.1%, the input impedance of the meter should be 10 times the cell resistance. To keep the overall measuring error at a level of a few per cent, the meter should have a resolution of the order of 0.1 mV. Modem instruments readily comply with these requirements. For a useful view of these aspects see [21]. 

Ruthenium alloyed to platinum, palladium, titanium and molybdenum have many apphcations. It is an effective hardening element for platinum and palladium. Such alloys have high resistance to corrosion and oxidation and are used to make electrical contacts for resistance to severe wear. Ruthenium-palladium alloys are used in jewelry, decorations, and dental work. Addition of 0.1% ruthenium markedly improves corrosion resistance of titanium. Ruthenium alloys make tips for fountain pen nibs, instrument pivots, and electrical goods. Ruthenium catalysts are used in selective hydrogenation of carbonyl groups to convert aldehydes and ketones to alcohols. 

If the three-electrode instrument is equipped with an iR-drop compensator, most of the iT-drop caused by the solution resistance can be eliminated. However, in order to minimize the effect of the iT-drop, a Fuggin capillary can be attached to the reference electrode with its tip placed close to the indicator electrode. Moreover, for a solution of extremely high resistance, it is effective to use a quasi-reference electrode of a platinum wire (Fig. 8.1(a)) or a dual-reference electrode (Fig. 8.1(b)), instead of the conventional reference electrode [12]. 

UMEs of 10 pm in diameter and voltammetric instruments for use with such UMEs are commercially available. Electrodes of smaller dimensions can be prepared in the laboratory, although this requires considerable skill [74], In order to use UMEs successfully for high-speed voltammetry in highly resistive solutions, care must be taken concerning the effects of the ohmic drop and the capacitance of the cell system [65 b, 74, 75]. Moreover, two types of voltammograms, i.e. curves (a) and (b) in Fig. 5.23, should be used appropriately, according to the ob-... 

It is costly to make and fabricate because temperature in excess of 1800°C is required lo manufacture it. However, its refractory character coupled with its very high resistance to thermal shock makes it ideal for special laboratory equipment, windows in high-temperature environments, and instruments. 

Another important challenge that remains to be addressed is the relatively low success rate in obtaining useable and consistent high quality patch clamp data from most of these instruments. The success rate currently stands at around 40-60% for most systems [64], excluding the Ion Works Quattro that incorporates population patch clamping and claims a success rate of > 95% [53]. Some of the factors that contribute to this lack of success include stable positioning of the cells over the pore of the planar electrode, lack of adequate suction control in some cases, and the inability to achieve high resistance seals or electrical access. 

The PatchXpress, the QPatch (Sophion Biosciences) and the Patchliner (Nanion Technologies) also use planar substrates, but recording from individual cells occurs in asynchronous fashion, limiting the throughput of these instruments. Commercially available instruments use 16-well or 48-well disposable cartridges. High-resistance seals between the cells and the substrate... 

Another important future direction is in the use of microelectrodes and microelectrode arrays. They are often easier to manipulate by the inexperienced, and instrumentation is simpler. They can be used in highly resistive dirty media where conventional electrodes may be unuseable and are able to probe localized concentrations. Composite electrodes42, of which carbon paste is an example, if conveniently prepared, can act as microelectrode assemblies. In a more general sense, lithographic and... 

The relationship between the solution resistance and the shortest relaxation time of the reaction that can be studied can perhaps be clarified by the following numerical example. Consider a small elec-trode of 0.05 cm, for which C =1.0 pF, and assume that the charge injected is 0.01 pC/cm yielding a value of T = 10 mV. employing high quality instrumentation one can measure the decay of overpotential with, sufficient accuracy if iR is n be expressed by the inequality... 

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