Instrumental limitations reference electrode

Potentiostats did not become commercially available until the late 1950s. Most earlier work was conducted either galvanostatically or potentiostatically, but with a two-electrode cell, in which one electrode served as both counter and reference electrode. Because of their complexity, potentiostats tend to have slower response times than galvanostats. It should be pointed out, though, that some of the limitations of potentiostats alluded to above are a matter of the past. With present day (1993) electronic components, it is possible to build home-made potentiostats, or to purchase commercial units, that make use of all the inherent advantages of potentiostatic measurements with little instrumental limitation, or none. 

The instrument (see Figure 27-4) is designed so that a manually or microprocessor actuated valve (V) admits calibrator gases, standard buffers, or a sample to a small chamber (C) maintained by a fluid or metal bath (B) to a constant temperature of 37 C 0.1 C, Measuring and reference electrodes (E) protrude into this chamber. In the pH calibration phase of the instrument, high pH standard buffer and low pH standard buffer are alternately admitted into the chamber and electronic responses of the upper and lower limits of a linear pH curve are established. In the gas calibration phase, gas mixtures with high and low fractional concentrations of O2 and CO2 are alternately admitted into the chamber and electronic responses of the upper and lower limits of linear PO2 and PCO2 curves are set. In the measurement phase, an anaerobically collected blood sample is admitted. 

The inhinsic disadvantage of this method is its two-terminal nature the facts that a dc potential cannot be applied to the electrode of interest with respect to a suitable reference electrode and that the potential e, across the specimen varies during the balance procedure. Since the in-phase and quadrature null signals usually are derived from a PSD, instruments of this type are limited at low frequencies to approximately 1 Hz due to the instability of analog filters with longer time constants. 

Instrumentation. The discussion here of instrumentation required for potentiometric measurements will be limited to a brief overview of suitable reference electrodes [51]. The conventional reference electrode consists of an electrode of the second kind, like the Ag/AgCl/KCl system. Much effort has been devoted to the development of miniaturized reference systems suited to direct integration into ISFETs. but most systems of this type fail to fulfill all the requirements for a true reference electrode with a well-defined and stable... 

A major advantage of this method is that it is a noncontact and nondestructive way of measuring. Additionally, it is not expected to influence the electrical or chemical structure of the material. The technique is very simple, but fast, accurate, and versatile it can, in principle, be used in many environments. However, experience has shown that for practical sensor applications Kelvin probes are not suitable due to the necessary instrumentation and the limits to miniaturization. Most probes are rather macroscopic with a reference electrode the size of a few millimeters or centimeters. In addition, this method of measurement is complicated in comparison with other methods used in gas sensing and requires specific conditions and equipment. As a result, these devices are not available on the sensor... 

Low control current (between working and reference electrode). Owing to practical considerations all real potentiostats affect a compromise of these ideals. A very fast response time, for example, implies some sacrifice of current capacity. Nonetheless, potentio-static circuitry has now been developed to a sufficient extent that purely instrumental limitations are rarely of concern in routine analytical applications. The practical magnitudes of the parameters listed above will be determined by the characteristics of the chemical system under observation and can, in many cases, be estimated by simple calculation. From the Nernst equation and Fick s first diffusion law one can calculate, for example, that the controlled-potential electrolysis of a 10 m solution of an ion M" which is reduced to will require an initial current of about 2A tmder typical diffusion controlled conditions. Thus, the potentiostat used should have a current capacity of at least 2 A. Actually, a potentiostat with a current capacity of approximately 10 A is desirable for the practical analysis of moderately concentrated solutions. 

Corrigan and Weaver employed the PDIR approach to study the potential-dependent adsorption of azide, N , at a silver electrode. The potential was switched between the reference value, —0.97 V vs. SCE (where adsorption is known to be limited) and the working potential every 30-60 scans, i.e. up to a minute per step, to a total of c. 1000 scans. The high number of scans was required in order to obtain the required S/N ratio hence the PDIR technique was employed to minimise instrumental drift. Since the electrochemical process under study was totally reversible on the timescale of the experiment, the PDIR technique was a viable option. 

Data obtained using these reference couples may be compared with analytical solutions for the peak current, limiting current or voltammetric waveshape (see Sections 3 and 4). Non-compliance of experiment and theory is indicative of malfunctioning instrumentation, poor experimental design (i.e. an unacceptably large uncompensated solution resistance) or a faulty electrode. 

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