Applications reference electrodes

This electrode is characterized by equilibria analogous to those typical for calomel electrode. Due to these processes potential measurements carried out at various temperatures may not be reproducible. Results with reproducibility lower than 40 pV were presented by Gupta et al. [131]. Hg2Br2 is less soluble than Hg2Cl2 however, it easily forms complexes, disadvantageous fi om the point of view of reference electrode applications [132]. 

The apparatus consists of a tip-position controller, an electrochemical cell with tip, substrate, counter and reference electrodes, a bipotentiostat and a data-acquisition system. The microelectrode tip is held on a piezoelectric pusher, which is mounted on an inchwomi-translator-driven x-y-z tliree-axis stage. This assembly enables the positioning of the tip electrode above the substrate by movement of the inchwomi translator or by application of a high voltage to the pusher via an amplifier. The substrate is attached to the bottom of the electrochemical cell, which is mounted on a vibration-free table [, and ]. A number... 

Table 3-1 Data and application range of important reference electrodes...

Reference electrode Me/Me" system Electrolyte Potential at 25°C (V) Temperature dependence (mV/°C) Application... 

In the same way that potential differences can occur due to different mobility, they can also occur due to different adsorption of ions. There are therefore a large number of possibilities for potential errors in the field of reference electrodes [2], which, however, are generally less than 30 mV. Such potential errors can be neglected in the application of protection potential criteria, but they can lead to increased error in the evaluation of voltage cones (see Section 3.3.1). Equation (3-4) can be used for their evaluation in this case. It explains, for example, the increased... 

In Fig. 3-25 the locational dependence of t/, and is shown together. For practical applications and because of possible disturbance by foreign fields (e.g., stray currents) and t/g are less amenable to evaluation than f/g, which can always be determined by a point of inflection between two extreme values [50]. Furthermore, it should be indicated by Fig. 2-7 that there is a possibility of raising the sensitivity by anodic polarization which naturally is only applicable with small objects. In such cases care must be particularly taken that the counter electrode is sufficiently far away so that its voltage cone does not influence the reference electrodes. 

Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

Fig. 19.36 Basic circuit for a poiemiostat. (a) Basic circuit for a potentiostat and electrochemical cell, (b) Equivalent circuit, (c) Circuit of a basic potentiostat. A.E. is the auxiliary electrode, R.E. the reference electrode and W.E. the working electrode (6 and c are from Polen-tiostat and its Applications by J. A. von Fraunhofer and C. H. Banks, Butlerworths (1972))...

C. Potentiometric methods. This is a procedure which depends upon measurement of the e.m.f. between a reference electrode and an indicator (redox) electrode at suitable intervals during the titration, i.e. a potentiometric titration is carried out. The procedure is discussed fully in Chapter 15 let it suffice at this stage to point out that the procedure is applicable not only to those cases where suitable indicators are available, but also to those cases, e.g. coloured or very dilute solutions, where the indicator method is inapplicable, or of limited accuracy. 

The titrations so far discussed in this chapter have been concerned with the use of a reference electrode (usually S.C.E.), in conjunction with a polarised electrode (dropping mercury electrode or rotating platinum micro-electrode). Titrations may also be performed in a uniformly stirred solution by using two small but similar platinum electrodes to which a small e.m.f. (1-100 millivolts) is applied the end point is usually shown by either the disappearance or the appearance of a current flowing between the two electrodes. For the method to be applicable the only requirement is that a reversible oxidation-reduction system be present either before or after the end point. 

The rate of ammonia production was enhanced by more than 1100% in the nitrogen rich regime (Figs 9.33 and 9.34), upon potential application of -IV between the working electrode and the Ag reference electrode. The extent of the NEMCA effect depends strongly on the kinetic regime of the reaction. Very pronounced non-faradaic behavior is observed in the regime 0.33

[Pg.470]

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

The microarray electrodes used for solid state electrochemistry are a slight variation of the transistor decribed in Sect. 5.2.2 The most appealing feature is the location of all the necessary electrodes on a single microchip, the reference electrode being provided by the application of a droplet of silver epoxy to one of the gold micro electrodes (Fig. 7). 

One barrel-tip contains the organic membrane phase and an internal reference electrode the other constitutes a second reference electrode. A four-barrel configuration with a 1-pm tip in which three barrels are liquid membrane electrodes for Na, Ca and and the fourth is a reference electrode has been reported Some representative applications of ion-selective electrodes for intracellular measurements are shown in Table 3. 

FIG. 13 Apparatus for measuring electrical potential oscillation across the octanol membrane with application of current through the interface between phases o and wl (a) octanol phase, (b) aqueous phase, (c) Ag/AgCl reference electrode, (d) KCl salt bridge, and (e) saturated KCl. 

Before dealing with various important applications of the electrochemical series with some practical examples, a break is given here to present a more detailed elaboration on the hydrogen electrode, reference electrodes, and some of the theoretical and general aspects pertaining to electrode potentials and free energy changes involved with cell reactions. 

Subsequently, Bos and Dahmen used in m-cresol65 (e = 12.29 at 25° C) a potentiometric titration method combined with conductometry. Essential precautions were the preparation of water-free m-cresol (<0.01% of water), the use of a genuine Bronsted base B, e.g., tetramethylguanidine (TMG), and the application of a glass electrode combined with an Ag-AgCl reference electrode filled with a saturated solution of Me4NCl in m-cresol. The ion product of the self-dissociation of m-cresol, Ks, was determined from the part beyond the equivalence point of the potentiometric titration curve of HBr with TMG comparison with titration curves calculated with various Ka values showed the best fit for Ks = 2 10 19... 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

Figure 4 shows the application (6) of potentials to the Pt and Au electrodes of the sandwich (vs. a reference electrode elsewhere in the contacting electrolyte solution) so that they span the E° of the poly-[Co(II/I)TPP] couple (Fig. 4B). There is a consequent redistribution of the concentrations of the sites in the two oxidation states to achieve the steady state linear gradients shown in the inset. Figure 4C represents surface profilometry of a different film sample in order to determine the film thickness from that the actual porphyrin site concentration (0.85M). The flow of self exchange-supported current is experimentally parameterized by applying Fick s first law to the concentration-distance diagram in Fig. 4B ...

One may think that the idea of detecting ionic compounds such as heparin using polymeric ion-selective electrodes seems very difficult due to the high charge of polyionic molecules, which makes the slope of the electrode function negligibly small for an analytical application. Indeed, for heparin-selective electrodes the theoretical slope is less than lmV decade 1 and the potential practically does not depend on heparin concentration, which means that this ISE can be useful as a reference electrode [33], Nonetheless, Ma and Meyerhoff noticed that the potential of polymeric membrane... 

Reference electrodes provide a standard for the electrochemical measurements. For potentiometric sensors, an accurate and stable reference electrode that acts as a halfcell in the measurement circuit is critical to providing a stable reference potential and for measuring the change in potential difference across the pH sensitive membrane as the pH concentration changes. This is especially important in clinical applications such as pH measurements in the blood, heart, and brain, where the relevant physiological pH range is restricted to a very small range, usually less than one unit. 

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