Reference electrodes stability

In general, restricted hypertonic junctions can introduce EMF errors due to protein contamination (amorphous or precipitate overcoats) and isotonization of the junction with a subsequent influence of ionic strength. Open junctions, on the other hand, are apparently affected by the ionic form of proteins. The problem of reference electrode stability is especially important in direct measurements, as discussed above and elsewhere [9,33]. 

Most of the reference electrodes embedded in concrete are used for control of cathodic protection (CP) systems. Potential stability is then less important, compared to corrosion state monitoring. Control of CP systems requires only short-term stability, maximum 24 hours. Corrosion rate measurement, like linear polarisation resistance (LPR) measurements, also requires short-term reference electrode stability. However, regardless of application, a reference electrode which is to be permanently embedded in the test solution, e.g. concrete, must have a long life when exposed to this environment. 

Accurate control of potential, stability, frequency response and uniform current distribution required the following low resistance of the cell and reference electrode small stray capacitances small working electrode area small solution resistance between specimen and point at which potential is measured and a symmetrical electrode arrangement. Their design appears to have eliminated the need for the usual Luggin capillary probe. 

Table 7 lists the electrochemical windows or the anodic stability limits of several nonaqueous electrolytes, or their anodic stabilty, the reference electrodes Rref used, the working electrode material Ew, the experimental conditions, and the references. It shows the following features ... 

Reference electrodes of Pt-air and In-In203 are useful to measure oxygen in sodium at 700-1100 K. Problems in the stability of the electrolyte tubes require the use of Th02 and 203 . 

Ag/AgCl electrodes were inserted in the plant at different positions. Following insertion of the electrodes, the plants were allowed to rest until a stable potential difference was obtained between the measuring and reference electrodes. Insertion of electrodes in plants induces action potentials across the stem and slow fluctuations of the resting potential (Fig. 5). After approximately 1-2 h, the resting potential stabilizes and action potentials induced by wounding disappear (Fig. 6). 

An inner filling solution and internal reference electrode are used in macro ISEs due to a very good stability of the potential at the inner membrane-solution interface in such a setup (see Fig. 4.4). However, the presence of a solution inside a sensor could be a serious limitation for development of microelectrodes and may be undesired for a variety of other reasons, including ionic fluxes in the membrane and limited temperature range of sensor operation. There are several requirements for such an inner contact. First of all, a reversible change of electricity carriers ions-electrons must take place at the membrane-substrate interface. The potential of the electrochemical reaction, ensuring this transfer, has to be constant, stable, and must not depend on the sample composition. At last, the substrate must not influence the membrane analytical performance. 

The three main requirements for a satisfactory reference electrode, given by Ives and Janz [107], are reversibility (non-polarizability), reproducibility, and stability. Hydrogen electrodes have been chosen as the primary reference electrode due to their excellent reproducibility [54], The electrode is represented schematically as... 

As a secondary reference electrode, the Ag/AgCl electrode is the most common due to its simplicity, stability, and capability of miniaturization. A conventional Ag/AgCl reference electrode is a silver wire that is coated with a thin layer of silver chloride either by electroplating or by dipping the wire in molten silver chloride. The electrode reaction is as follows... 

Commonly used cells use Pt/C>2 or Pt/air as reference electrodes. At very low partial pressures of oxygen, care must be taken to avoid direct permeation of oxygen through stabilized zirconia from the air (or reference electrode) [74, 75]. The effect may be avoided by applying reference electrodes with activity near that observed at the working electrode. A well-defined buffer system like a metal-metal... 

Stability The SSCE has the smallest temperature voltage coefficient of any common reference electrode... 

Electrochemical sensors have several disadvantages with respect to optical sensors (i) they are based on electrodes and require a reference electrode (ii) the liquid-liquid junction is easily perturbed by external factors (iii) they are sensitive to electrical interferences (iv) miniaturization is not easy and their cost is relatively high. However, optical sensors also have some disadvantages (i) ambient light can interfere (ii) the range over which the concentration of an analyte can be accurately measured is often limited (iii) they have generally limited long-term stability. 

CycUc voltammetry was carried out on the Sycopel Work Station AEW 2, a three electrode assembly glassy carbon electrode as working, graphite rod as counter and anHg-pool as reference electrode were used. The stability and drift of Hg-pool was checked from time to time and found satisfactory. The scan rate was either 50 or 100 mV/s. 

Figure 48. Anodic stability as measured on a spinel LL-Mn204 cathode surface for EMS-based electrolytes (a) Lilm (b) LiC104 (c) LiTf. In all cases, 1.0 m lithium salt solutions were used, and slow scan voltammetry was conducted at 0.1 mV s with lithium as counter and reference electrodes and spinel LiJV[n204 as working electrode. (Reproduced with permission from ref 75 (Figure 3). Copyright 1998 The Electrochemical Society.)...

The precision and accuracy of the measurement also depend strongly on the reference electrode, which affects the results through fluctuations in its own potential and through the liquid-junction potential at the test solution-liquid bridge interface. This subject is extensively treated in [158]. Common electrodes of the second kind have sufficiently stable potentials at a constant temperature, but difficulties can be encountered due to temperature hysteresis. Silver chloride electrodes are preferable to calomel electrodes, because their temperature hysteresis is substantially smaller with a calomel electrode, potential stabilization after a change in the temperature may even take several hours. Negligible temperature hysteresis is exhibited by the thallamide reference electrode [26,... 

The simplest flow-through cells are actually caps fitted at the end of the ISE and the reference electrode and connected by a tube (see fig. 5.5). Some researchers [80, 92] claim that stirring the solution inside the cap by a rod driven by an external magnet (fig. 5.56) accelerates the ISE response and improves the potential stability (see also section 5.1.2). The solution can also be led... 

Ever since an ISFET that was chemically modified by a valinomycin-containing PVC membrane was reported [141], there has been general consensus on the advantages of this type of microsensor over conventional ISEs. Some serious problems have also been acknowledged, though e.g. the low mechanical stability of the membranes, the interference of COj in the potentiometric response, the lack of a stable micro-reference electrode and the relatively high drift rate of ISFETs). Attachment of the membrane can... 

Silver-Silver Ion Electrode This is the most popular reference electrode used in non-aqueous solutions. Since Pleskov employed it in acetonitrile (AN) in 1948, it has been used in a variety of solvents. It has a structure as shown in Fig. 6.1(a) and is easy to construct. Its potential is usually reproducible within 5 mV, if it is prepared freshly using pure solvent and electrolyte. The stability of the potential, however, is not always good enough. The potential is stable in AN, because Ag+ is strongly solvated in it. In propylene carbonate (PC) and nitromethane (NM), however, Ag+ is solvated only weakly and the potential is easily influenced by the presence of trace water and other impurities. In dimethylformamide (DMF), on the other hand, Ag+ is slowly reduced to Ag°, causing a gradual potential shift to the negative direction.2) This shift can reach several tens of millivolts after a few days. 

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