Temperature effects, reference electrode

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

Values of the electrode potentials for the more common reference electrodes are collected in Table 15.1 together with an indication of the effect of temperature for the most important electrodes. 

FIGURE 10.2 A schematic diagram of a combination glass pH electrode. A thin glass bulb with an inner Ag/AgCI electrode responds to pH changes in the test solution. A second Ag/AgCI in an outer jacket with a liquid junction serves the reference electrode for potentiometric measurement. An attached temperature probe is used to compensate for temperature effects. 

The cyclic voltammograms of these systems display quasi-reversible behavior, with AEv/v being increased because of slow electrochemical kinetics. Standard electrochemical rate constants, ( s,h)obs> were obtained from the cyclic voltammograms by matching them with digital simulations. This approach enabled the effects of IR drop (the spatial dependence of potential due to current flow through a resistive solution) to be included in the digital simulation by use of measured solution resistances. These experiments were performed with a non-isothermal cell, in which the reference electrode is maintained at a constant temperature... 

One of the first observations one is likely to make when carrying out low-temperature voltammetric measurements is that the effects of solution iR drop that may have been scarcely noticeable at room temperature are suddenly alarmingly pronounced. The iR drop, of course, is governed by the cell current and the effective resistance between the working electrode and the Luggin capillary of the reference electrode. For example, the solution resistance for an embedded circular disk electrode of radius r with a distant reference electrode is given by Equation 16.13, where p is the resistivity of the solution. 

By far the biggest problems with the stability and the magnitude of the liquid junction potentials arise in applications where the osmotic or hydrostatic pressure, temperature, and/or solvents are different on either side of the junction. For this reason, the use of an aqueous reference electrode in nonaqueous samples should be avoided at all cost because the gradient of the chemical potential of the solvent has a very strong effect on the activity coefficient gradients of the ions. In order to circumvent these problems one should always use a junction containing the same solvent as the sample and the reference electrode compartment. 

Engstrom and Carlsson already introduced in 1983 an SLPT [119] for the characterisation of MIS structures, which was extended to chemical gas sensors by Lundstrom et al. [26]. Both SLPT and LAPS base upon the same technique and principle. However, due to the different fields of applications in history, one refers to LAPS for chemical sensors in electrolyte solutions and for biosensors, and the SLPT for gas sensors. A description of the development of a hydrogen sensor based on catalytic field-effect devices including the SLP technique can be found, e.g., in Refs. [120,121]. The SPLT consists of a metal surface as sensitive material which is heated by, for instance, underlying resistive heaters to a specific working-point temperature, and a prober tip replaces the reference electrode (see Fig. 5.10). 

Table 1. References for specific materials, temperature effects and electrode stability.

Thus, online measurements of composition are usually limited to some overall property. A typical example is pH, defined as the absolute value of the logarithm of the molar concentration (or, more exactly, activity) of hydrogen ion pH can be measured by exploiting the electric potential established between two proper electrodes immersed in the sample fluid, usually a glass membrane electrode and a reference electrode [15], Notwithstanding the temperature dependence and the alkaline error (at high pH, a marked sensitivity to the effect of Na+ and of other monovalent... 

The hydrogen electrode. The hydrogen electrode is discussed first because it is the primary reference electrode used to define an internationally accepted scale of standard potentials in aqueous solution. By convention, the potential of an electrode half-reaction that is measured with respect to the normal hydrogen electrode (NHE also written as SHE, standard hydrogen electrode) is defined as the electrode potential of the half reaction. This convention amounts to an arbitrary assignment for the standard potential of the hydrogen electrode as zero at all temperatures. Thus, there is in effect a separate scale of electrode potentials at each temperature level. 

Temperature effects. The ordinary glass electrode-reference electrode pair that is used for pH measurements is not well suited to measurements far removed from room temperature. This is because the electrodes are immersed only partially, with the tips of the electrodes in the solution and the tops of the electrodes at ambient temperature. This creates a thermal gradient in the body... 

In practice, the value of k is never obtained as such, because the meter is adjusted so that the standard reads the correct value for its pX, the scale being Nernstian. As k contains in addition to the reference electrode potentials, a liquid-junction potential and an asymmetry potential, frequent standardization of the system is necessary. The uncertainty in the value of the junction potential, even when a salt bridge is used, is of the order of 0.5 mV. Consequently the absolute uncertainty in the measurement of pX is always at least 0.001/(0.059// ) or 0.02 if n = I, i.e. a relative precision of about 2% at best. For the most precise work a standard addition technique (p. 32) and close temperature control are desirable. All measurements should be made at constant ionic strength because of its effect on activities. Likewise,... 

The constant term depends on the environmental conditions such as temperature, pH, concentration of oxygen and the reference electrode offset. But the differential method without the term has advantages on them, in case that the same reference electrodes are used in the short-time measurement. This formulation easily eliminates the effect of open circuit corrosion potential and reference electrode offset. If the potential or current density are constant in two boundary conditions, the differential boundary conditions are zero according to Eqn. (12) or Eqn. (13). 

In another study [35], the electrochemical emission spectroscopy (electrochemical noise) was implemented at temperatures up to 390 °C. It is well known that the electrochemical systems demonstrate apparently random fluctuations in current and potential around their open-circuit values, and these current and potential noise signals contain valuable electrochemical kinetics information. The value of this technique lies in its simplicity and, therefore, it can be considered for high-temperature implementation. The approach requires no reference electrode but instead employs two identical electrodes of the metal or alloy under study. Also, in the same study electrochemical noise sensors have been shown in Ref. 35 to measure electrochemical kinetics and corrosion rates in subcritical and supercritical hydrothermal systems. Moreover, the instrument shown in Fig. 5 has been tested in flowing aqueous solutions at temperatures ranging from 150 to 390 °C and pressure of 25 M Pa. It turns out that the rate of the electrochemical reaction, in principle, can be estimated in hydrothermal systems by simultaneously measuring the coupled electrochemical noise potential and current. Although the electrochemical noise analysis has yet to be rendered quantitative, in the sense that a determination relationship between the experimentally measured noise and the rate of the electrochemical reaction has not been finally established, the results obtained thus far [35] demonstrate that this method is an effective tool for... 

As stated earlier, the reference electrode in a cell used for electroanalysis is designed so that its potential is independent of the composition of the test solution. There are several general properties that reference electrodes should have in order to be useful in analysis (1) they should be reversible with an electrode potential which is independent of time and reproducible (2) they should have a small temperature coefficient (3) they should be ideally non-polarizable with negligible effects from the flow a small current through the system and (4) they should be easily constructed. The most commonly used reference electrodes are those based on on the mercury calomel system and the silver silver chloride system. The electrolyte most commonly used in these systems is KCl. Relevant parameters for commonly used reference electrodes are given in table 9.4. 

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