Reference-electrode potentials, temperature coefficients

A typical set of experimental data290a,290b is shown in Fig. 11. All measurements converge to the value measured by Grahame.286 At present, the of Hg in water can be confidently indicated5 as -0.433 0.001 V (SCE), i.e., -0.192 0.001 V (SHE). The residual uncertainty is related to the unknown liquid junction potential at the boundary with the SCE, which is customarily used as a reference electrode. The temperature coefficient of of the Hg/H20 interface has been measured and its significance discussed.7,106,1 8,291... 

As the temperature is varied, the Galvani potentials of all interfaces will change, and we cannot relate the measured value of d"S dT to the temperature coefficient of Galvani potential for an individual electrode. The temperature coefficient of electrode potential probably depends on the temperature coefficient of Galvani potential for the reference electrode and hence is not a property of the test electrode alone. 

Reference electrode potentials change with temperature. Both electrochemical reactions (Nernstian thermodynamics) and chemical solubilities, e.g. of the inner reference electrode solution, are affected. Accordingly, the temperature coefficient, dE/dT (mV °C4), varies from one type of reference electrode to another. To minimise errors in potential readings the coefficient should be low and at least known. Examples of temperature coefficients are given in Table 2.2. 

This book contains extensive tables of standard electrode potentials covering the periodic chart. For each electrode reaction Is given the standard potential, the temperature and the pressure, the solvent, and a literature reference. Occasionally the temperature coefficient of the electrode potential Is given together with an estimate of uncertainty. Much of the tabulated data Is taken from secondary sources (such as Item [149]). 

We might try to measure the temperature coefficient of the Galvani potential for an individual electrode under nonisothermal conditions then only the temperature of the test electrode would be varied, while the reference electrode remains at a constant temperature and retains a constant value of Galvani potential (Fig. 3.2). 

Thus, the temperature coefficient of Galvanic potential of an individual electrode can be neither measured nor calculated. Measured values of the temperature coefficients of electrode potentials depend on the reference electrode employed. For this reason a special scale is used for the temperature coefficients of electrode potential It is assumed as a convention that the temperature coefficient of potential of the standard hydrogen electrode is zero in other words, it is assumed that the value of Hj) is zero at all temperatures. By measuring the EMF under isothermal conditions we actually compare the temperature coefficient of potential of other electrodes with that of the standard hydrogen electrode. 

The temperature coefficient of the ISE potential has received relatively little attention. As follows from (3.1.7), the constant term (the ISE standard potential), the determinand and interferent activity coefficients and the selectivity coefficient, liquid-junction potentials and, of course, also the RTIZfF coefficient, depend on the temperature [118]. When the internal reference electrode and the reference electrode in the test solution are identical, the interferent activity sufficiently low and the liquid-j unction potentials negligible, then the constant term depends on the determinand activity in the electrode internal solution alone and thus the temperature coefficient of the measured EMV depends only on the temperature coefficient of the determinand activity coefficient and on the/ 77z,F coefficient. Measuring instruments are usually... 

E° [equation (15.4)] is also referred to as the offset, the zero potential point, or the isopotential point, since theoretically it is defined as the pH that has no temperature dependence. Most pH electrode manufacturers design their isopotential point to be 0 mV at pH 7 to correspond with the temperature software in most pH meters. The offset potential is often displayed after calibration as an indication of electrode performance. Typical readings should be about 0 30 mV in a pH 7 buffer. In reality, E° is composed of several single potentials, each of which has a slight temperature coefficient. These potentials are sources of error in temperature compensation algorithms. 

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. 

The standard potential difference of the Ag/AgCl reference electrode E° is determined in cell (I) filled with HC1 at a fixed molality. For the molality of 0.01 mol kg-1, the values for the mean activity coefficient of the HC1 are given in [7] at various temperatures. 

In general a necessary part of a potentiometric measurement is the coupling of a reference electrode to the indicating electrode. The ideal reference electrode has a number of important characteristics (1) a reproducible potential, (2) a low-temperature coefficient, (3) the capacity to remain unpolarized when small currents are drawn, and (4) inertness to the sample solution. If the reference electrode must be prepared in the laboratory, a convenient and reproducible system is desirable. 

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

To be considered a suitable reference electrode, an electrode must have a known and reproducible potential versus the NHE. This potential must be nearly an invariant of the current flowing through the electrode, which implies that the electrode reaction is extremely fast and the electrode reactants are extremely concentrated (see Secs. III.C.3 and IV.B.2). It should also have a small temperature coefficient and should be easily constructed in a reproducible way. A large variety of electrodes meeting these requirements to different degrees have been devised Table 2 presents a few of the most frequently used. 

For references on electrochemical reaction kinetics and mechanism, see, e.g., Newman and Thomas-Alvea, Electrochemical Systems, 3d ed., Wiley Interscience, 2004 Bard and Faulkner, Electrochemical Methods Fundamentals and Applications, 2d ed., Wiley, 2001 Bethune and Swendeman, Table of Electrode Potentials and Temperature Coefficients, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, New York 1964, pp. 414-424 and Bethune and Swendeman, Standard Aqueous Electrode Potentials and Temperature Coefficients, C. A. Hampel Publisher, 1964. 

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. 

Remember that even if a molar concentration of a strong mono-acid is chosen (the HE is called a NHE in this instance), then this reference electrode is not a SHE because the real compounds are not in their standard state. Figure 3.13 shows this difference on the mean activity coefficient of hydrogen chloride acidic solutions. In practice, if one wishes to use a hydrogen electrode whose potential is as close as possible to that of the SHE, then one would use an acidic solution whose concentration is slightly higher than 1 mol For example, if you take the case of HCI, a concentration equal to 1.2 mol L is chosen because of a mean activity equal to 1.2x0.84 = 1 (0.84 is the mean activity coefficient found in figure 3.13). These values all depend on the system s temperature. 

Consequently, two different temperature coefficients of the electrode potential can be obtained the isothermal and the thermal temperature coefficient. The thermodynamic implications of these coefficients were clearly established by de Bethune, who also evaluated the values of the temperature coefficients of a wide variety of reference electrodes. While the use of a non-isothermal cell presents the clear advantage that only the temperature effect on the reaction on the working electrode is evaluated, it also presents the problem that the measurements will be interfered by the appearanee of a thermodiffiision potential, arising from temperature differences within the electrolyte solution. This thermodiffusion potential can be experimentally minimized (by using, for example, a saturated potassium chloride bridge for the liquid unions) or, alternatively, the effect of the thermodiffusion potential can be subtracted by calculating its numerical value from ... 

Once total charge density curves are referred to a constant-temperatnre reference electrode, this data can be used to evaluate the temperature coefficient of the potential drop across the inter-... 

---