Standard hydrogen electrode, temperature dependence

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 SHE. The H" " H2 couple is the basis of the primary standard around which the whole edifice of electrode potentials rests. We call the H H2 couple, under standard conditions, the standard hydrogen electrode (SHE). More precisely, we say that hydrogen gas at standard pressure, in equilibrium with an aqueous solution of the proton at unity activity at 298 K has a defined value of of 0 at all temperatures. Note that all other standard electrode potentials are temperature-dependent. The SHE is shown schematically in Figure 3.3, while values of Eq r are tabulated in Appendix 3. 

Measurements of the open circuit potential (OCP) were performed by linear sweep voltammetry with the anode of the electrolyser set as the working electrode, and the cathode set as both counter and reference electrodes. The hydrogen reference electrode condition was created by saturating the catholyte with H2 gas at the room temperature. The measured OCP values were refered to the standard hydrogen electrode (SHE). Since the potential of the hydrogen reference electrode varied from SHE depending on the HC1 concentration used in the experiement, this correction was taken into account for all measured OCP. 

It is impossible to measure directly the electrode potentials. Only the electromotive force (emf) of a voltaic cell arising from a combination of two electrodes can be directly measured, which is given as the arithmetical sum or difference of the two electrode potential depending upon their signs. If one of the electrode potential be accurately measured, that of the other may be calculated. The reference electrode arbitrarily chosen for this purpose is the standard hydrogen electrode. Hydrogen gas at 1 atm. pressure and at a temperature of 25°C is slowly bubbled over a platinised platinum electrode which is immersed in a solution of hydrogen ions of unit activity. By convention potential of the half cell reaction... 

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. 

The value of this electric field depends on the electrochemical potentials of the electrode metals and is related to their position in the electrochemical series. Table l7.3 shows some values for some common electrode metals at romn temperature. Electrochemists have by convention adopted the hydrogen electrode as a standard of reference potmitial and assigned it a value of 0 V. All other metals have a ncmzero potential with respect to it Metal half-cell potentials depend on their electrochemical oxidation state, and they are usually arranged in a table showing their activity relative to others, such as seen in Table 17.3. 

A metal in equilibrium with its ions in solution will establish a potential difference which depends on the concentration of the metal ion in solution and the temperature. This potential is best determined by comparison with a standard which is arbitrarily set to zero—the hydrogen electrode ... 

Also, for proper technique, the effect that temperature has on hydrogen ion activity should be considered. The label on a bottle of buffer lists the standardization values at specific temperatures for that buffer (see Table 4.1). The operator should always observe the buffer temperature and corresponding standardization value before calibrating the meter. For the most accurate results, the buffer and the sample should have as nearly equal a pH as possible and be brought to the same temperature. This minimizes any errors arising from differences between the ideal temperature-dependent slope factor and the actual electrode response due to temperature changes. 

In the discussion of the Daniell cell, we indicated that this cell produces a voltage of 1.10 V. This voltage is really the difference in potential between the two half-cells. The cell potential (really the half-cell potentials) is dependent upon concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard state of 298 K (25°C) and all components in their standard states (1M concentration of all solutions, 1 atm pressure for any gases and pure solid electrodes). Half-cell potentials appear in tables as the reduction potentials, that is, the potentials associated with the reduction reaction. We define the hydrogen half-reaction (2H+(aq) + 2e - H2(g)) as the standard and has been given a value of exactly 0.00 V. We measure all the other half-reactions relative to it some are positive and some are negative. Find the table of standard reduction potentials in your textbook. 

Been varies only as a function of the test solution pH if the temperature is constant. It is significant to point out that the potential of modern glass electrodes is a linear function of pH (equation 39). By using a test solution of known pH it is possible to relate the cell potential to hydrogen ion activity of a test solution. This standardization must be done each time a pH meter is used because of subtle changes in the various potentials owing to aging of the electrode. Therefore, the accuracy of a pH determination depends on the accuracy of the standard buffer. Table 1-2... 

However, the active dissolution of titanium depends markedly on temperature in acid solution. At lower temperatures, the picture is not so clear. It is necessary to have a quantitative measure of the rate of the hydrogen reaction and the titanium dissolution reaction. The complete set of current-potential and impedance-potential data has been tested against the theory given above. The best strategy seems to be to fit to a single electrode reaction and then to look for deviations from the expected behaviour for a perfect redox reaction. A convenient way of doing this is to represent the electrochemical data as a standard rate constant-potential curve in conjunction with a double layer capacity-potential curve [21]. 

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