Hydrogen reference electrode

The thermodynamics of electrochemical reactions can be understood by considering the standard electrode potential, the potential of a reaction under standard conditions of temperature and pressure where all reactants and products are at unit activity. Table 1 Hsts a variety of standard electrode potentials. The standard potential is expressed relative to the standard hydrogen reference electrode potential in units of volts. A given reaction tends to proceed in the anodic direction, ie, toward the oxidation reaction, if the potential of the reaction is positive with respect to the standard potential. Conversely, a movement of the potential in the negative direction away from the standard potential encourages a cathodic or reduction reaction. 

FIGURE 12.2 Half-cell with hydrogen reference electrode. 

Figure 5.9 Schematic cyclic voltammogram showing the electro-oxidation of the electrode (dashed box). The curve was generated from measurements by Jerkiewicz et al. [2004] of Pt in 0.5 M H2SO4 with a reversible hydrogen reference electrode (RHE). For each separable potential range, an atomistic model of the electrode structure is shown above.

An interesting correlation exists between the work function of a metal and its pzc in a particular solvent. Consider a metal M at the pzc in contact with a solution of an inert, nonadsorbing electrolyte containing a standard platinum/hydrogen reference electrode. We connect a platinum wire (label I) to the metal, and label the platinum reference electrode with II. This setup is very similar to that considered in Section 2.4, but this time the metal-solution interface is not in electronic equilibrium. The derivation is simplified if we assume that the two platinum wires have the same work function, so that their surface potentials are equal. The electrode potential is then ... 

The potential of this electrode is defined (Section 5.2) as the voltage of the cell Pt H2(l atm) H (a = 1) M M, where the left-hand electrode, = 0, is the normal hydrogen reference electrode (described in Section 5.6). In Chapter 6, we derive the Nemst equation on the basis of the electrochemical kinetics. Here we use a simplified approach and consider that Eq. (5.9) can be used to determine the potential E of the M/M electrode as a function of the activity of the products and reactants in the equilibrium equation (5.10). Since in reaction (5.10) there are two reactants, and e, and only one product of reaction, M, Eq. (5.9) yields... 

Potentiodynamic Technique. Adsorption of methanol on Pt in acid solution was studied by Breiter and Gilman (3) using a potentiostatic technique. The anodic sweep, with a sweep rate of 800 V/s, was started at rest potential and extended to 2.0 V with respect to a hydrogen reference electrode in the same solution. As shown in Figure 10.8, the current was recorded as a function of potential (time) in the absence (curve A) and in the presence (curve B) of methanol. The increase in current in curve B is due to oxidation of the adsorbed methanol on the platinum electrode. Thus, shaded area 2 minus shaded area 1 (Fig. 10.8) yields the change 2m (C/cm ) required for oxidation of the adsorbed methanol ... 

However, at least two other reference electrodes, calomel (Hg. 7.42) and silver silver chloride electrodes, are in common use as secondary reference electrodes (they are easier to set up than die hydrogen reference electrode). Potentials of electrodes measured using one of die secondary reference electrodes can be directly converted to values on die hydrogen scale, if die potential of die secondary reference electrode with respect to the hydrogen electrode is known (see also Section 7.5.73). 

In amperometric detection, a reference electrode was usually employed. However, in one report, a platinized Au electrode was used as a pseudoreference electrode in a three-electrode system for amperometric detection. The operation principle follows that of the hydrogen reference electrode [242]. 

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. 

In principle, we can measure the potential of an electrode with a hydrogen reference electrode. We can also calculate the reversible potential of the cell composed of the electrode of interest and the hydrogen reference electrode. In practice, a hydrogen electrode is difficult to operate properly and is rarely used in engineering measurements. Instead, commercially available reference electrodes (e.g., calomel, Ag/AgCl, and Hg/HgO) are used. 

The hydrogen reference electrode is shown on the right, and the electrode system being measured is on the left. The two halfcells are joined by a salt bridge. 

Ktxver A, Vogel I, Vielstich W (1994) Distinct performance evaluation of a direct methanol SPE fuel cell. A new method using a dynamic hydrogen reference electrode. 

The impedance spectra of the DMFC cathode electrodes are obtained by subtracting the anode impedance from the total cell impedance. The cell impedance, ZDMFC, was obtained from normal operation of the DMFC (i.e., the cathode side was fed with air or 02 and the anode side was fed with methanol solution). The anode impedance was measured by supplying H2 to the cathode compartment, which was used as a dynamic hydrogen reference electrode. Since the impedance of the H2 electrode is negligible, the measured impedance is considered to be the anode impedance, Zanode. The cathode impedance is therefore... 

The reduction of benzoquinone in NMA at a platinum wire electrode has been studied143). In unbuffered solutions the reduction gave two fast, one-electron waves of almost equal height ( 1 2 = —0.04 V and —0.38 V versus a normal hydrogen reference electrode). It was proposed that these represented the formation of the radical anion and of the hydroquinone dianion, respectively. In buffered solutions an irreversible two-electron reduction (probably to hydroquinone) was found. It thus appears that certain radical anions may be much more stable in NMA than in water. 

The electrochemical cell consists of a Teflon block with three interconnected cylindrical wells (See Fig. 2.2). The counter electrode is a polycrystalline nickel disk (12.54 mm diameter) cast in Kel-F and then press fitted into an orifice bored on the Teflon block. The dynamic hydrogen reference electrode, is formed by inserting a platinum wire into... 

The reference electrode used in this experiment, as already mentioned, was a dynamic hydrogen reference electrode consisting... 

Although the vacuum level is of no practical reference, the question arises of whether the standard potential or energy of the hydrogen reference electrode (NHE) can be quantitatively related to the vacuum level. The first quantitative approach, which was a straightforward one, was published by Lohmann [9]. He examined free energy changes associated with the reduction of silver ... 

Thus, the E values (relative interface potential differences, or interface potentials) represent differences in potentials across the metal/solution interface minus the potential difference across the standard hydrogen reference electrode interface. The E values are physically measured by attaching one lead of an electrometer to the metal, the other lead to a reference electrode in the solution and very close to the metal surface (a point discussed further in Chapter 6). If the positive electrometer lead is... 

All values are relative to the standard hydrogen (reference) electrode ... 

The first suggestion of a compound with a CN/Co ratio between 2 and 5 was in an early titration with a platinum electrode and a hydrogen reference electrode. Figure 1 shows the small peak at 7 = 4.5 disturbing an otherwise smooth increase in free-cyanide concentration from 72 = 0 to R = 80. There is no evidence here for the existence of a hexacyanide ion. 

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