Hydrogen - water electrodes

The pH describes the composition of water pH 7 indicates neutral water, lower values indicate acid water, and higher values indicate basic water. The pH is determined by means of a glass hydrogen ion electrode of known potential. Calibration is carried out with special solutions of known pH. 

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 single cells consist of a dense solid electrolyte membrane and two porous electrodes. In most cases, at least one of the electrodes is exposed to an oxygen-containing gas (often, ambient air), while the other electrode is exposed to an inert gas, a liquid metal, a partial vacuum, or a reacting mixture (hydrogen, water vapor, hydrocarbons, CO, CO2, etc.). The single-chamber reactor (SCR) has been also proposed either as a membrane reactor or as a fuel cell. In this case, the solid-electrolyte disk, with two different electrodes that are coated either on opposite sides or on the same side of the pellet, is suspended in a flow of the reacting mixture (see Section 12.6.3). 

In a buffered methanol/water solution the electrochemical reduction of3,5-dichloro-/V,/V-diiso-propyl-1 A4,2,4,6-thiatriazin-l-amine shows a strong dependence on the measured half-wave potential ( — 0.86 to —1.30 V) from pH 1.87 5.72 of the electrolyte.80 This indicates protonation of the thiatriazinc, ring opening and secondary reactions of the intermediate. All isolated and identified reduction products are identical with those of the catalytic hydrogenation. The electrode reactions are not reversible. 

Figure 25-2. U/T diagrams for oxygen and hydrogen, water vapor electrodes for gases saturated with water vapor at 25 °C and for water vapor electrodes at various pressures [11].

Figure 25-20. Potential differences measured on air/hydrogen, water vapor cells with platinum electrodes prepared with different layer thickness resistivity ratios d/p (a), or after annealing at different temperatures between 500 and 1300 °C (b) [73].

Qtdnhydrone Electrode. The quinhydrone electrode is an important hydrogen-ion electrode, and is perhaps typical of a whole class of such electrodes which function as pH sensors owing to a reversible organic oxidation-reduction pair involving protons. Quinhydrone (an equimolar compound of benzoquinone and hydroquinone) is only slightly soluble in water. The reversible oxidation-reduction couple... 

The hydrogen electrode behaves reversibly in many of the protic solvents. Some problems are encountered in the amides where the presence of platinum may lead to catalytic decomposition of the solvent, especially in the presence of HCl. Feakins and French report that the platinum-hydrogen gas electrode in acetone-water mixtures functions well on a mechanically roughened platinum surface. The use of platinum-black was found to be deleterious. 

The solvent (water) can sometimes itself be the reactant in an electrode half reaction. This is the case in neutral or alkaline solutions where, instead of H+, H2O is the source of hydrogen (water is reduced to hydrogen). On the other hand, in neutral or acidic solutions, instead of OH , H2O is the source of oxygen (water is oxidised to oxygen). 

The Hydrogen Electrode Process. There are three oxidation states of hydrogen, as shown in Table 4.1.4. The hydrogen-water couple in solution at pH = 0, in contact with hydrogen gas at a partial pressure of 1 atm at a given temperature, is called the SHE. The electrode reaction is given by Eq. (36) ... 

Yamauchi, S., Mitsuda, K., Maeda, H., and Takai, O. (2000) Application of a hydrogen reference electrode to a solid state water removal device. J. Appl. Electrochem., 30, 1235-1241. 

Holtappels P, Vinke IC, de Haart LGJ, Slimming U (1999) Reaction of hydrogen/water mixtures on nickel-zirconia cermet electrodes. 2. AC polarization characteristics. J Electrochem Soc 146 2976-2982... 

The composition of the hydrogen + water vapour mixture, which had a total pressure of 1 atm, was adjusted until the cell produced zero e.m.f., that is, until the same chemical potential of oxygen existed at both electrodes. This oxygen potential was calculated from the known hydrogen and water vapour pressures. Thus, by arranging the conditions in such a way that the method becomes a null-point method, interference by electronic conduction is made as small as possible and the range of utility of stabilized zirconia as an electrolyte is extended. 

The problem of LJP correction of pH values for highly diluted solutions remains essential, and empiric corrections are still under discussion in practical analytical chemistry [76]. In relation to this problem, the study of electrochemistry in pure water [77] is of interest. In this case (no electrolyte added to high purity water) the LJP is unpredictably high and sensitive to occasional solution composition. The sometimes used Nafion membrane to separate the working electrode and a hydrogen reference electrode is certainly to be discouraged because of the introduction of an additional membrane potential. 

The flame ionization detector works by measuring the current in a flame (into which the gas to be analyzed is passed) across which a potential difference of 200-400 V is applied by means of platinum electrodes. This sensor is not temperature sensitive and is capable of measuring very low concentrations. However, its primary usefulness is in detecting organic carbon atoms, and it is not suitable for hydrogen/water vapor, carbon monoxide/carbon dioxide, or oxygen/sulfur dioxide mixtures. Calibration is required for each gas mixture. 

The measurement of a from the experimental slope of the Tafel equation may help to decide between rate-determining steps in an electrode process. Thus in the reduction water to evolve H2 gas, if the slow step is the reaction of with the metal M to form surface hydrogen atoms, M—H, a is expected to be about If, on the other hand, the slow step is the surface combination of two hydrogen atoms to form H2, a second-order process, then a should be 2 (see Ref. 150). 

By analogy, ammonium salts should behave as acids in liquid ammonia, since they produce the cation NH4 (the solvo-cation ), and soluble inorganic amides (for example KNHj, ionic) should act as bases. This idea is borne out by experiment ammonium salts in liquid ammonia react with certain metals and hydrogen is given off. The neutralisation of an ionic amide solution by a solution of an ammonium salt in liquid ammonia can be carried out and followed by an indicator or by the change in the potential of an electrode, just like the reaction of sodium hydroxide with hydrochloric acid in water. The only notable difference is that the salt formed in liquid ammonia is usually insoluble and therefore precipitates. 

---