Hydrogen in solution

D. V. Sokolskii, Hydrogenation in Solutions, Israel Program for Scientific Translations, Jerusalem, 1964, pp. 345-355. 

Where R is the gas constant, T the temperature (K), Fthe Faraday constant and H2 is the relative partial pressure (strictly, the fugacity) of hydrogen in solution, which for continued evolution becomes the total external pressure against which hydrogen bubbles must prevail to escape (usually 1 atm). The activity of water a jo is not usually taken into account in elementary treatments, since it is assumed that <7h2 0 = U nd for dilute solutions this causes little error. In some concentrated plating baths Oh2 0 I O nd neither is it in baths which use mixtures of water and miscible organic liquids (e.g. dimethyl formamide). However, by far the most important term is the hydrogen ion activity this may be separated so that equation 12.1 becomes... 

In the case of hydrogenase, the substrate hydrous are always present and so at reducing potentials the enzyme will generate hydrogen. In solutions of H2, the hydrogen-oxidizing activity can also be observed. Because measurements can be made over timescales of milliseconds to hours, it was possible to observe both the extremely rapid reaction of the enzyme with hydrogen, and the slow activation/deactivation processes at more positive potentials. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

Starting from point 1, a small amount of hydrogen goes into solution in the metal phase as the H2 pressure increases. At point 2, the hydriding reaction begins (Eq, l) and H2 is absorbed at nearly constant pressure. This pressure Pp is termed the "plateau pressure" and corresponds to a two-phase mixture of metal. Me, and metal hydride, MeHx. At point 3, the metal has been completely converted to the hydride phase. Further increases in H2 pressure (point h) result in only a small addition of hydrogen in solution in the hydride phase. In principle this curve is reversible. Extraction of H2 from the gas phase results in the dissociation of the hydride phase in an attempt to maintain the equilibrium plateau pressure. 

Dicobalt Octacarbonyl. The chemistry of this interesting and versatile catalyst has been the subject of several recent reviews (Orchin, 38 Wender, Sternberg, and Orchin, 39). Dicobalt octacarbonyl reacts readily with molecular hydrogen in solution to form cobalt hydrocarbonyl ... 

A comparison of the electron configurations of the various catalytic species shows more promise. Thus, it can be seen from Table III that all the metal ions which react with hydrogen in solution, have outer electron configurations corresponding to nearly filled or just filled d-shells ... 

The cobalt-59 NMR spectra of various Co(R2Dtc)3 complexes and of Co(RXant)3 have been reported. For B0 =450 cm-1 a linear correlation between the resonance frequency (p/) and the Tig - Aig absorption maximum is observed (430). In an electrochemical study, Toropova et al. (592) reported on the catalytic liberation of hydrogen in solutions of Co(R2 Dtc)2 complexes R2 = (hydroxyethyl)2, //-hydroxyethyl, carboxymethyl, etc.). The following mechanism was proposed for this very interesting reaction ... 

Figure 26 Photosensitized oxidation of alkenes with two distinct allylic hydrogen in solution and within NaY zeolite.

Castellan, A., Nourmamode, A., Fomier de Violet, P., Colombo, N. and Jeager, C., "Photoyellowing of Milled Wood Lignin and Peroxide-Bleached Milled Wood Lignin in Solid 2-hydroxypropyl cellulose after Borohydride Reduction Catalytic Hydrogenation in Solution An UV-Visible Absorption Spectroscopic Study", J. Wood Chem. Technol., 1992,12(1) 1. 

The effect of hydrogen pressure on the reaction was measured. Figure 14 shows that the first-order rate constant is directly proportional to the hydrogen partial pressure, PB,. Since the solubility of hydrogen in most aqueous solutions obeys Henry s law over this range of pressure, this relation also implies that the first-order rate constant is proportional to the concentration of molecular hydrogen in solution (H2)... 

One type that is a bit more complex but that shows up a fair amount is the addition of a metal to a strong oxoacid solution (usually nitric or sulfuric). The reason these are selected as often as they are is that they violate the typical rules for the replacement of hydrogen in solution. This is because the acids act as strong oxidizing agents that will ionize the metal. Copper or silver typically appear on the test because they won t react with other acids but they will with the oxoacids. Both metals are oxidized by these acids. 

Pt, which is not trivial over the projected lifetime for a PAFC. Migration of the platinum from the cathode towards the anode is due to the platinum being deposited, not on the anode catalyst, but rather on the silicon carbide matrix, adjacent to the anode, as a consequence of the small solubility of hydrogen in solution at the electrode/matrix interface. 

For unsaturated solutions the amount of hydrogen in solution is governed by the pressure. Over this region the Sievert s constant, K, is slightly affected by temperature and is given by the equation 32... 

While Eq. (11) is relevant for relatively low pH values (pH < 3), Eq. (12) applies at higher values of the pH. The adsorbed hydrogen atoms, formed as intermediates, can recombine and evolve as gas bubbles (molecular hydrogen) in solution, following one of two possible mechanisms ... 

JV,JV -Dioxides or IV-oxides may be formed as intermediates in these reactions, depending on the stage at which N—N bond formation takes place (Scheme 1) and on the vigor of the subsequent reduction. By suitable choice of reaction conditions these may be isolated for example, reduction of 2,2 -dinitrobiphenyl with sodium hydrosulfide gives high yields of benzo[c]-cinnoline 5-oxide (5),11 while catalytic hydrogenation in solutions containing sodium hydroxide can be stopped at the 5,6-dioxide (4), or carried further.151618... 

Sokolskii, D. V., Hydrogenation in Solutions, Oldbourne Press, 1965 (comprehensive Russian source book). 

---