Hydrogen fugacity

Figure 6. SEM micrographs showing the reduced ductility observed at high hydrogen fugacities (region 3 of Figure 4(b)) in the parent metal of the (a) HY-100 and (b) HSLA-100 alloys and the transgranular cleavage-like (TCL) fracture morphology in (c) HY-100 and (d) HSAL-100.

Figure 5.46 shows the stability field of the annite end-member at 1 kbar total pressure, based on equilibria 5.131, 5.133, and 5.135, as a function of temperature and hydrogen fugacity in the system. The figure also shows the positions of the iron-wuestite (IW) and wuestite-magnetite buffer (WM). 

The latter equation for the NHE also is the defining basis for the potentiometric measurement of hydronium ion activity [H30+] and molecular hydrogen fugacity (PH2) ... 

Figure 9.18. Oxygen, water, and hydrogen fugacity relationships at 1.65 GPa and 800°C. (From Ord and Hobbs 1986.)...

Therefore in aqueous systems, in which aH2o(z) 1, the oxygen and hydrogen fugacities are inversely related, so that as fo2 becomes very small, /h2 (or h2 ) becomes large, and in many cases is actually a measurable quantity, i.e., a partial pressure (or a molality). 

Fig. 18.6. Experimental control and measurement of hydrogen fugacity. (a) Schematic arrangement of an oxygen-buffered experiment, (b) Shaw membrane for external control of hydrogen fugacity. Apparatus can also be used as a hydrogen sensor to measure H2 pressure externally, (c) Double capsule method of controlling both oxygen fugacity and HCl activity within an inner, Pt capsule.

Because the overpotential is negative for hydrogen evolution, increases in r) can markedly increase the effective fugacity and hence the concentration of dissolved hydrogen. Fugacities of the order of 10 atm can be achieved with cathodic charging [36]. For some mechanisms (e.g. slow discharge-fast electrochemical desorption), however, /hj will be less than 1 atm when an overpotential is applied. 

Figure 11 (left). Fugacity of SO2 of the Ag + Ag2S buffer at 2000 bars. The hydrogen fugacity is controlled externally by magnetite + hematite. Arrows indicate individual experiments. Unpublished data of Rudert and Eugster. 

Figure 4. Potential-pH diagram for iron (A) and nickel (B) in supercritical aqueous solutions at 400"C and 450°C, respectively, P = 500 bar. The hydrogen equilibrium line for unit hydrogen fugacity is coincident with the Ni/NiO equilibrium line. The diagram for iron shows the approximate regions in potential-pH space for the operation of SCWO reactors and supercritical thermal power plants (SCTPPs). Reprinted from Ref. 5, Copyright (1997) with permission from Elsevier.

From this last equation, it is apparent that we can estimate the hydrogen fugacity in the solution, independently of the solution pH, as ... 

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