Upper limiting potential

Potential cycling has been found to accelerate Pt dissolution compared with poten-tiostatic conditions. The dissolution mechanisms and dissolved species involved in this process are unclear [Johnson et al., 1970 Kinoshita et al., 1973 Ota et al., 1988 Rand and Woods, 1972]. Darling and Meyers have developed a mathematical model based on (9.5)-(9.7) to smdy Pt dissolution and movement in a PEMFC during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005]. Severe Pt dissolution occurs when the potential switches to the upper limit potential (1.2 V), and then stops once a monolayer of PtO has formed. The charge difference between the anodic and cathodic cycles was found to be consistent with the amount... 

Ruthenium was electrochemically deposited on platinum foil at a potential of 50 mV for 10 s. The cyclic voitammogram of this Pt—Ru electrode in 3 M H2SO4 is shown in Fig. 4—2. The voitammogram shows the hydrogen adsorption-desorption features from 50 mV to 200 mV and the oxidation and reduction current over 300 mV. The voltanunogram seemed stable when the upper limit potential was 800 mV. When the upper limit was higher than 800 mV, the voitammogram became slowly like pure... 

The h-pH diagram of galena is constructed through the reactions (2-12) to (2-14) and (2-21) to (2-23) and presented in Fig. 2.15. It may be seen from Fig. 2.15 that the lower limit potential of collectorless flotation of galena at various pH are well defined by the conditions producing elemental sulphur. The upper limit potential of self-included collectorless flotation of galena at various pH is... 

Considering the formation of 8203" needing 0.5 V over-potential, assuming the concentration of all dissolved species to be 10 mol/L, the reaction potential are 0.28 V and 0.37 V respectively, corresponding to reactions (2-57) and (2-58), the upper limit potential of flotation of sphalerite depends on reaction (2-57). 

Only limited studies on the electrochemical behavior of sphalerite have been reported, perhaps due to its high electrical resistivity. The Relation between recovery of sphalerite and pulp potential is presented in Fig. 4.17 with an initial butyl xanthate concentration of 10 mol/L. It can be seen from Fig. 4.17 that flotation begins at 0 V, the upper limit potential is 0.31 V. 

For the collector flotation of sphalerite, the upper limit potential of flotation may be corresponding to the following decomposition reactions ... 

Figure 6.4 shows that the initial potential of marmatite flotation is around 0.26 V and is almost independent of pH, but the upper limit potential of flotation varies with pH, which is higher at acidic pH value. The recovery of marmatite can be above 90% only at certain pulp potential ranges at given pH. The optimal flotation potential range is 0.35 - 0.6 V at pH=4.5, 0.28 - 0.5 V at pH = 6.5 and 0.25-0.3 V at pH =9.2. It shows the stronger activation of copper ion on marmatite flotation. 

Similarly, in the presence of 150mg/L TX4, copper ion still activates the xanthate flotation of marmatite as shown in Fig. 6.17. Marmatite starts flotation at a potential around 0.3 V with recovery of above 80%. At pH=4.5, the upper limit potential of flotation of marmatite can be high to above 0.6 V with a recovery about 90%. At pH = 6.5 and 9.2, the upper limit potential of flotation of marmatite decreases to about 0.5 V. Arsenopyrite can not be floated in the same conditions with a recovery of below 30% as seen from Fig. 6.18. This result also suggests that the flotation separation of marmatite from arsenop)aite may be accomplished by using TX4 as a depressant and xanthate as a collector in the presence of copper ion through the control of pulp potential and pH. 

The results of the polarization and complex impedance measurements for (2) and (3) are shown in Figs. 18 and 19, respectively. The leveling off of /(oc) was also observed in (2) and (3). The upper limit potentials of the ohmic portion were 0.5 V for (2) and 0.1 V for (3). Thus, the polyelectrolyte-type polymer (2) had a higher limit, compared with the other two polymer... 

Here the point p belongs to the spherical surface A of radius R. In order to find the upper limit on the left hand side of this equality, let us recall that T is the disturbing potential. In other words, it is caused by the irregular distribution of masses whose sum is equal to zero. This means that its expansion in power series with Legendre s functions does not contain a zero term. The next term is also equal to zero, because the origin coincides with the center of mass. Therefore, the series describing the function T starts from the term, which decreases as r. This means that the product r T O if oo and... 

The changes in surface concentrations of the components caused by current flow have two important effects They produce a change in electrode potential, and they imply that there is an upper limit to the cell currents when the diffusion flux attains its iimiting value. The first of these effects is considered in Section 6.3 the second, in the present section. 

It was shown later that a mass transfer rate sufficiently high to measure the rate constant of potassium transfer [reaction (10a)] under steady-state conditions can be obtained using nanometer-sized pipettes (r < 250 nm) [8a]. Assuming uniform accessibility of the ITIES, the standard rate constant (k°) and transfer coefficient (a) were found by fitting the experimental data to Eq. (7) (Fig. 8). (Alternatively, the kinetic parameters of the interfacial reaction can be evaluated by the three-point method, i.e., the half-wave potential, iii/2, and two quartile potentials, and ii3/4 [8a,27].) A number of voltam-mograms obtained at 5-250 nm pipettes yielded similar values of kinetic parameters, = 1.3 0.6 cm/s, and a = 0.4 0.1. Importantly, no apparent correlation was found between the measured rate constant and the pipette size. The mass transfer coefficient for a 10 nm-radius pipette is > 10 cm/s (assuming D = 10 cm /s). Thus the upper limit for the determinable heterogeneous rate constant is at least 50 cm/s. 

The very slow dissociation rates for tight binding inhibitors offer some potential clinical advantages for such compounds, as described in detail in Chapter 6. Experimental determination of the value of k, can be quite challenging for these inhibitors. We have detailed in Chapters 5 and 6 several kinetic methods for estimating the value of the dissociation rate constant. When the value of kofS is extremely low, however, alternative methods may be required to estimate this kinetic constant. For example, equilibrium dialysis over the course of hours, or even days, may be required to achieve sufficient inhibitor release from the El complex for measurement. A significant issue with approaches like this is that the enzyme may not remain stable over the extended time course of such experiments. In some cases of extremely slow inhibitor dissociation, the limits of enzyme stability will preclude accurate determination of koff the best that one can do in these cases is to provide an upper limit on the value of this rate constant. 

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