Absolute potential difference

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

The above discussion has been given as may perhaps have been gathered, a little bit with tongue in cheek it is not quite complete. The reason is that it started talking about absolute potential differences (i.e., thc/, >M /sand then with a slight change in phraseology slid into an electrode potential. 

Fig. 6.35. The meaning of the relative potential difference across the Cu/CuS04 interface, i.e.. the relative electrode potential, (a) The electrochemical cell corresponding to the relative potential difference, (b) The relative potential difference includes a platinum-copper contact potential and the unknown potential difference across the SHE, apart from the absolute potential difference across the Cu/CuS04 interface.

Does the impossibility of measurement of a quantity preclude further thought about it Discussion of a concept, even if it cannot be measured, often leads to better understanding of it. With this view, attempts will be made to probe further into the question of the absolute potential difference across an individual metal/solution interface. 

Is the Inner Potential Difference an Absolute Potential Difference ... 

Thus, surface tension changes have been related to changes in the absolute potential differences across an electrode/electrolyte interface and to changes in the chemical potential of all the species, i.e., to changes in solution composition. Only one other quantity is missing, the surface excess. This can be easily introduced by recalling the definition of surface excess [Eq. (6.66)], i.e.,... 

Even though one is considering the simplified situation of equilibrium, one is still faced with the fact that the absolute potential differences across single interfaces are experimentally inaccessible (Section 6.3.1). However, one can always resort to the convention (see Section 6.3.4) of referring all potentials to an SHE which would consist, e.g., of a platinized platinum electrode in contact with hydrogen gas at 1 atm pressure and a solution of hydrogen ions at unit activity. 

The vertical shift has arisen from the application of an absolute potential difference of d to a hypothetical interface, initially with zero potential difference across it, i.e., zhj) = 0. But the argument is valid for any change of potential across the interface. Thus, if the double-layer potential is initially Atye (i.e., the interface is at equilibrium) and then the potential is change to zf< ), the Morse curve for the initial state is shifted vertically through an energy F(Aty — Atye), or Ft],... 

Measurement of electrode potentials. It is not practical to attempt to measure the absolute potential difference between each element and a... 

Apart from this important feature of a corroding system, there is another characteristic that arises from the short-circuit condition of the corrosion cell and of the equivalent cell. (It will be recalled that the electron sources and sinks in the corroding metal are internally short-circuited the two electrodes in the equivalent cell are externally short-circuited.) The total potential difference V across the equivalent cell is zero. But this cell potential is composed of the absolute potential differences across the interfaces at the two electrodes and the potential drop IR in the electrolyte ... 

Absolute Potential Difference between Mercury and an Aqueous... 

To verify under which conditions the dipole terms in Eq. (79) can be disregarded, the extra-thermodynamic absolute potential difference must be estimated. On mercury an approximate estimate of... 

Afaj 2e) can be made by considering a mercury electrode coated with a self-assembled lipid monolayer having a neutral polar head, such as dioleoylphosphatidylcholine.87 In this case the extrathermodynamic absolute potential difference across the whole mercury/solution interphase can be approximately expressed by the equation ... 

It follows that the absolute potential difference i// between mercury and the aqueous phase can be obtained by increasing the applied potential... 

After making an approximate estimate of the absolute potential difference y/ across the mercury/aqueous solution interphase, let us detennine the electrosorption valency of three mercury-supported thiol monolayers for which uM was measured by potential-step chronocoulometry. As already reported in Sec. IV.1, n-octadecanthiol... 

Absolute Single Electrode Potentials.— The electrode potentials discussed hitherto are actually the e.m.f. s of cells resulting from the combination of the electrode with a standard hydrogen electrode. A single electrode potential, as already seen, involves individual ion activities and hence has no thermod3mamic significance the absolute potential difference at an electrode is nevertheless a quantity of theoretical interest. Many attempts have been made to set up so-called null electrodes ... 

It is quite impossible to determine the absolute potential difference across a sin e met /solution interface, and the potential must be evaluated indirectly from the e.m.f. of a cell comprising the interface under consideration and another electrified interface. The e.m.f. of the cell can be determined readily by a suitable measuring device such as a potentiometer, vacuum-tube voltmeter or an electrometer, which are capable of measuring the e.m.f. with the minimum passage of electrical charge. This is essential since if a significant current is allowed to pass, the electrodes (electrified interfaces) become polarised and the e.m.f. will be less than the equilibrium value. Consider the determination of the interfacial potential at the surface of a zinc electrode in equilibrium with Zn ions in solution. In order to determine the potential it is necessary to couple it with another electrode, and for the purpose of this discussion the equilibrium between ions in solution and gas will be chosen, i.e. the reversible hydrogen electrode in which the equilibrium between and H takes place at a platinised-platinum surface. The spontaneous cell reaction will be... 

The absolute potential difference between half-wave potentials for oxidation and reduction of a given (OEP)M or (TPP)M complex was initially claimed to be a constant value of 2.25 0.15 V [51, 52], but there are now many examples in the literature where this is not the case. 

Application of Eq. (2) to a DOPC SAM, which does not contain ionizable groups inside the hydrocarbon tail region (uin = 0), allows an extratherm odynamic estimation of the absolute potential difference, A(p, across the whole mercury-aqueous solution interphase [37]. Upon ignoring the small potential difference, < a> across the diffuse layer, Eq. (2) can be written as follows ... 

The potential difference across the floating lipid bilayer on top of the thiolspacer SAM (i.e. the transmembrane potential) can be estimated by noting that its zero value is practically attained at the applied potential E at which the absolute potential difference across the whole interphase, A (f/SCE-t-0.250 V) (see Eq. 9), is equal to the calculated value of Xt. In fact, from Eq. (21) it is apparent that, under these conditions the charge density q = ctm + aoi experienced by the diffuse-layer ions is zero, just as the electric field across the floating lipid bdayer. The applied potential at which the transmembrane potential equals zero can also be approximately estimated by incorporating valinomycin in the lipid bilayer [37, 121]. Valinomycin is a K+-selective ion carrier that may shuttle K+ ions across... 

Nernst s early vievi that the separate terms of his whole-ceJI equations gave the absolute potential difference across each individual interface was later modified by the recognition that the condition of zero interfacial charge is not necessarily the condition of zero potential difference. His eponymous equations for whole cells in any case relate to relative not absolute differences, and remain the crucial cornerstone of equilibrium electrochemistry. 

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