Junction voltage.

Jongsma HJ, Wilders R, Takens-Kwak BR, Rook MB Are cardiac gap junctions voltage sensitive in Hall JE, Zampighi GA, Davies RM (eds) Gap Junctions, Progress in Cell Research, vol 3. Amsterdam, Elsevier, 1993, pp 187-192. 

From the fact that 633-nm light passing under the detector was not observed behind the detector, it was concluded that the beam was effectively absorbed. Operating in the photovoltaic mode, the detector voltage was a linear function of the guided wave power (Fig. 2). The rise time of these detectors was 350 nsec. From assumed values for thfe mobility, junction voltage, and depletion width, the dominant limiting factor for the speed of these detectors was the transit time. 

Electrical effects. Electrical methods are convenient because an electrical signal can be easily processed. Resistance thermometers (including thermistors) and thermocouples are the most widely used. Other electrical methods include noise thermometers using the Johnson noise as a temperature indicator resonant-frequency thermometers, which rely on the temperature dependence of the resonant frequency of a medium, including nuclear quadrupole resonance thermometers, ultrasonic thermometers, and quartz thermometers and semiconductor-diode thermometers, where the relation between temperature and junction voltage at constant current is used. 

Minority Carrier - A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device. 

If the electrochemical chain includes several successive electrolyte media, then the abbreviated notation 11 is often used to denote the separation zone between two electrolytes. Such a notation represents either the porous material filled with a mixture of the two liquid electrolytes, or all of the various phases of a salt bridge (see example below). The nature of this intermediate zone is often such that the global zero current junction voltage can be neglected... 

Zn I aqueous solution of ZnS0411 aqueous solution of CUSO41 Cu neglecting junction voltages and assuming that both interlaces are at thermodynamic equilibrium lead to the following equation ... 

Generally speaking, in systems with localised interfaces, the voltage between the terminals in an electrochemical chain can be split into the sum of the interfacial voltages and the ohmic drops . Let us recall that an interfacial voltage is the difference between the potentials of both sides of the interface, whose thickness is about a few nanometres most of the time. It is therefore either an ionic or electronic junction voltage or an electrochemical interface voltage. The ohmic drop is the potential difference between... 

Fora system which starts in equilibrium at open circuit and with negligible ionic junction voltages, the word overpotential usually refers to the difference between the interfacial voltages observed with and without current flow. In connection with the general definition , the ohmic drop terms are therefore excluded from overpotential terms. It all comes down to being able to imagine that one can place two references infinitely close to each of the interfaces. The following equation is therefore commonly used ... 

Subsequently, this chapter will only describe situations of this type. Notably, what will not be touched upon are electrochemical systems in which the ionic junction voltages undergo large changes when there is a current flow. 

Equilibrium is rapidly reached and the ionic junction voltage is given by ... 

The order of magnitude of this type of junction voltage can be relatively high (mV to V). However, one should keep in mind that it cannot be measured directly, just as in the case of electronic junctions. 

Now imagine a junction where there is a selective membrane separating two solutions in the same solvent with different concentrations. Given that the standard chemical potentials are equal, then the constant is zero in the equation for the junction voltage, and the voltage is therefore directly linked to the activities ratio of the ion being exchanged between the two solutions. 

The junction voltage can also be presented in an eguation as a function of the activity of CT ions in the solution. For these purposes, one uses the solubility eguilibriutn of AgCI ... 

The junction voltage of this type of interface (single exchange) is non-zero in equilibrium. Assuming that the activity coefficients of the anions and cations are equal in same phase and taking into account the electro-neutraiity of the media a and p, then this junction voltage can be written in the following simple equation ... 

As for a junction where several species can be exchanged, the ionic junction voltage in equilibrium relates to the differences between the chemical potentials of each charged species, based on the following equations ... 

The equilibrium is only reached once the Galvani potential and the chemical potentials of each type of species are both identical in the two phases. The junction voltage is therefore zero in equilibrium, which indeed can take a long time to reach. 

In this case, when the multiple junction reaches equilibrium, the iatter s junction voltage is not zero. 

Numerous applications have been developed in the field of chemical analysis using potentiometric measurements as indicators, including the production of potentiometric sensors and titration devices. In this chapter, we will focus on the defining principles of these potentiometric methods at zero current when these systems are in thermodynamic equilibrium, which is not necessarily true for all potentiometric measurements. In particular, the following description is confined to electrochemical cells with no ionic junction. In practice, these results will also be applied to many experimental cases in which ionic junction voltages can be neglected . 

Appendix A.3.2 addresses these questions in more thorough detail even if the ionic junction voltage is negligible in numerical terms, it may nonetheless be important to consider it from a fundamen tal point of view. 

To take a more general case, let us imagine an electrochemical cell, which may possibly be fictive, where the sum of the ionic junction voltages is zero. This would give the following equations ... 

V For example, the standard emf of the following cell, at pH= 0, can be given using thermodynamic data (assuming the ionic junction voltage is zero ) ... 

By definition, the potential of a redox couple vs SHE is the emf of the Active electrochemical cell, whereby the working electrode is in the half-cell involving the redox couple in question. The counter-electrode is the standard hydrogen electrode at the same temperature. The terminals are made of the same metals and the sum of the possible ionic junction voltages are considered to be equal to zero. In this case we therefore have E,she =... 

In the case of solutions separated by a membrane or a porous material, the equilibrium state corresponds to a perfect mbtture of the two solutions. However, it takes a very long time to reach this state, if the porosity and/or the geometric configuration of the interfacial zone are well chosen (see section 4.4.2). In such a case, the key phenomenon to consider therefore is the fact that the quasi-steady state is rapidly reached. This latter state has different solutions compositions and non-zero junction voltages. From an experimental point of view, the aim is to minimize this quasisteady-state ionic junction voltages. Different examples are outlined in appendix A. 1.1. 

Appendix A. 1.1 highlights the points addressed in this paragraph by giving a selection of different situations where the ionic junction voltages are numerically estimated. 

In electrochemistry, on usual time scales ( 10" s) it can be assumed that electron exchange is immeasurably fast (no energy barrier) at the junction between two metals. In these conditions, the current flow through a metallic junction does not modify the latter in any way. In particular, the electronic junction voltage is the same as in equilibrium, when thecurrent iszero, as represented in figure 4.13. 

---