Reference interface

In modern investigations of the electrochemical properties of immiscible electrolyte solutions mixed cells are used, i.e., cells containing one interface, e.g., that under investigation - Nernstian or polarizable, and a second reference interface of the Haber types (Scheme 7 or 10). 

Another opportunity to realize constant activity of the potential determining ion at the reference interface appears when one chooses the solid electrolyte in such a way that the ion of the redox couple is the same as one ion of the major component of the electrolyte. In that case, the change of the activity due to the electrode reaction with the gas can be neglected against the overall constant activity of that ion in the salt. This is the solid-state reference arrangement. An example is the chlorine sensor (Fig. 6.40), in which the reference potential is set up by the constant activity of CP in the solid AgCl electrolyte. This arrangement is equivalent to a reference electrode of the second kind, discussed in Section 6.2.2.1. 

In Scheme 2.2b interface Wj/M is the outer or working interface, and interface M/w2 is the inner interface (not a reference interface). 

Figure 4 Load schedule for reference interface - HTE temperatures...

Figure 12 Reference interface power conversion unit with bypass...

The performance of the reference interface is characterised in Vilim (2007). In that work the GPASS/H code was used to determine the full power condition, the combined plant efficiency and the partial power load schedule. The values of the main operating parameters are summarised in Table 1. 

Figure 1 Combined plant with reference interface...

Now we consider the important fact that our reference electrode responds to one of the components of the aqueous phase. From the equilibrium at the reference interface, we have... 

Reference Interface. Any system that measures the potential of the liquid-liquid interface has to have a pair of reference electrodes connected to it. The aqueous phase is usually connected to a simple electrode of the second kind the state of the nonaqueous phase is usually explored by the so-called reference interface, which is, in essence, an ion-selective electrode. The reference interface is a liquid-liquid boundary that shares a common ion (usually a cation, denoted here Bx) of a constant and usually known concentration. The interfacial potential can be calculated as... 

Other Ions in the System. Consider what happens to the potential of the reference interface if another ion is added to the nonaqueous phase. This experimental condition will arise from ion redistribution on the working interface. Iterative calculations account for the effect of the dye in the nonaqueous phase on the reference interface. The result shows that if the dye concentration changes from 10-2 to 10 6 mol/L while the supporting electrolyte concentrations are held constant at 0.01 mol/L, the interface experiences only a 1-mV change. This change is so small that for actual work the potential of this interface can be considered constant. 

Repartitioning. When the two phases are brought into contact, repartitioning equal to 0.0017% of all ions present has to take place to establish equilibrium. The equilibrium potential will be 58.2 mV. When a 100-mV step is applied on the interface from an outside source, only 0.035% of the total ions will have to repartition to achieve the new equilibrium. This situation contrasts markedly with the reference interface, where only a 10-mV potential step requires a 20.5% concentration change. 

The interface of experimental interest, the working interface, is labeled WI the constant potential or reference interface (vide supra) is labeled RI. The aqueous phase of the RI is 0.01-mol/L tetrabutylammonium chloride. The interfacial potential of the reference interface is poised by the concentration ratio of tetrabutylammonium, a constant concentration ion shared by both phases. The exact potential of the reference interface can be calculated using eq 1. The electrochemical cell is connected to an electric circuit via two Ag-AgCl electrodes imersed in an aqueous chloride solution. 

Figure 7. Relationship between interfacial potential and the logarithm of the initial DiOC/3) concentration in the nitrobenzene phase. Curve A shows the calculated potential of the working interface between 0.01 mol/L LiCl in water and 001 mol/L TBATPB in the presence of DiOC/3), with the reference interface potential subtracted. Curve B is the calculated potential of the reference interface between 0.01 mol/L TBACl in water and 0.01 mol/L TBATPB in nitrobenzene. The experimentally determined potential differences between the reference interface and the working interface are given for DiOC2(5) (O) and...

That the distribution potential of the water-immiscible solvent (in particular, nitrobenzene) system, in the presence of TEAPi distributed between two phases, is close to zero. It can be concluded from the above assumptions that the EMF of cell XII is equal to the value of the distribution potential (defined by Eqs. (14,18-20) of the salt MX. It is noteworthy that salt bridges containing TEAPi were often used, and as mentioned above, both in direct studies of the distribution systems or in measurements of the ion transfer energy, e.g. of Ag, between two miscible or immiscible solvents [116-119]. In some works a bridge with TEAPi in di-isopropyl ketone was most frequently used but unfortunately, different reference interfaces or electrodes, e.g. calomel and picrate electrodes, were employed [48,62]. 

At a number of electrodes, the equilibrium state of certain electrode reactions can be observed. The electrical state of these interfaces can be reproduced experimentally with a relatively high precision. However, any absolute specification of the electrical state at these interfaces is inaccessible. On the other hand, the equilibrium state is thermodynamically well defined. Thus, under isothermal conditions it is possible to experimentally prepare interfaces where equilibrium of an electrode reaction can be assured, which is characterized by a constant and unknown GPD but is at the same time a well-defined thermodynamic state. As a consequence, A(A0) can be determined experimentally in accordance with Eq. (11), if A0/ = A0g. However, any experimental determination of A(A0) in accordance with Eq. (11) assumes the use of at least two interfaces and the formation of an electrochemical cell that contains an experimental electrical reference interface. Then,... 

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