Electrode, reversible

Electrode reversible Standard reference Salt bridge... 

ION-SELECTIVE ELECTRODES REVERSIBLE TO DOUBLE-CHARGED ANIONS PROBLEMS AND TRENDS... 

ION-SELECTIVE ELECTRODES REVERSIBLE TO PHYSIOLOGICALLY ACTIVE AMINE CATIONS THE MAIN WAYS FOR CONTROLLING POTENTIOMETRIC SELECTIVITY... 

The indicator electrode must be reversible to one or the other of the ions which is being precipitated. Thus in the titration of a potassium iodide solution with standard silver nitrate solution, the electrode must be either a silver electrode or a platinum electrode in the presence of a little iodine (best introduced by adding a little of a freshly prepared alcoholic solution of iodine), i.e. an iodine electrode (reversible to I-). The exercise recommended is the standardisation of silver nitrate solution with pure sodium chloride. 

The sign of the electrode potential is arbitrarily defined as follows. A kation electrode (e.g., Zn in ZnS04 aq.) is said to be positive when it is positive to a unimolar (f — 1) solution of its ions an anion electrode e.g., CI2 in KC1) is said to be positive when it is positive to a unimolar solution of its ions. If a cell is made up of electrodes reversible with respect to any kinds of ions, its electromotive force is the algebraic difference of its electrode potentials, provided the electromotive force at the contact of the two solutions, due to diffusion (cf. Jahn, Elcktro-chcmie) is neglected. 

Finally, we may observe that measurements of electromotive force can often serve to distinguish which kind of ions are really present in a solution. A concentration cell containing a solution of a known ion with an electrode reversible to the latter on one side, and the given solution with a similar electrode on the other side is taken. From its electromotive force, the concentration of the particular ion is calculable. In this way, for example, it was found... 

For simplicity, they treated a solution containing only one supporting electrolyte in addition to dissolved metal ions. Then, using a reference electrode (indicator electrode) reversible to one of the three kinds of ions (a dissolved metal ion, and an anion and cation of the supporting electrolyte), they examined the difference in potential between the working and indicator electrodes. Furthermore, the expressions derived were rewritten to correspond to the actual case when the potential of the working electrode was measured with respect to a conventional reference electrode as follows ... 

Rabinovich et al. have shown that it is possible to propose an extrather-modynamic definition of single-ion activity, a, as a function of the real potentials of those particles. "" By carrying out the measurements of voltaic cells containing electrodes reversible to the same ionic species in solutions of different concentrations in the same solvent. 

Among potentiometric methods of analysis that are important for ecological applications, the one most widely used is that of pH measurements with an indicator electrode whose potential is a function of the hydrogen ion concentration. More recently, ion-selective electrodes reversible to other cations such as those of heavy metals have become available. 

The third limitation is concerned with the numerous contributions to the cell voltage Vceii, which, along with the difference in the electrode reversible potentials AEeq, comprises overpotentials at the cathode, tjc, and the anode, as well as the ohmic drop A ohmic ... 

Ag(p%+ are fulfilled. When in the system (13) the reference electrodes reversible to anion X and cation are connected to the phase w and o respectively, the Gibbs adsorption... 

The potentiometric determinations have been achieved by the use of a glass-calomel electrode assembly described elsewhere (Lanoque et ai, 1976). The potential of a glass electrode, reversible with respect to H, immersed in a solution whose proton activity is ah is given by... 

Similarly, the calomel electrode, reversible with respect to Cl, has a potential... 

The combined effects of electroneutrality and the Donnan equilibrium permits us to evaluate the distribution of simple ions across a semipermeable membrane. If electrodes reversible to either the M+ or the X ions were introduced to both sides of the membrane, there would be no potential difference between them the system is at equilibrium and the ion activity is the same in both compartments. However, if calomel reference electrodes are also introduced into each compartment in addition to the reversible electrodes, then a potential difference will be observed between the two reference electrodes. This potential, called the membrane potential, reflects the fact that the membrane must be polarized because of the macroions on one side. It might be noted that polarized membranes abound in living systems, but the polarization there is thought to be primarily due to differences in ionic mobilities for different solutes rather than the sort of mechanism that we have been discussing. We return to a more detailed discussion of the electrochemistry of colloidal systems in Chapter 11. 

In lithium rechargeable batteries carbon materials are used that function as a lithium reservoir at the negative electrode. Reversible intercalation, or insertion, of lithium into the carbon host lattice avoids the problem of lithium dendrite formation and provides a large improvement in terms of cycleability and safety (111). 

Fig. 5. Cyclic Voltammograms for the Catalytic Electroreduction of Benzene H) sessile hanging mercury drop electrode, reversal potential —3.2 V (SCE), v = 10 V s 1, a) 0.5 M (C4H,)4NBF4 in THF-2% HzO b) a + benzene (0.25 M)...

Finally, two remarks regarding terminology. If an electrochemical reaction displays a negligible resistance, the corresponding electrode is called a reversible electrode . Reversible electrodes are known for cation conductors, but have not been reported for oxide ion conductors. The term electrode resistance denotes the electrical resistance due to the electrochemical reaction, or to the transfer through the space charge, rather than the resistance of the electrode material itself. 

Now consider a soil suspension into which is immersed an electrode pair consisting of a glass electrode reversible to Na and a silver-silver chloride electrode ... 

To yireparc electrodes both metals and nonmctals can be applied. Iodine in a solid state in contact with iodide solution can, for example, become an electrode reversible with respect to the iodide ions. When making such an electrode the iodine is placed into a glass vessel and the iodide solution poured over it current connection is made by a platinum wire, which is in contact with the iodine and acts as an indifferent electrode. Equilibrium at the electrode is attained according to the formula ... 

If an electrode reversible to the ions of metal M is introduced into a solution of known M, and L, in a medium of constant ionic strength and the solution is combined with a reference electrode through a suitable conducting bridge (such as a WUhehn type electrode ), the measured emf at a temperature T K is given by the Nemst equation (see Nemst Equation) ... 

The system thus behaves as an electrode reversible with respect to calcium ions. This result is of great interest since a reversible calcium electrode employing metallic calcium is difficult to realize experimentally. 

Concentration Cells with Transference.—When two solutions of the same electrolyte are brought into actual contact and if identical electrodes, reversible with respect to one or other of the ions of the electrolyte, arc placed in each solution, the result is a concentration cell with transference for example, the removal of the AgCl(s) Ag AgCl(s) system from the cell on page 196 gives... 

Activity Coefficients from Cells With Transference.—In order to set up a cell without transference it is necessary to have electrodes reversible with respect to each of the ions of the electrolyte this is not always possible or convenient, and hence the use of cells with transference, which require electrodes reversible with respect to one ion only, has obvious advantages. In order that such cells may be employed for the purpose of determining activity coefficients, however, it is necessary to have accurate transference number data for the electrolyte being studied. Such data have become available in recent years, and in the method described below it will be assumed that the transference numbers are known over a range of concentrations. ... 

Measurement of Liquid Junction Potentials with Different Electrolytes.—If the same assumption is made as on page 209, that the potential of an electrode reversible with respect to a given ion depends only on the concentration of that ion, then in cells of the type... 

This equation is seen to be identical with equation (14) of Chap. VI, and in fact the E derived on page 201 by suitable extrapolation of the e.m.f. data of cells of the type shown above, containing hydrochloric acid at different concentrations, is identical with the E of equations (15) and (16). It follows, therefore, that the standard e.m.f. of the cell under consideration is + 0.2224 volt at 25 , and hence the standard e.m.f. of the corresponding cell with the electrodes reversed, i.e.,... 

Since kxjk is a constant at definite temperature, this equation is obviously of the same form as the electrode potential equations derived by thermodynamic methods, e.g., equation (85) for an electrode reversible with respect to positive ions. The first term on the right-hand side of equation (30) is clearly the absolute single standard potential of the electrode it is equal to the standard free energy of the conversion of solid metal to solvated ions in solution divided by and its physical significance has been already discussed. 

In this equation we added a new subscript, which has not been discussed so far. Instead of the potential E, used thus far, we have introduced E, to specify that the cell for which this equation is valid has a reference electrode that is reversible with respect to the anion in solution. What would have been the result if a reference electrode reversible with respect to the cation had been used We shall spare the reader the tedium of working through the equations again, presenting only the final result, which is similar to Eq. 28H, namely ... 

In the equation for the e.m.f. of a cell with electrodes reversible for the cation, the transference number of the anion appears as Sit factor, and vice versa. 

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