Electrode of the third kind

An electrode of the third kind consists of a metal whose surface is in reversible contact with a small, fixed dissolved amount of one of its chelates, while the chelating agent Y concerned can also yield in a competitive way a chelate... 

Electrode of the third kind — Electrodes of the third kind contain a metal and two salts with a common anion. For instance, Zn(s) ZnC2C>4(s), CaC204(s) Ca(NC>3)2(aq). In these electrodes simultaneous equilibria with respect to anions and cations exist. The electrode reaction is... 

Occasionally electrodes of the third kind are encountered these consist of a metal, one of its insoluble salts, another insoluble salt of the same anion, and a solution of a soluble salt having the same cation as the latter salt, e.g.,... 

Under some circumstances, a metal electrode can be made to respond to a different cation. It then becomes an electrode of the third kind. As an example, a mercury electrode has been used for the determination of Ihe pCa of calcium-containing solutions. As in the previous example, a small concentration of the EiDTA complex of Hg(ll) is introduced into the solution. As before (Equation 23-4). the potential of a mercury electrode in this solution is given hy... 

Thus, the mercury electrode ha.s become an electrode of the third kind for calcium ion. 

C rystalline membranes arc also available thal consist of a homogeneous mixture of silver sulfide with sulfides of copper(U), lead, or cadmiunt. Toward these divalent cations, electrodes from these materials have electrical responses similar to electrodes of the third kind (Section 23C-3). Note that these divalent sulfides, by themselves, are not conductors and thus do not exhibit ion-selectivc activity. 

A metal electrode of the third kind uses two equilibrium reactions to respond to a cation other than that of the metal electrode. EDTA complexes many metal cations, with different stabilities for the complexes but a common anion (the EDTA anion) involved in the equilibria. A mercury electrode in a solution containing EDTA and Ca will respond to the Ca ion activity, for example. The complexity of the equilibria make this type of electrode unsuitable for complex sample matrices. 

Rather exotic electrodes of the third kind (with simultaneous equilibria with... 

An electrode of the third kind involves a metallic phase in contact with an electrolyte solution in which the activity of the metal ions in equilibrium with the metallic phase is controlled by a different cationic, solution-phase species through the presence of two insoluble salts of the respective cations, for example... 

Electrodes of the third kind were first reported by Le blanc and Hamapp (20), and the particular type of third-kind electrode outlined above was used by Jakuszewski and Tanieuska-Osinka (2I) to determine the Gibbs energies of formation of Ca +Caq). 

The cell diagrani for a double cell Involving electrodes of the third kind, for which the net cell reaction is equation (1. 6) is (22)... 

It is convenient to classify metallic indicator electrodes as electrodes of the first kind, electrodes of the second kind, electrodes of the third kind, and inert redox electrodes. 

In this way pellets which contain CuS, PbS or CdS in addition to Ag2S behave as though they were Cu, Cd or Pb electrodes (electrodes of the third kind). By substituting Eq. (38) into Eq. (33) and combining the constant terms in E the electrode potential can be expressed ... 

---