Electrode silver iodide

As mentioned before, Stern had a metal electrode in mind when he described the surface-solution interface then (7q referred to the electronic charge on the surface of the metal itself, ato the charge formed by electrostatically (or chemically) bound electrolyte ions at the IHP, and a to the charge in the diffuse layer. In the case of silver iodide, the surface charge ctq is assumed to be made up of the adsorbed "potential determining ions"... 

A silver wire is dipped into an aqueous solution containing HNO3 and AgNC>3 at equal concentrations. Hydrogen gas is bubbled around the wire so that we have a hydrogen electrode on one side. The other electrode consists of a silver wire coated with Agl. Let us assume a negative potential is applied to the right electrode. At the silver iodide electrode the reactions... 

The silver chloride electrode gave poor response to iodide and bromide, and so did the silver bromide electrode to iodide. Although the silver iodide electrode responded to all three halides, the peaks are not sufficiently resolved and they are asymmetric. Further, there was a drift of the base line after detection of a halide ion which was not a component of the electrode and this drift caused disturbance in the following peak. This difficulty is eliminated by using hydrous zirconium oxide instead of the anion exchange resin for the chromatography since it reverses the elution order for halide ions. The silver bromide electrode is then the most suitable as the detector for both bromide. 

The reversible iodine/molten silver iodide electrode was used for the first time by Sternberg, Adorian and Galasiu [108], To obtain this electrode it was necessary to construct an electrochemical cell which maintained iodine in the gaseous state from the moment it was generated until it was removed from the cell. The cell, which is shown in Figure 12, was constructed of heat resistant... 

I) For materials that can be made Into an electrode, or that can be deposited on an electrode, the differential capacitance can sometimes be measured directly. From this, the surface charge follows by Integration. A number of technical problems have to be surmounted, to be discussed In sec. 3.7c. One of these Is that Faradaic currents (currents across the Interface) have to be suppressed or accounted for. Another Intrinsic problem Is whether the surface properties of the electrode are Identical to those of the dispersed particles. For silver Iodide and some oxides the capacitance approach has worked well. It Is recalled that for polarizable, conducting Interfaces, with mercury as the prototype, this Is virtually the sole method. 

Establishing the surface area is the next concern. Electrode surfaces are seldom flat. Instead, they tend to display a set of different crystal faces. Sliver Iodide electrodes, prepared by amalgamation of silver with mercury, followed by vapour deposition of iodine, look smooth and shiny to the naked eye but reveeil crystallites under the electron microscope. Surface Irregularities not only complicate the assessment of the real area, they may also Interfere in the analysis of impedance spectra In terms of equivalent circuits. After drying, the surface may be studied by the usual optical methods (sec. 1.2) with the famlllrir caveat that drying may change these properties. Anyway, for a number of oxides and silver iodide It Is now established that electrodes can be made which have... 

Figure 3.32. Electrical double layer on silver iodide in IO m Na or K (1-1) salts. Comparison of results obtained by different authors, different techniques and different sols. Curves 1-5, potentlometric colloid titration curve 6, capacitance method for electrodes. References 1) E.L. Mackor, Rec. Trav. Chim. 70 (1951) 763 2) J.A.W. van Laar, PhD. Thesis, State Unlv, of Utrecht. NL (1952) 3) J. Lyklema, Trans. Faraday Soc. 59 (1963) 418 4) B.H. Bljsterbosch. J. Lyklema. J. Colloid Set 20 (1965) 665 5) B.H. Bljsterbosch, unpublished 6) J.H.A. Pleper, D.A. de Vooys. J. Electroanal. Chem. 53 (1974) 243. (Redrawn from B.H. Bljsterbosch, J. Lyklema, Adv. Colloid Interface Set 9 (1978) 147).

Use Substitute in solution for glass-calomel electrode systems, coagulant for silver iodide solutions, stereospecific catalyst. 

From Thomsen s determinations of the heat of formation of silver iodide it may be calculated, by means of the approximation formula, that iodine should have a measurable dissociation pressure at moderately high temperatures. Naumann (n) found, however, that this was too small to be measured even at 6oo°. Analogous differences occurred in the theoretical calculation of the E.M.F. of the silver-iodine electrode. U. Fischer then found the heat of formation by three independent methods as 15,200, 14,800, and 15,000, whereas Thomsen had given 13,800 so the differences between theory and observation are reconciled (cf. further, p. 113). 

From the argument above it can be seen that the more stable the species in which the silver ion is bound, the lower will be the electrode potential of the silver. A group of 0° s for various silver couples is given in Table 17.2. From the values in Table 17.2, it is clear that iodide ion ties up Ag" more effectively than bromide or chloride Agl is less soluble than AgCl or AgBr. The fact that the silver iodide-silver couple has a negative potential means that silver should dissolve in HI with the liberation of hydrogen. This occurs in fact, but the action ceases promptly due to the layer of insoluble Agl that forms and protects the Ag surface from further attack. 

The Nernst relationship for the silver electrode in a solution saturated with silver iodide can then be written as... 

Thus, when in contact with a solution saturated with silver iodide, the potential of a silver electrode can be described either in terms of the silver ion activity (with the standard electrode potential for the simple silver half-reaction) or in terms of the iodide ion activity (with the standard electrode potential for the silver-silver iodide half-reaction). The silver -silver iodide half-reaction is usually more convenient. 

Figuerola et al.[37] determined free cyanide and cyanide present in weak complexes sequentially using two silver iodide/silver sulfide electrodes with an intervening gas> diffusion separator. Following the potentiometric determination of free cyanide with the first flow-through electrode, the effluent was acidified, and the evolved hydrogen cyanide from weakly complexed species w as transferred through the membrane of the gas-diffusion separator, collected in an alkaline acceptor stream and determined with the second electrode. 

The fact that silver iodide is an ion conductor was first observed in 1914 by Tubandt and Lorentz. They noted that when a current was passed between Ag electrodes separated by solid Agl, the mass of the electrodes changed. At room temperature, Agl can exist in either the P- or y-form. On heating to 419 K, both phases transform to a-Agl, and this is accompanied by a dramatic increase in ion conductivity. In ot-Agl, the 1 ions are in a body-centred cubic arrangement. The Ag ions randomly occupy tetrahedral... 

Solid-state sensors for chloride, iodide, and fluoride are based on the solubility product of silver chloride or silver iodide particles in silicone rubber and a doped lanthanum fluoride single crystal, respectively. The fluoride-selective electrode was applied for the analysis of urine and bone tissue of people exposed to industrial sources as well as for control of therapeutic fluoride application for osteoporosis, whereas the chloride-selective sensor was applied to the analysis of sweat for the diagnosis of cystic fibrosis. In solid-state contact electrodes the solvent polymeric membrane is directly contacted to the solid field transducing element, although the reference electrode is separated from the ion-selective sensing pad. 

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