Silver iodide surface potential

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"... 

In this section we discuss five different materials as examples with different charging mechanisms mercury, silver iodide, oxides, mica, and semiconductors. Mercury is one example of an inert metal. Silver iodide is an example of a weakly soluble salt. Oxides are an important class of minerals. For most biological substances like proteins or lipids a similar charging process dominates. Mica is an example for a clay mineral. In addition, it is widely used as a substrate in surface force measurements and microscopy. We also included a general discussion of semiconductors because the potential in the semiconductor can be described similarly to the diffuse layer in electrolytes and there is an increasing effort to make a direct contact between a liquid or a living cell and a semiconductor. 

Silver iodide particles in aqueous suspension are in equilibrium with a saturated solution of which the solubility product, aAg+ai, is about 10 16 at room temperature. With excess 1 ions, the silver iodide particles are negatively charged and with sufficient excess Ag+ ions, they are positively charged. The zero point of charge is not at pAg 8 but is displaced to pAg 5.5 (pi 10.5), because the smaller and more mobile Ag+ ions are held less strongly than-the 1 ions in the silver iodide crystal lattice. The silver and iodide ions are referred to as potential-determining ions, since their concentrations determine the electric potential at the particle surface. Silver iodide sols have been used extensively for testing electric double layer and colloid stability theories. 

In many colloidal systems, the double layer is created by the adsorption of potential-determining ions for example, the potential 0o the surface of a /Silver iodide particle depends on the concentration of silver (and iodide) ions in solution. Addition of inert electrolyte increases k and results in a corresponding increase of surface charge density caused by the adsorption of sufficient potential-determining silver (or iodide) ions to keep 0O approximately constant. In contrast, however, the charge density at an ionogenic surface remains constant on addition of inert electrolyte (provided that the extent of ionisation is unaffected) and 0O decreases. 

Figure 3.28. Illustration of a seminal colloid titration result obtained after pioneering work by E.J.W. Verwey and H. de Bruyn. Silver iodide in (l-l) electrolytes drawn curves 7 1 KNOg + NaNOg mixture, O NaClO, A NaNOg. The surface charge could not be exactly established because the surface area was not well known. pAg and units are convertible because Nemst s law applies. The (7 l)-KNOg + NaNOg mixture was chosen to suppress the liquid Junction potential (sec. F5.5d) with the salt bridge. Source Redrawn from data by J.A.W, van Laar, PhD Thesis. State Unlv. Utrecht (1952) E.L. Mackor. Rec. Trau. Chim. 70 (1951) 763, as collated by J.Th.G. Overbeek In Colloid Science Vol. 1, H.R. Kruyt, Ed., Elsevier (1952) 162. Older references include E.J.W. Verwey, H.R. Kruyt, Z. Phys. Chem. A167 (1933) 149 E.J.W. Verwey. Rec. Trav. Chim. 60 (1941) 887 and H. De Bruljn. Rec. Trav. Chim. 61 (1942) 5, 21.

The surface charge on a solid surface can be obtained by determining the adsorption of potential-determining ions at various potentials of the interface [1]. For example, in the case of a silver iodide sol the adsorption of Ag+ and I ions is determined at various concentrations of Ag" " and I" ions in bulk solution. Similarly, for an oxide the adsorption of H" " and OH" ions Fand respectively) is determined as... 

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. 

More detailed information on ion exchange involved in protein adsorption can be derived from titration experiments in systems where the charge of the protein and the sorbent can be varied independently. Currently, we are studying such systems, using bovine plasma albumin (BPA) and cytochrome c as the proteins and silver iodide (Agl) particles as the sorbent. In these systems there are two potential determining ion couples, the H" /oh" couple for the protein and the Ag" "/ " couple for the sorbent. They enable independent control of protein and surface charge. Below, we will briefly discuss some results obtained with the BPA - Agl system. 

Adsorption-desorption of lattice ions. Silver iodide particles in Ag" " or solutions are the typical example the crystal lattice ions can easily find their way into crystal sites and become part of the surface. They are called potential-determining ions (p.d.i.). 

For the completely reversible electrode the surface charge in the model is identified with the charge of the adsorbed potential-determining ions So in the case of silver iodide... 

Just as in the electrocapillary curve the state of zero charge on the surface is important as a point of reference This point is not equal to the equivalence point in the solution as a completely stoichiometric crystal of silver iodide may show quite different escaping tendencies for the two constituent ions It is possible, however, to find a certain concentration of silver and iodide ions for which this difference in escaping tendency is just compensated In very dilute ueous solutions the point of zero charge is then found at a Ag" " ion concentration of CAgo 10- JV and an I "ion concentration JV Assuming provisionally that our considerations. are restricted to dilute solutions where the /12 potential (cf 3 d, p 124) is a constant we may relate the double layer potential D to the concentration (activity) of the potential-determining ions. 

Fig, 40. Exchange of monovalent for divalent electrolyte as a function of the composition of the solution and of the surface potential Oo Circles describe exchange of HNO3 against Ba(N03)2 in a silver iodide sol. 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

We may consider, for instance, the system Agl/w,atcr in some detail. In this case the double layer potential appears to be determined by the concentration of the silver- or iodide ions, respectively, in the solution. When Agl particles, as present in an Agl sol, are in equilibrium with an aqueous solution containing 10 eq. Agf"ions per litre (and therefore 10 i eq. l ions, as the solubility product of Agl is about 10 eq./l at room temperature), the Agl is just about uncharged, hence no double layer is present ( o ) When the Ag+ions arc brought to ten times this concentration (and accordingly the 1 to one-tenth their original concentration) , the distribution equilibrium of the Ag+ions about the particle surface and the solution is shifted in the direction of more Ag" at the surface. Hence, wh[en equilibrium has been newly established, we will find that the particle surface no longer contains equivalent amounts of Ag+ and I ions, but a small excess of the former. 

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