Mixed silver electrode

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

The silver-silver Ion electrode. Of the reversible metal electrodes, silver has been most often employed. There is only one stable oxidation state of silver above 300°C there is no danger of oxide formation because Ag20 is unstable.57 The metal has no observable tendency to dissolve in molten silver salts and is highly reversible in mixed chloride and nitrate eutectics. The Ag(I) ion can be introduced into the melt by either adding silver nitrate to a nitrate melt (AgCl to a chloride melt) or by anodizing a silver electrode. The potentials of silver nitrate concentration cells show ideal thermodynamic behavior up to 0.5 mol % in (Na,K)N03 eutectic and in NaN03.58... 

Example 34 20 ml OlMNaCl and 2 ml 0lMAgNO3 are mixed and diluted to 100 ml. Calculate the potential of a silver electrode dipped into this solution. 

In the first series solutions of silver nitrate and sodium selenite were mixed to the total concentrations 5.0 x 10 and 48.0 x 10 M, respectively. The pH was adjusted by addition of HCIO4 or NaOH to values between 1.2 and 13.1 (53 experimental points). After equilibration for 6 days, the phases were separated. The pH and pAg were obtained from potentiometric measurements with a glass and a silver electrode. The latter electrode was calibrated on the concentration scale. The following equilibrium constants were evaluated from the data on the molar scale ... 

The chief restriction of these mixed-salt electrodes is that the solubility of the second salt must be much larger than that of Ag2S but, on the other hand, it must be sufficiently insoluble that its dissolution does not limit to relatively high values the test ion concentration that can be detected. As long as the membrane contains sufficient silver sulfide to provide silver ion conducting pathways through the membrane, it will function as a silver electrode. The potential, then, is related... 

At open circuit the interface with the silver electrode is In thermodynamic equilibrium. The interface with the zinc electrode has a mixed potential, which is defined by adding the zinc oxidation and proton reduction currents, with the latter reaction characterised as being very slow at the zinc electrode. The polarities of the electrodes in open-circuit conditions are defined by the experimentally measured potentials of each electrode vs a saturated calomel reference electrode. 

SERRS and voltammetry techniques were used to elucidate the mechanism of oxygen reduction on a silver electrode with iron(III) tetra-4-iV-methylpyridyl-porphyrin " . The results indicate that after the oxidation-reduction cycle at pH 10 and pH 4 the iron porphyrin is adsorbed on the Ag surface as a high-spin, five-coordinated -oxo-bridged dimer. SERRS spectra show that the first electron transfer forms a mixed-valence Fe(n)-0-Fe(III) //- oxo-bridged dimer. 

Flildebrandt and Stockburger found that there are two conformational states of cyt. c upon adsorption on a silver electrode [38-40]. Cytochrome c adsorbed on the silver electrode at negative potentials (<0.04 V) exhibits the state I (cyt. C ) conformation and that adsorbed at positive electrode potentials (>0.04 V) exhibits the state II (cyt. cn) conformation. The structure of the RR spectrum of cyt. c in the solution is fully maintained in the state I, which must be the same for the whole cyt. c molecules. In the state II, the spin state of the heme iron is in the mixed 5cHS and 6c LS conformation. The conformational states I and II are at potential-dependent equilibrium. The most stable species of ferro-cyt. c is the reduced form of state I (cyt. ci +) in the electrode potential range, more negative than 0.2 V and that of ferri-cyt. c is the oxidized form of state II (cyt. cn +) in the electrode potential range more positive... 

It was shown that cleavage of the S—S bonding of 4-PySSPy takes place upon adsorption on both gold and silver electrodes, and a stable chemisorbed film is formed through sulfur to these electrodes [63, 65]. Cytochrome c adsorbed on a gold or silver electrode is displaced entirely by 4-PySSPy and the electrode reaction of cyt. c in the solution takes place through the PyS- film on the electrode surface [63, 64]. Added purine partially displaces cyt. c from a silver electrode surface and a mixed adsorbed layer of purine and cyt. c is formed, at which a reversible electrode reaction of cyt. c takes place [63]. 

Fan and coworkers studied using SERS the conformation of pyridine derivatives at a silver electrode in the presence of cyt. c [67]. When the 4,4 -bipy modified silver electrode is transferred in a cyt. c solution, 4,4 -bipy partially displaces cyt. c from the electrode and a mixed adsorbed layer of 4,4 -bipy and cyt. c is formed. The spin state of the coadsorbed cyt. c is a mixture of the 5cHS and 6cLS states. The formal potential of the coadsorbed cyt. c estimated from the shift of the oxidation... 

Cytochrome c is found to exhibit a quasireversible voltammetric response at an iodine-modified silver electrode. S ERRS spectroscopy indicates that cyt. c immobilized on the iodine-modified silver electrode is in the 6cLS state [74, 75]. The shift of the oxidation state marker band was plotted against the electrode potential and the formal potential of cyt. c immobilized on the iodine-modified silver electrode is estimated to be 0.19 V versus NHE, which is somewhat negative relative to that of the native state. A similar shift is also seen when cyt. c interacts with mitochondrial membranes (50-60 mV) [76, 77]. It can be concluded that the SERRS results indicate that the adsorbed cyt. c is structurally similar to the native protein in solution. The SERRS spectrum of cyt. c adsorbed on the iodine-modified gold electrode is different from that on the iodine-modified silver electrode. The SERRS spectrum of cyt. c (ox) excited by 550 nm at open-circuit potential reveals that cyt. c is present in the mixed 5cHS and 6cLS state [78]. 

Because of the electronic conductivity of the compound in the centre of the device, its potential versus the silver electrodes can be measured directly without further reference electrodes. On the other hand, this potential is determined by the Ag" " activity in the mixed conductor. If the stoichiometry of the mixed conductor varies this potential will change. 

The principle of the operation of the analogue memory is thus A writing or erase current injects or pumps Ag" out of the mixed conductor. One silver electrode is used for that purpose. The other one is used to read the information stored as silver ions in the mixed conductor. 

It has been shown (U3) that a rather high overall capacitance (Table III) could be obtained by optimising the total interface area in a given volume. Such a capacitance is constituted by a carbon electrode mixed with Rb Agi I5 and another silver electrode. Tension on it is limited to about 0.5 V where electrolysis occurs. 

The curve is experimentally obtained with a silver electrode that directly gives pAg, or it can be calculated using Eqs. (36.4) and (36.5) by mixing activities and concentrations. The important point to be stressed here is the abrupt change in pAg close to the equivalence point. 

At the Prague Institute of Chemical Technology, F. Jirsa studied anodic oxidation of gold [15] later he published an important paper about silver electrode for a silver-iron battery [16], Jaroslav Chloupek (1899-1975), partly with V. Danes (1907-1980) and B. Danesova, studied the electrode potential in solutions of mixed manganese salts [17], the solubility and activity coefficient of Ag2S04 in some solutions [18], the ions and deviations from the approximation of Debye-Hiickel theory [19], the liquid potentials [20], and the anomalous valency effect of strong electrolytes in aqueous solution [21]. 

Flg. 11.9.9 a, b. a Schematic presentation of anodic and cathodic partial current densities (dashed lines), net current densities (thin solid line) and the position of the equilibrium electrode potential of a silver electrode and a glass electrode (one glass-solution surface). Subscript gl indicates the surface of a glass membrane and sol indicates the electrolyte solution, b Schematic presentation of anodic and cathodic partial current densities (dashed lines), net current densities of the single reactions (thin solid lines) and the position of the mixed potential Emixed at a zinc electrode in an acidic solution... 

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