Standard electrode potentials metal complexes

The review of Martynova (18) covers solubilities of a variety of salts and oxides up to 10 kbar and 700 C and also available steam-water distribution coefficients. That of Lietzke (19) reviews measurements of standard electrode potentials and ionic activity coefficients using Harned cells up to 175-200 C. The review of Mesmer, Sweeton, Hitch and Baes (20) covers a range of protolytic dissociation reactions up to 300°C at SVP. Apart from the work on Fe304 solubility by Sweeton and Baes (23), the only references to hydrolysis and complexing reactions by transition metals above 100 C were to aluminium hydrolysis (20) and nickel hydrolysis (24) both to 150 C. Nikolaeva (24) was one of several at the conference who discussed the problems arising when hydrolysis and complexing occur simultaneously. There appear to be no experimental studies of solution phase redox equilibria above 100°C. 

We now return to the case of codeposition of metals whose standard electrode potentials are wide apart. As stated, the deposition potentials [Eq. (11.2)] are brought together by complexing the more noble metal ions, as illustrated below for the case of the codeposition of copper and zinc as brass. 

SOURCES The most authoritative source is S. G. Bratsch, J. Phys. Chem. Ref. Data 1989,18, 1. Additional data come from L. G. Sillen and A. E. Martell, Stability Constants of Metal-Ion Complexes (London The Chemical Society, Special Publications Nos. 17 and 25. 1964 and 1971) G. Milazzo and S. Caroti, Tables of Standard Electrode Potentials (New York Wiley, 1978) T. Muss ini,... 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

This equation has a standard electrode potential of -0.447 V. Thus, the solution containing mercuric and chloride ions in contact with iron forms a battery. The reduction of the complex ions to metallic mercury is the cathodic reaction. The dissolution of iron is the anodic reaction. The overall reaction in the battery is given by the addition of Equation (13.42) and Equation (13.43). Due to the high value of its reversible cell voltage under standard conditions (0.85 V), it is expected that a very low equilibrium concentration of the complex ion can be achieved. 

One of the first questions one might ask about forming a metal complex is how strong is the metal ion to ligand binding In other words, what is the equilibrium constant for complex formation A consideration of thermodynamics allows us to quantify this aspect of complex formation and relate it to the electrode potential at which the complex reduces or oxidizes. This will not be the same as the electrode potential of the simple solvated metal ion and will depend on the relative values of the equilibrium constants for forming the oxidized and reduced forms of the complex. The basic thermodynamic equations which are needed here show the relationships between the standard free energy (AG ) of the reaction and the equilibrium constant (K), the heat of reaction, or standard enthalpy (A// ), the standard entropy (AS ) and the standard electrode potential (E for standard reduction of the complex (equations 5.1-5.3). 

Active catalysts should therefore be able to form surface metal-ethyne and metal-HCl complexes. One of the most extensive studies of metal chloride catalysts was carried out by Shinoda [254] 20 metal chlorides supported on carbon were investigated for ethyne hydrochlorination and it was proposed that a correlation existed between the catal3dic activity and the electron affinity of the metal cation, divided by the metal valence. The correlation consists of two straight lines and, for this reason, it cannot be used predictively. However, electron affinity is, necessarily, a one-electron process, whereas the hydrochlorination of ethyne is more likely to be a two-electron process, involving 27t electrons of ethyne. Many of the metal cations investigated in the original study of Shinoda [254] are divalent and, consequently, the standard electrode potential was proposed as a more suitable correlation parameter. 

Inert-gas-type ions, such as Ca", La", are much more stable than noninert-gas-type ions, such as Cu, Ag, Zn", Ga-. This statement is true for the ease of reduction to the metal, measured either by the ionization potentials or by the standard electrode potentials, and it is also true for the ease of forming covalent bonds. Atoms whose ions would be of the noninert-gas type are more prone to form covalent bonds, other things being equal, than those which give inert-gas-type ions. Many complex ions and covalent compounds of Cu", Ag", and Zn" are known, but very few of Na, K", and Ca". Thus the fourth covalency rule reads ... 

The anaerobic oxidation of a range of dithiocarbamates and xanthates by inert metal complexes has been shown to proceed by an outer-sphere process, in which the key step involves the formation of a sulfur-centered radical. The standard electrode potential for the Et2NCS2/Et2NCSJ couple is estimated to be 0.425 0.33 V vs. SCE. 

Changing the ligands not only changes the colour of a complex ion, but can also affect the standard electrode potential (Chapter 19) of a complexed d-block metal. This is because different ligands provide different environments for the ion. 

It is also important to consider that the emitter electrode material may participate in electrochemical reactions (e.g., anodic corrosion of iron in a stainless steel emitter). Table 3.2 lists the standard electrode potentials for some of the possible oxidation processes of metals used as ES emitters. The potential at which oxidation of a metal may take place strongly depends on the solution composition and follow-up reactions (e.g., precipitation, complexation, etc.) as well. As an example, the standard electrode potential for the reversible Ag/Ag couple is 0.80 V vs. SHE. However, at pH 14, Ag20 forms during the... 

Voltammetric methods also provide a convenient approach to establish the thermodynamic reversibility of an electrode reaction and for the evaluation of the electron stoichiometry for the electrode reaction. As outlined in earlier sections, the standard electrode potential, the dissociation constants of weak acids and bases, solubility products, and the formation constants of complex ions can be evaluated from polarographic half-wave potentials, if the electrode process is reversible. Furthermore, studies of half-wave potentials as a function of ligand concentration provide the means to determine the formula of a metal complex. 

Complex constants and standard potentials of some metal ion complex electrodes with various complex agents are given in Table 3.4. ... 

The high adsorption capacity of Ag+ ions by all the activated carbons was attributed to the reduction of Ag+ ions to metallic silver by the hydroquinone groups present on the carbon surface, which in turn are oxidized to quinone groups. This redox process is supported by the standard reduction potentials of Ag+ (Ag+ + e Ag, E = 0.7996 V) and quinhydrone electrode, = 0.6995 V. The increase in adsorption of Ag+ ions by the ammonia-treated sample was attributed to the formation of silver amino complexes which are quite stable under the conditions used in these studies. 

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