Oxidation states and emfs

The Chemistry of the Heavier Transition Metals 587 Oxidation States and EMFs of Groups ]-12 588 The Lanthanide and Actinide Elements 599 Coordination Chemistry 608 The Transactinide Elements 613... 

Worked Example 3.4. A sample of iron-containing ore is crushed and the powder extracted to form a clear aqueous solution. A clean iron rod is immersed into the solution and the cell Fe Fe " (aq) 11 SCE is therefore made. The emf at equilibrium was measured as 0.714 V at 298 K, and the SCE was the positive electrode. What is the concentration of the iron (Assume that all the iron exists as a simple aquo ion in the -1-2 oxidation state and that the solution is quiet .)... 

The first member of this family, manganese, exhibits One of the most interesting redox chemistries known thus it has already been discussed in detail above. Technetium exhibits the expected oxidation states, and associated with these are modest emf values. All of the isotopes of technetium are radioactive but "Tc has a relatively long half-life (2.14 k 10s years) and is found in nature in small amounts because of the radioactive decay of uranium. Oxidation slates of rhenium range from +7 to - 3, with some species ReOj and Re3+) unstable with respect to disproportionation. 

The elements copper, silver, and gold show such anomalies that there sometimes appears to be little congruence as a family, with the member that is least reactive as a metal (Au) being the only one that has an appreciable chemistry in the +3 oxidation state and also the only one to reach the - I and +5 oxidation slates (CsAu and AuFj), although both copper and silver may be oxidized to +4. The members of the family more or less routinely (silver less frequently) violate the very useful rule of thumb you have seen earlier The maximum oxidation state of an element is equal to or less than its group number (IB, IVB, VIIB, etc.). Thus we have CUSO4, AgF, and [AuClJ . Each member of the family has a different preferential oxidation state (Cu, +2 Ag, + 1 Au, +3). The one property they do have in common is that none has a positive emf for M -> M therefore, the free metals arc not affected by simple acids, nor are they readily oxidized otherwise, leading to their use in materials intended to last.31... 

Having compared in general terms the properties of transition metals both on the basis of the d-electron configuration and the properties of the light versus heavier metals, we shall now look more specifically at the stabilities of the various oxidation slates of each element in aqueous solution. Every oxidation state will not be examined in detail, but the emf data to make such an evaluation will be presented in the form of a Latimer diagram. 

Other applications of emfs include the prediction of thermodynamically possible redox reactions [e.g., will Sn4+oxidize Fe2 to Fe3+ ] and the stabilization of oxidation states through the formation of complexes. The former is a straightforward application of thermodynamics and will not be discussed further here. The second is of great importance. It was introduced in Chapter 11 and will be discussed further below. 

The highest oxidation state is listed on the left and the reduction emfs are fisted between each species and the next reduced form, with the lowest oxidation state appearing on the right.9... 

Thermoelectric power measurements indicated that all the compounds are p-type semiconductors and thermo - emf increases with an increase in Zn2+ content (Table 1). In this system Zn + ions with stable oxidation state occupy A sites. With the introduction of Zn + in the lattice the concentration of Cu occupying octahedral site decreases, however the concentration of Mn ions at the octahedral sites of the series Cuj. Zn MnCr04 remains same. Therefore, with the increase in value of Zn2+, the number of ion pairs of Mn3+ - Mn + start decreasing thereby increasing the resistivity of Zn rich compositions. Seebeck coefficient ( a ) varied between +26 to +66 pV/K which shows with increase in zinc contents of the spinel, the Seebeck coefficient increases. 

Because vanadium ions exist in four different oxidation states (as V2+, V3+, V02+ and V02+) in aqueous solution, redox couples can be formed by all vanadium ions. The emf of a vanadium redox battery is 1.4 V and the electrode kinetics are higher than those of the Fe-Cr battery. Also, the energy density of the battery can be increased due to the high solubility of vanadium salts.257 In the battery, vanadium sulfate solution is used,... 

Reversible potentials of partially charged a- and P-Ni(OH)2 electrodes have been measured. Experimental emf-oxidation state measurements for both the P P- and the a y-phase couples agree well with theoretically derived expressions. From considerations of the homogeneous emf-composition regions it is deduced that the oxidised species are dissociated in both the p P- and the a y-phase systems. 

However, all of the emf results by Chiotti and Dock [1975CH1/DOC], Yoshida et al. [1967YOS/OYA], and Srinivasan and Flengas [1964SR1/FLE] failed to identify lower oxidation states of thorium in the Th-ThCLt system. The voltametric determina-tiorrs by Martinot [1986MAR4] also showed no evidence of the existence of Th(ll) in the eutectic system, even as an intermediate species. 

The solvent polymeric membranes used for EMF measurements typic ly consisted of 1 wt.% metalloporphyrin, 33 wt. % poly-(vinyl chloride) (PVC) and 66 wt % plasticizer (dibutylsebacate). If the porphyrin complex central metal oxidation state was (-I- 2) or (-t- 4), addition of lipophilic cationic sites or anionic sites respectively was required to achieve optimal anion response and selectivity. Tridodecyl-methylammonium cation (TDMAC) or teti s[bis (3,5-trifluoromethyl) phenyl] anion (at a concentration 10-20% mol relative to the metalloporphyrin) are used for this purpose. The total weight of the components varies according to the desired memln ane thickness (typically 150-250 pm) and diameter (7-10 mm). 

A rich series of M Cg2 EMFs has been X-ray characterized (M = Sc [13], Y [14], La [15a-g], Ce [16], Sm [17], Gd [18], and Dy [19]). We will limit our discussion to those derivatives that display well-defined electrochemical responses. It must be premitted that the oxidation state of the encaged metal in EMFs of Y, La, Ce, Gd, and Dy is commonly assumed as -1-3, whereas in EMFs of Sm is -f2. In reality, as we will note briefly, in some cases such assumption are controversial. 

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