Aqueous reduction potentials

Reducibility of the Cations. The removal of lattice oxygen from the active site is accompanied by the reduction of the neighboring cation. Thus another way to view the ease of removal of lattice oxygen is to look at the reduction potential of the cations at the active site. This concept was tested using a series of orthovanadates of the formula M3(V04)2, where M = Mg, Zn, Ni, and Cu, and MVO4, where M = Fe, Sm, Nd, and Eu. The cations in these two series were chosen to span a range of (aqueous) reduction potentials from -2.40 V to +0.77 V. Orthovanadates are made up of isolated VO4 units that are separated from each other by MO units. Thus there are only M-O-V bonds in these structures and no V-O-V bonds, and the difference in the ease of removal of a lattice oxygen should depend on the difference in the reduction potential of the M ion. 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

Table 6 Standard aqueous reduction potentials for protons, water, and oxygen species and the free energy of bond formation for several hydrogen - oxygen species...

The electrochemical potential for redox reaction controls the situation where atoms of one element are available to be sorbed by a zeolite containing exchangeable cations of another element. Within the zeolite and even in the absence of water, aqueous reduction potentials are usually capable of deciding whether reaction will occur, with an error due to the difference between the zeolitic environment and aqueous solution of no more than 0.1 (or perhaps 0.2) V. Accordingly there is no question that alkali-metal vapors will reduce transition-metal ions within a zeolite, and that vapors of zinc, mercury, or sulfur will not reduce the cations of the alkali or alkaline-earth metals. 

Table I. Aqueous Reduction Potentials for Reductants and Oxidants...

Standard Aqueous Reduction Potentials in Aqueous Solution at 2S C... 

Table 3-7 Standard Aqueous Reduction Potentials for Protons, Water, and Oxygen Species and the Free Energy of Bond Formation (-AGbf) for Several Hydrogen-Oxygen Species...

Enthalpies of formation of several rare-earth dichlorides (Morss and Fahey 1976) and of dysprosium diiodide (Morss and Spence 1992) have been published and have been used to calculate the aqueous reduction potentials (R /R ) see section 4.5 for details. Others (Kim and Oishi 1979, Johnson and Corbett 1970, Johnson 1974, 1977) have calculated enthalpies of formation and used them as part of a cycle to calculate aquo-ion properties see section 3.3 for details. 

Aqueous reduction potentials °(M /M ) have been assessed from thermodynamic measurements and estimates as described in section 2.4.2. Among the actinides, dihalides of Am, Cf, and Es have been characterized but no thermochemical measurements have been made. The synthesis conditions are consistent with the E° An /An ) values (Martinet and Fuger 1985). Within these limitations we compare the °(Ln /Ln ) and °(An /An " ) in fig. 9. Similarly, some °(M /M ) have been measured electrochemically and others estimated using thermochemical or spectroscopic measurements. We compare ° (Ln Ln ) with ° (An /An ) in fig. 10. Note that E° refers to a formal potential (as measured in 1 mol dm acid solution, HCIO4 if available) rather than a standard potential (as... 

Fig. 5a. Standard (or formal) reduction potentials of actinium and the actinide ions in acidic (pH 0) and basic (pH 14) aqueous solutions (values are in volts...

Ozone can be analyzed by titrimetry, direct and colorimetric spectrometry, amperometry, oxidation—reduction potential (ORP), chemiluminescence, calorimetry, thermal conductivity, and isothermal pressure change on decomposition. The last three methods ate not frequently employed. Proper measurement of ozone in water requites an awareness of its reactivity, instabiUty, volatility, and the potential effect of interfering substances. To eliminate interferences, ozone sometimes is sparged out of solution by using an inert gas for analysis in the gas phase or on reabsorption in a clean solution. Historically, the most common analytical procedure has been the iodometric method in which gaseous ozone is absorbed by aqueous KI. 

The peroxodisulfate ion in aqueous solution is one of the strongest oxidising agents known. The standard oxidation—reduction potential for the following reaction is 2.08 V (77,78). 

The chemistry of plutonium ions in solution has been thoroughly studied and reviewed (30,94—97). Thermodynamic properties of aqueous ions of Pu are given in Table 8 and in the Uterature (64—66). The formal reduction potentials in aqueous solutions of 1 Af HCIO or KOH at 25°C maybe summarized as follows (66,86,98—100) ... 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

Reactions in Water. The ionization potential for bromine is 11.8 eV and the electron affinity is 3.78 eV. The heat of dissociation of the Br2 molecule is 192 kj (46 kcal). The reduction potentials for bromine and oxybromide anions in aqueous acid solutions at 25°C are (21) ... 

The standard reduction potential of Cr " (Table 2) shows that this ion is a strong reducing agent, and Cr(II) compounds have been used as reagents in analytical chemistry procedures (26). The reduction potential also explains why Cr(II) compounds are unstable in aqueous solutions. In the presence of air, the oxidation to Cr(III) occurs by reaction with oxygen. However, Cr(II) also reacts with water in deoxygenated solutions, depending on acidity and the anion present, to produce H2 and Cr(III) (27,28). 

Table 5.1 lists some of the atomic properties of the Group 2 elements. Comparison with the data for Group 1 elements (p. 75) shows the substantial increase in the ionization energies this is related to their smaller size and higher nuclear charge, and is particularly notable for Be. Indeed, the ionic radius of Be is purely a notional figure since no compounds are known in which uncoordinated Be has a 2- - charge. In aqueous solutions the reduction potential of... 

TII3 is an intriguing compound which is isomorphous with NH4I3 and Csly (p. 836) it therefore contains the linear I3 ion and is a compound of Tl rather than Tl . It is obtained as black crystals by evaporating an equimolar solution of Til and I2 in concentrated aqueous HI. The formulation Tl (l3 ) rather than Tl (I )y is consistent with the standard reduction potentials °(T1"VT1 )1.26 V and °(il2/I )-(-0.54 V,... 

Table 11.4 Standard reduction potentials for nitrogen species in acidic aqueous solution (pH 0, 25°C)...

When an element can exist in several oxidation staie.s it is sometimes convenient to display the various reduction potentials diagramaltcally. the corresponding half-reactions under standard conditioas being implied. Thus, in acidic aqueous soiultons... 

The aqueous solution chemistiy of nitrous acid and nitrites has been extensively studied. Some reduction potentials involving these species are given in Table 11.4 (p. 434) and these form a useful summaiy of their redox reactions. Nitrites are quantitatively oxidized to nitrate by permanganate and this reaction is used in titrimetric analysis. Nitrites (and HNO2) are readily reduced to NO and N2O with SO2, to H2N2O2 with Sn(II), and to NH3 with H2S. Hydrazinium salts yield azides (p. 432) which can then react with further HNO2 ... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Figure 14.12 Variation of the reduction potentials of the couples O2/H2O and H /Hi (or O2/OH" and H2/H2O) as a function of pH (full lines). The broken lines lie 0.5 V above and below these full lines and give the approximate practical limits of oxidants and reductants in aqueous solution beyond which the solvent itself is oxidized to 02(g) or reduced to Hi(g).

In H2O2 the oxidation state of oxygen is —1, intermediate between the values for O2 and H2O, and, as indicated by the reduction potentials on p. 628, aqueous solutions of H2O2 should spontaneously disproportionate. For the pure... 

Anhydrous NaC102 crystallizes from aqueous solutions above 37.4° but below this temperature the trihydrate is obtained. The commercial product contains about 80% NaC102. The anhydrous salt forms colourless deliquescent crystals which decompose when heated to 175-200° the reaction is predominantly a disproportionation to C103 and Cl but about 5% of molecular O2 is also released (based on the C102 consumed). Neutral and alkaline aqueous solutions of NaC102 are stable at room temperature (despite their thermodynamic instability towards disproportionation as evidenced by the reduction potentials on p. 854). This is a kinetic activation-energy effect and, when the solutions are heated near to boiling, slow disproportionation occurs ... 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

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