Thallium redox potentials

Mercury(II), thallium(III), and lead(IV) are isoelectronic and, as can be seen from the data in Eqs. (19)-(22) (77) the redox potential for thallium is intermediate between those of mercury and lead. Consequently, the relative oxidizing ability of the three metal ions should be in the order Hg(II) <... 

The Tl -Tl relationship is therefore a dominant feature of thallium chemistry. The standard reduction potentials at 25 °C and unit activity of H+ are TIVtI = —0.336 V, T1 /T1 = +0.72 V, and Tl /Tli = +1.25V. Estimates have also been made for the couples T1 /T1 = +0.33 V and Tl /Tl = 2.22 V. The generally valid limitations concerning the use of standard electrode potentials to predict the redox chemistry of real systems are especially important in the case of thallium factors such as complex formation in the presence of coordinating anions or neutral ligands and pH dependence due to hydrolysis do affect the actual or formal redox potentials. For example, redox potentials have been measmed for TICI/TICI3 =+0.77 V in IM HCl and T10H/T1(0H)3 = —0.05 V in alkaline soluhon. These formal potentials differ from the standard value for Tiin/Tii = +1.25 V. The difference can be attributed to the substanhal difference between the complex forming abilities of Tl and Tl , which will be discussed in detail later. The... 

In contrast to the other elements of group 13, thallium is considered a soft acid in Pearson s soft/hard acid classification (see Hard Soft Acids and Bases).This makes the element and its derivatives unique, and leads to its potentially most outstanding feature the properties of thallium are a subtle blend of some of the most desirable properties of numerous other metals (e.g. heavy alkali metals, silver, mercury, and lead). Thallium compounds are stable in both oxidation states (-1-1 and -1-3). The trivalent cation is quite a strong oxidation reagent, since it is reduced to T1+ easily (standard redox potential E°(TP+ — Tl" ") = -1.25 V). The ease of this reduction is utilized in certain organic reactions. 

In contrast to the general assumption in the literature 15), based on the redox potentials of the TP /TF and (CN)2/CN couples (see discussion in Section IV,B), it has been recently shown that thallium-... 

Complete oxidation of thallium(I) usually took about 20 minutes. The solution, which contained 4 10" M T1(I) and more than 3 M Cl , was saturated in oxygen In solutions deoxygenated by blowing nitrogen gas for one hour, no oxidation of thallium(I) could be observed. Chloride ions are necessary for the reaction which is assisted by Tl(III) the redox potential of the thallium(III)/thallium(I) couple is reduced at high chloride concentration, and this explains why the oxidation does not occur in the absence of chloride. 

Similar observations on the oxidation of the thallium atom or on the reduction of T1+ have been made by pulse radiolysis. They are in agreement, as for silver, with the value determined from the electrode potential and the sublimation energy of the bulk metal into atoms, i.e. °(T1 /T1 ) = —1.9 Vnhe-Silver ions complexed by cyanide, ammonia, or EDTA, Ag L, are not reduced by the radical (CH3)2C OH, even under basic conditions, and the redox potential of these complexed forms must be more negative than —2.1 According... 

Although the redox potentials favour electron transfer between cerium(iv) and thallium(i), the reaction is very slow. Catalysis by osmium(vm) has, however, been observed in both sulphate and perchloric acid media. In the latter system, although perchlorate complexing is not completely ruled out, the reactive species is considered to be the hydrolysed cerium(iv) ion, the mechanism... 

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

We will use the example of thallium ion. The potential of the working electrode will be stepped from a potential at which only Tl" " is the stable form to a potential at which only is the stable form. Figure 6.2(a) shows a plot of potential against time - note that the rise in potential here is essentially vertical. It would be completely vertical but for the requirement to charge the doublelayer around the electrode. The potential before the step is, e.g. 0 V, i.e. it is well cathodic (negative) of, .,+(= 1.252 V), so Tl is the only stable redox form, and no Tl " " will form. Thie potential after the step is, e.g. 1.6 V, i.e. it is well anodic of 3+. p,+, so Tl is the only stable redox form here, thus causing Tl" to oxidize to Tl +. 

Thallium (Tl), which appears to exhibit conservative behaviour in seawater, has two potential oxidation states. As Tl1, thallium is very weakly complexed in solution. In contrast, Tl111 should be strongly hydrolysed in solution ([T13+]/[T13+]t — 10 20 5) with Tl(OH)3 as the dominant species over a very wide range of pH. The calculation of Turner et at. (1981) indicated thatTl111 is the thermodynamically favoured oxidation state at pH 8.2. Lower pH and p()2 would be favourable to Tl1 formation. Within the water column, pH can be considerably less than 8.2 and /)( )2 lower than 0.20 atm. In view of these factors, and the observation that redox disequilibrium in seawater is not uncommon, the oxidation state of Tl in seawater is somewhat uncertain. The existence of Tl in solution as Tl+, a very weakly interactive ion, would reasonably explain the conservative behaviour of Tl in seawater. However, the extremely strong solution complexation of Tl3+ suggests that Tl3+ may be substantially less particle reactive than other Group 13 elements (with the exception of boron). 

Fig. 6. Model pH-potential diagram calculated for the thallium oxide-water system with consideration of the existence of the mixed-valence oxide. Equilibrium potentials for the redox systems Tl" /Tl203 (1), Tl" /mixed oxide (2) and mixed oxide/Tl203 (3) are given for 1 mm TI2SO4 solution.

Other Thallium Compounds.—Tl2 ions may be generated by flash photolysis of Tl111 solutions.626 Some of their redox reactions have been studied, and they give values of the standard reduction potentials for the reactions ... 

Propylene was similarly oxidized electrochemically to acetone and glycol in the presence of mercurous and thallium salts 334,335). It is noteworthy that increasing anodic potentials (> +1.8 V) favored cleavage of the propylene chain to yield formic and acetic acid. It is conceivable, then, that unexplored low anodic potentials or electrogenerative oxidation at 0.6-1.0 V could minimize or eliminate these undesirable side reactions. The significance of the nature of the redox couple and its concentration in solution for controlling reaction selectivity has already been emphasized earlier. 

Another type of thallium compounds which have potential applications in converting light into electrochemical energy are the hi- or oligometallics of the type shown in Fig. 19. Similar compounds, such as (CN)5-Fe -(/u-CN)-Pt (NH3)4-(/L-NC)-Fe -(CN)5, have recently been shown to undergo redox reaction when irradiated by visible light (356). Research in this field has been active during the last few years, and... 

In the most sensitive eiectroanaiytical methods, exclusively treated within this context, the analyte ion is electrodeposited on an electrode from an electrically conducting sample solution. Current and potential of subsequent redissolution are due to the concentration and the kind of ion to be determined. For thallium, the reversible redox couple TI /TI° at about -0.5 V versus saturated calomel electrode is used (Bellavance and Miller, 1975). Infinite tolerance towards alkali, alkaline earths and halogenides are great merits for the analysis of biological materials. Because of the preconcentration step included, thallium determination is more sensitive than atomic spectrometric methods. For thallium, the multielement capabilities of the method can hardly be used, because lead and frequently cadmium have to be masked with excess of complexants, leaving just Tl in the potential... 

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