Redox potentials thallium ions

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) <... 

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 +. 

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. 

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). 

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 ... 

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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