Thallium, reduction potentials

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... 

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 solids such as TlBr2, the valence state of thallium is +2, judging from the stoichiometry. Reduction potentials do not favor the conclusion that this valence state exists in the ground state, however, so disproportionation into T1+ and TP+ must take place. This is confirmed by alternating TIO distances. In fact, two different valence states appear in a number of cases with TP+, for example, in TIS. A spectrum that can be explained by Figure 10.18 can be seen and corresponds to T1 TP+ T1 T12+. 

Recall from the discussion in Chapter 12 (pp. 310-311) that these half-reactions do not stand alone but rather must be measured versus another half-reaction such as the standard hydrogen electrode. (See Problems 14.24—14.26 for additional insight into the utility of the standard reduction potentials in thallium chemistry.)... 

B Use the standard reduction potential for thallium given in Table 14.2 for... 

Colloids of more electronegative metals such as cadmium and thallium also act as catalysts for the reduction of water. In the colloidal solution of such a metal, an appreciable concentration of metal ions is present. The transferred electrons are first used to reduce the metal ions, thus bringing the Fermi potential of the colloidal particles to more negative values. After all the metal ions have been reduced, excess electrons are stored as in the case of silver. 

For practical applications it is important to minimize the production of the intermediate peroxide, and to ensure that the reaction goes all the way to water. Sometimes this can be ensured by the addition of a suitable catalyst. A case in point is oxygen reduction on gold from alkaline solutions. At low and intermediate overpotentials the reaction produces only peroxide in a two-electron process at high overpotentials the peroxide is reduced further to water. The addition of a small amount of Tl+ ions to the solution catalyzes the reaction at low overpotentials, and makes it proceed to water. Thallium forms a upd layer at these potentials it seems that a surface only partially covered with T1 is a good catalyst, but the details are not understood [3]. 

For secondary alkyl iodides, the two one-electron polarographic waves are more separated. Reduction of 2-iodooctane at the potential of the first wave alfords the dialkylmercury and 7,8-dimethyl-tetradecane by reactions of the sec-octyl radical. At the potential of the second wave only octane and octenes are isolated [37]. 2-Bromooctane shows only one polarographic wave and yields octane and octene on reduction at any potential [37]. Radicals generated by reduction of primary and secondary iodoalkanes will react with other cathode materials including tin, lead and thallium to form metal alkyls [48,49],... 

Dropping indium and thallium amalgam electrodes [41] were used to determine kinetic parameters of Zn(II) reduction as a function of the amalgam composition. The formal potentials were shifted to more negative values with increasing thallium and indium amalgam concentrations. 

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. 

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... 

The half-wave potential for the reduction of lead in acid (HCl) solution is about —0.49 V vs SCE. Thallium(I) and tin(II) have polarographic waves close to this. The lead wave can be distinguished from the thallium wave by a plot of log (id — )/ vs E. This gives a slope of 0.059/n volt and for lead n will be equal to 2 while for thallium it will be unity. Alternatively, the waves can be separated chemically. 

The potential of the thallium reduction is practically unaflEected by chelating agents. The lead wave is shifted to —0.80 V in sodium hydroxide solution. Tin (II) is reduced at —1.26 V in this medium. Alkaline tartrate is often used as the supporting electrolyte. In this medium, lead is reduced at —0.79 V, thallium at —0.50 V, and tin (II) at —1.20 V. In tartaric acid medium, the half-wave potentials are —0.52, 0.49, and —0.68 V, respectively. 

VS. SCE in 0.1 M hydrochloric acid. The polarographic reduction waves of thallium (I) and lead (II) in molar hydrochloric acid overlap so closely that direct coulometric separation is not convenient, but Meites (227) ingeniously combined polarographic and controlled-potential coulometric techniques to yield two sets of data which can be resolved to the individual concentrations of the two components through simultaneous equations. 

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