Cerium stability constants

An opportunity to use the thermodynamic cycle shown in Fig. 7 was provided by the requirement to estimate stability constants for cerium(IV) complexes of a series of hydroxypyridinones. As stability constants for their cerium(III) analogues had been measured and F °(Ce /Ce ) values established, stability constants for one bidentate and two tetradentate 3-hydroxy-2-pyridinones could be obtained. Log P4 for the former was calculated to be 40.9, log P2 for the complexes of the tetradentate ligands 40.6 and 41.9. These very high values, expected for a 4+ cation, are paralleled by high pCe values between 37 and 38 for the tetradentate ligands (147). 

Thiophen Derivatives of Analytical Interest.—2-Thenoyltrifluoroacetone has maintained its position as a chelating agent in analytical chemistry. Papers describing its use in the extraction or determination of thorium, copper, europium, thallium, niobium, and molybdenum have appeared. The effect of copper(n) on the formation of monothenoyltri-fluoroacetonatoiron(iii) has been studied. The stability constants of some bivalent metal chelates of di-(2-thenoyl)methane have been determined. 3-Thianaphthenoyltrifluoroacetone has been proposed as a reagent for the spectrophotometric determination of iron(iii) and cerium(iv). The stabilities of metal chelates formed from derivatives of thiophen-2-aldehyde and of rare-earth carboxylates of thiophen-2-carboxylate have been studied. 

Hydrolysis studies of the tetravalent cations are limited to Ce and the light (Pa, Th, U, and Pu) actinides (tables 8B and 9). Only in strong acid media do the tetravalent ions exist free of hydrolysis. Similarly, only at very low concentrations are mononuclear hydroxides of significance. For Ce(I V), stability constants have been determined for bi-, ter-, tetra-, and dodeca-cerium units and for species with molecular weights as large as 40,000 (Hardwick and Robertson 1951, Danes 1967, Louwrier and Steemers 1976). X-ray measurements of thorium and uranium solutions show polynuelear complexes built from... 

Leal RS (1959) Composition and stability constant of cerium and ammonium citrate at alkaline pH. J Inorg Nucl Chem 10 159-161... 

Equation (3.8) can, however, be easily extended to include polymeric species. The same technique can then be utilised to derive any hydrolysis species stability constant from the average ligand number formulation. Where polymeric species form, the average ligand number is dependent upon the metal concentration utilised in each experiment as is shown in Figure 3.4. These data have been obtained for the hydrolysis of cerium(IV). When polymeric hydrolysis species are to be determined, it is necessary to perform a series of experiments at different total metal concentrations. In this case, it is necessary to assume the stoichiometry of the species likely to form, and typically, a computer program is then used to investigate which of the proposed species, and their associated... 

Kragten and Decnop-Weever (1978) studied the solubility of Ce(OH)3(s) and in their experiments estimated the stability constants of the first three monomeric species of cerium(III). Their data for both the solubility and stability constants are not consistent with other available data and are not retained by this review. Sarma... 

A number of studies of the hydrolytic reactions of cerium(III) have reported stabihty constants for Ce(OH)3(aq) (Fatin-Rouge and Biinzh, 1999 Bentouhami et al., 2004 Teksoz, Acar and Onak, 2009). None of the constants given are consistent with the stability constant selected in this review for CeOH and, as such, are not retained. This is also the case for the constant derived by Bilal and Koss (1981) for Ce(OH)2 and, as such, this datum is also not retained (this will also be the case for other data reported by Bilal and Koss). 

Cerium(IV) is a strong oxidising agent and, as such, it has a number of chemical applications as an oxidant. In studying the hydrolysis of cerium(IV), a number of studies have utilised oxidation-reduction reactions in determining the relevant stability constants. As a tetravalent cation with a relatively small ionic radius, the hydrolysis of cerium(IV) occurs at very low pH. Consequently, data have been provided that show that the metal ion is extensively hydrolysed even at a pH of around 0 (i.e. Imoll" H" ) (Baes and Mesmer, 1976). Cerium(IV) is used as an analogue for plutonium(IV) in nuclear fuel manufacturing studies. 

There are a number of studies that have obtained data for the second stepwise stability constant of cerium(IV) at 25 "C and in perchloric acid media, that is, for the formation of Ce(OH)2 from CeOH (Sherrill, King and Spooner, 1943 Baker, Newton and Kahn, 1960 Offner and Skoog, 1966 Everett and Skoog, 1971). The stepwise stability constant determined for zero ionic strength is given in Section 8.4.3. This constant can be combined with the first stepwise constant given earlier to determine a stability constant for reaction (2.5) M = Ce, p= 1, q = 2). The constant derived is... 

There have been a number of studies that have given stability constants for the hydrolysis species of cerium(lV). There is ample evidence available in the literature that cerium(lV) forms both monomeric and polymeric hydrolysis species. The data available are listed in Table 8.55. 

The data given in Table 8.55 are only for perchlorate media. There is also some data reportedin nitrate media. Wadhawan, Sankhla and Mehrotra (1973) reported a stability constant (log j) for CeOH of 0.99 in 1 moll HNOj. This value appears to be in reasonable agreement with that given by Everett and Skoog (1971) in perchloric acid at the same ionic strength and temperature. The datum of Wadhawan et al. is retained in the present review. Danesi (1966) studied the hydrolysis of cerium(IV) in a 3moll (H,Na)N03 medium and at 25 C. Only polymeric species were postulated, with evidence provided for the formation of Ce2(OH)3, Ce2(OH) and Ceg(OH)22. There is no confirmation of the formation of these species elsewhere in the literature, and these species have not been proven to form for other tetravalent cations. As such, they are not retained in this review. 

This solubility is very similar to that retained for cerium(IV). Based on the assumption that the magnitude of solubility and stability constants for a metal ion is related (Baes and Mesmer, 1976), it is likely that the hydrolysis of lead(IV) occurs at very low pH and that the magnitude of the hydrolysis constants is similar to those of cerium(IV). However, there does not appear to be any reported constants for the cationic hydrolysis species of lead(IV) in the hterature. 

Fig. 7.27. It is seen from Fig. 7.27 that the rate constant for the exchange reaction [Ce. +-Ce(EDTA)-] is much lower than the expected value. It is to be noted that the crystal field stabilization energy including spin-orbit interaction reaches a maximum for cerium in the cerium sub group and the ligand field strength is in the order H2O < CH3COO. The...

The TPR profiles for supported ceria present two to three poorly resolved peaks, in the so-called low temperature domain (600-950 K), and a slowly decreasing consumption of hydrogen after stabilization of the temperature at 1073 K. The low temperature H2 consumption phenomenon increases as the cerium content in the samples increases, but shows also that more than one chemical species is concerned. Since, the temperature for the begiiming of this first phenomenon is rather constant, at ca. 600 K, the low temperature reduction peaks must always deal with the same type of cerium species. 

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