Hydrolysis beryllium

The many attempts to quantify beryllium hydrolysis are summarized in Table III. 

The first modern study was performed by Kakihana and Sillen (54). These authors studied an extensive range of solutions in the pH range from the onset of hydrolysis to the onset of precipitation. They made a convincing case for the formation of the trimer, Be3(OH)j]+, as the principal product of hydrolysis, with the species Be2(OH)3+ being formed to a minor extent under the experimental conditions used. They also found that some Be(OH)2 was present in solution before precipitation had occurred. This model was subsequently confirmed by various authors (57-65, 74, 79, 86). The same model was found to 

Subsequent studies showed that other species are formed to a small extent just before precipitation begins these species have a hydroxide to beryllium ratio greater than 1, but because their concentrations are always relatively low their identification has been dogged by controversy. Mesmer and Baes obtained data at a range of temperatures and from this data proposed the minor species Be5(OH)7+ (66). Later they also suggested that the species Be6(OH)3+ was formed (10), in agreement with earlier work by Lanza and Carpeni (69, 72). These last had also proposed the additional species Be3(OH)4+ and Be6(OH)ir. 

From a structural point of view Bruno s hypothesis is sound. In chemical terms the hydrolysis proceeds with the formation of hydroxide bridges up to the formation of the trimer. Then at high concentrations trimeric units are fused together to form the higher aggregates, that can be seen as precursors to the structure adopted by Be(OH)2 in the solid state. (See Section IV, C, precipitation equilibria). Speciation diagrams for Bruno s model are shown in Fig. 3. Two points are noteworthy in these diagrams. In the first place the concentrations of the 

1 Hydrogen-ion concentration measurements cannot distinguish between a bis-hydroxy species and a mono-oxy species because the equilibrium 2 OH — 02 + H20 

ALDERIGHI, CANS, MIDOLLINI, AND VACCA TABLE III Continued) 

NMR spectroscopy has been a useful tool for confirming the model deduced by potentiometry. Akitt et al. (27, 28) have characterized the species [Be2(0H)(H20)e] and [Be3(0H)3(H20)e]. Very concentrated solutions were prepared by dissolving a known weight of Na2C03 in a certain volume of concentrated BeCb solution, and spectra were recorded at temperatures between —45 and — 55°C. The ratio [H20]/[Be ] varied between 18.47 and 22.7, which corresponds 

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Carell and Olin (58) were the first to derive thermodynamic functions relating to beryllium hydrolysis. They determined the enthalpy and entropy of formation of the species Be2(OH)3+ and Be3(OH)3+. Subsequently, Mesmer and Baes determined the enthalpies for these two species from the temperature variation of the respective equilibrium constants. They also determined a value for the species Be5(OH) + (66). Ishiguro and Ohtaki measured the enthalpies of formation of Be2(OH)3+ and Be3(OH)3+ calorimetrically in solution in water and water/dioxan mixtures (99). The agreement between the values is satisfactory considering the fact that they were obtained with different chemical models and ionic media. 

C. Notation Properties of Beryllium The Beryllium Ion Beryllium Hydrolysis... 

Figure 7.1 Structures of the beryllium hydrolysis species. The open bonds are in the plane of the page, the closed bonds come out of the page, and the hatched bonds go into the page. Each coloured area denotes a separate species.

In comparison with other metal ions, the stability of BeOH is relatively weak at 25 °C. Polymeric species predominate in acidic solutions at beryllium concentrations above 10 molkg . Consequently, few studies of beryllium hydrolysis have been able to measure the stabUity of BeOH at this temperature. At higher temperatures, the monomers become more stable than the polymers, and, as a result, only monomers have been detected at high temperatures. 

The predominant polymeric beryllium hydrolysis species that forms is Be3(OH)3 . The formation of the species has been confirmed by nuclear magnetic resonance studies (Akitt and Duncan, 1980 Akitt and Elders, 1988) and has also been identified in the solid state (Cecconi et al., 1998). The beryllium ion is tetrahedral and contains four waters of hydration, and in the Be3(OH)3 species each beryllium atom is bound to two hydroxide molecules with the other two tetrahedral positions occupied by water molecules (see Figure 7.1). Thus, the complete formula of the species is Be3(0H)3(H20)g with the three beryllium and three hydroxide ions forming a six-membered ring that has a... 

Soboleva etal. (1977) also provided data on the solubility of bromellite (BeO(s)). They studied the solubility of this phase over the temperature range of 150-250 °C but provided thermodynamic data over the larger range of 25-300 C. From the solubility study, they provided data for the stability constants of the beryllium hydrolysis species, BeOH", Be(OH)2(aq) and Be(OH)3 , as well as that of the... 

Data reported for the stability constant of polymeric beryllium hydrolysis species are listed in Table 7.4. For the majority of species, the data cover a relatively narrow range of temperature. Data have also been acquired for the enthalpy of reaction for all of the polymeric species that are believed to form. The accepted species include Be20H , Be3(OH)3 ", Beg(OH)g and Beg(OH)g. ... 

There have been three studies that have used calorimetry to determine the enthalpies of reaction of the polymeric beryllium hydrolysis species (Carell and Olin, 1962 Ishiguro and Ohtaki, 1979 Alderighi et al., 1998). In addition, Mesmer and Baes (1967) measured the stability constants of these species over the temperature range of 0-60 "C from which the enthalpies of reaction were deduced. Finally, Ciavatta and Grimaldi (1973) determined the polymeric stability constants at 60 "C and in 3 moll NaClO. The stability constant values obtained in their study can be compared with those obtained at 25 "C in 3 mol 1" NaClO and the enthalpy data of Carell and Olin (1962) acquired under the same conditions. These data are presented in Table 7.6. 

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