Aqueous boron species

Figure 4. Change in pK values with temperature and salinity (a) and the effect on the isotopic composition of the aqueous boron species (b).

The fundamental chemical principle behind the isotopic offset between the two aqueous boron species relates primarily to vibrational and rotational energy differences between the two isotopes, such that the heavier isotope is preferentially incorporated into the trigonal species. The motivation for studying this fractionation was for the nuclear industry, as has a very high cross section for neutron captme, and so is used as a neutron flux absorber. Mechanisms for enriching from natural boron led to studies that showed it could be separated from "B on ion exchange columns. Theoretical approaches were used to calculate the fractionation factor a, which describes the isotopic offset between two phases. 

Vapor phases ia the B2O3 system iaclude water vapor and B(OH)3(g) at temperatures below 160°C. Appreciable losses of boric acid occur when aqueous solutions are concentrated by boiling (43). At high (600—1000°C) temperatures, HB02(g) is the principal boron species formed by equiUbration of water vapor and molten B2O3 (44). At stiU higher temperatures a trimer (HB02)3(g) (2) is formed. 

Alkoxides are usually more difficult to hydrolyze than halides, although hydrolysis can be rapid in activated systems. Pyrimidinethiones can sometimes be hydrolyzed directly to pyrimidinones, but it is often better to convert the thiones into alkylsulfenyl, alkylsulfmyl, or alkylsulfonyl derivatives before hydrolysis <1994HC(52)1, 1996CHEC-II(6)93>. The formation of 5-hydroxypyrimidines is not normally performed using hydrolytic procedures, although it can be achieved by the oxidation of boronate species in aqueous solution <1996CC2719, 2006TL7363>. 

Equilibrium studies in dilute aqueous solutions cannot alone help to distinguish between species that differ only by varying amounts of water. One may learn, for example, that a boron species exists with one boron atom and no charge, and another with one boron atom and the charge —1. One may write them (because of other evidence) as B(OH) and B(OH)4", but as far as the equilibrium data are concerned what we call [B(OH)4 ] might well be a sum of the concentration of B02, H2BO3, and B(OH)4". This must also be understood when we use conventional formulas for the concentrations of, say H+ and H2CO ... 

The mode of dissolution of metal borates in aqueous solution is complex. Hydrolysis of the borate anion can result in completely different boron species that are stable only under particular conditions of pH, temperature, and concentration. 

Direct activation methods are not normally successful for the oxidation of alcohols using hydrogen peroxide. An advantage of this is that alcohols can often be used as solvents for other oxidations, which do not use direct activation methods. However, hydroxy ketals can be cleaved with aqueous hydrogen peroxide, and the ene-diol of L-ascorbic acid can be oxidized with sodium perborate.185 Under the conditions employed, it is likely that the active oxidant is free hydrogen peroxide rather than any boron species. 

Boron species. Aqueous [CuCL] " and [BH4] form a somewhat stable intermediate, possibly [CuHCl ]" , and H2. 

Boron species. Aqueous [BH4]" reduces Sb completely to stibane, SbHs, perhaps as ... 

Properties. Fluoroboric acid is stable in concentrated solutions, and hydroly2es slowly in aqueous solution to hydroxyduoroborates. For the stabihty of the duoroborate species, see Reference 3. The equiUbrium quotients (4,5) in 1 molal NaCl at 25°C show the strong affinity of boron for duo ride ... 

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