Polyborates

In aqueous solutions of borate at low concentrations the main anionic species are B(0H)4 and B303(0H)4, with higher species likely to be present when the total borate concentration exceeds 0.2 M. With such solutions, temperature-jump studies reveal the presence of two relaxation times longer than 10 sec (at pH 6 and 7.4). One of these is evident in the whole range pH 5-10 and is attributed to reaction (1), for which pK is 6.87, 

Interpretation is limited since these are overall coefficients. Measurements have also been made by ultrasonic absorption and nmr methods. The former gives an interpretation in terms of reaction (2) 

No measurements attributable to (3) appear to have been made. 

Measurements were based on spectrophotometry at 292 m/x (a reactant peak). Reaction leads to formation of B2oHi70H (III). The following rate coefficients were reported, for ionic strength 0.20. 

Energy of activation Entropy of activation 22 1 kcal.mole —7 3 cal.deg .mole 14 2 kcal.mole — 24 5 cal.deg .mole  

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Boron also has a high affinity for oxygen-forming borates, polyborates, borosiUcates, peroxoborates, etc. Boron reacts with water at temperatures above 100°C to form boric acid and other boron compounds (qv). 

The apparent acid strength of boric acid is increased both by strong electrolytes that modify the stmcture and activity of the solvent water and by reagents that form complexes with B(OH) 4 and/or polyborate anions. More than one mechanism may be operative when salts of metal ions are involved. In the presence of excess calcium chloride the strength of boric acid becomes comparable to that of carboxyUc acids, and such solutions maybe titrated using strong base to a sharp phenolphthalein end point. Normally titrations of boric acid are carried out following addition of mannitol or sorbitol, which form stable chelate complexes with B(OH) 4 in a manner typical of polyhydroxy compounds. EquiUbria of the type ... 

Polyborates and pH Behavior. Whereas bode acid is essentiaHy monomeric ia dilute aqueous solutions, polymeric species may form at concentrations above 0.1 M. The conjugate base of bode acid in aqueous systems is the tetrahydroxyborate [15390-83-7] anion sometimes caHed the metaborate anion, B(OH) 4. This species is also the principal anion in solutions of alkaH metal (1 1) borates such as sodium metaborate,... 

This anomalous pH behavior results from the presence of polyborates, which dissociate into B(OH)2 and B(OH) as the solutions are diluted. Below pH of about 9 the solution pH increases on dilution the inverse is tme above pH 9. This is probably because of the combined effects of a shift in the equihbrium concentration of polymeric and monomeric species and their relative acidities. At a Na20 B202 mol ratio equal to 0.41 at pH 8.91, or K20 B202 mol ratio equal to 0.405 at pH 9 the pH is independent of concentration. This ratio and the pH associated with it have been termed the isohydric point of borate solutions (62). 

The presence of metal salts, particularly those containing alkaline-earth cations and/or haUdes, cause some shifts in the polyborate equiUbria. This may result from direct interaction with the boron—oxygen species, or from changes in the activity of the solvent water (63). 

The Polyborate Species. From a series of very rigorous pH studies, a series of equihbrium constants involving the species B(OH)2,... 

A rapid equiUbrium exists among the various polyborate species in aqueous solutions. 

Other Sodium Borates. SOLUBOR, TIM-BOR, or POLYBOR (63), proprietary products of United States Borax Chemical Corp., have the approximate composition of disodium octaborate tetrahydrate, Na20 4320 4H2O. This material is produced by spray-drying mixtures of borax and boric acid. 

Other Sodium Borates. Prices for disodium octaborate tetrahydrate (SOLUBOR, TIM-BOR, POLYBOR), sodium metaborate tetrahydrate, and sodium metaborate dihydrate in 1985 were 924/1, 880/t, and 1034/1, respectively. The corresponding prices in 1990 were 1056/t, 990/t (technical grade), and 1298/t (technical grade), respectively (33). 

Disodium octaborate tetrahydrate (TiM-BOR) is registered for a variety of pests including termites, wood destroying beeties, and carpenter ants. This same compound is also used to control fly larvae in manure piles and is marketed as POLYBOR 3 for this apphcation. 

The general procedure for the preparation of vanadium borates consists in heating a concentrated H2O solution of boric acid and vanadium oxide in an autoclave at 170 °C for several days [143]. Two different vanadium borate clusters 105 and 106 are obtained, one with two polyborate chains coordinated to a contorted vanadium oxide ring (105) and another one with a macrocyclic Bi8036(0H)6 ring (106). The latter ring is composed of six B306(0H) units and has a chair-like conformation (Fig. 27) [143]. 

Heller, G. A Survey of Structural Types of Borates and Polyborates. 131, 39-98 (1985). 

PolyCy-benzyl L-glutamate) (PBLG), 15 109 Poly(y-ketosulfide)s, optically active, 23 711 Poly(P-alanine), 1 292 Poly-P-hydroxybutyrate (PHB), 12 482 Polybetaines, 20 479-482 applications of, 20 482 preparation of, 20 480-481 solution properties of, 20 481-482 synthesis of, 20 479-481 Polyborates, 4 256-258 Polyborosiloxanes, in silicon carbide manufacture and processing, 22 533 Polybrominated diphenyl ethers (PBDEs), 13 142-143 20 56... 

Amongst the hydro-closo-polyborates B H 2- members with n = 6 to 12 are known. All hydro-closo- but also hydro-nido-polyborates act as bases they are, however, not typical Lewis-bases as they miss free electron pairs. However, the negative charge at the hydrogen atoms allows for an interaction with Lewis acids A by formation of hydride bridge bonds (3c2e bonds) ... 

In dilute aqueous solutions, the boric acid molecule remains undissociated B(0H)3 but in concentrated solutions, B(OH)4 and complex polyborate anions, such as B303(0H)4 are formed. Reactions with fluoride ion produce fluoroborates, BF(OH)3, BF2(OH)2, BFsCOH), BF4, and B3O3F63 in stepwise sequence. It forms similar polyions with amides and borates. 

The most important polyborate species observed in solution are the triborate anions [B303(0H)4] and [B303(0H)5] , the pentaborate anion [B506(0H)4] , and the tetraborate anion [B405(0H)4] . The population distributions of these species found at 25 °C in 0.4 molar boric acid equivalent solution as a function of pH is shown in Fig. 2 [8]. Concentrations of these species may vary considerably with temperature and overall boron concentration, and other minor polyborate species not represented are also likely to exist in solution. Fig. 2 should be regarded as a snapshot of a complex system taken under one set of conditions. 

The polyborate oxoanions can be regarded as acid anhydrides, formed by elimination of water following nucleophilic attack of a B-OH oxygen on a trigonal boron center. The polyborate anions contain one or more tetrahedral boron centers and have a negative charge resulting from acceptance of the OH ion that is shared by boron atoms within the polyborate. Polyborate... 

Another example of cation control over borate structure is found in the recently elucidated structure of the industrially important synthetic zinc borate Zn[B304(0H)3] [2 ZnO 3 B3O3 3 H2O], shown in Fig. 12 [45]. This compound has a polytriborate structure reminiscent of that found in colemanite (Fig. 7a) [29]. Yet, due to the different coordination requirements of the Ca and Zn cations, the anionic polyborate chains in these two compounds have different spatial arrangements. In the case of colemanite, parallel triborate chains are linked together into extended sheets by coordination with interstitial (and H2O). These sheets are bound... 

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