Polyborate species

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. 

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. 

Vibrational spectroscopy has proved to be the most useful technique yet for identifying which boron species are present in solution. Close agreement between solution spectra and those of crystalline borates of known structure have confirmed the presence of hitherto postulated polyborate ions. Details of the IR spectra of the NajO-BaOs-H O system at 26°C with absorptions assigned to polyborate species are shown in Table III (416, 417). In a more recent study (126), the major ions in 0.5 M boron solution were identified as BsOefOH), B405(0H)4 , and B(OH)4-. 

The interaction between sodium and alkaline-earth metal ions and borate has attracted recent attention, particularly from the point of view of association of ions in seawater. Several studies (69, 114,168, 169, 340) have shown that the boron content of seawater (4-5 x 10 4 M) is too low to support appreciable concentrations of polyborate species. The increase in acidity of boric acid in the presence of metal ions results from ion-pair formation ... 

Boric acid is a relatively weak acid compared to other conunon acids, as illustrated by the acid equilibrium constants given in Table 4. Boric acid has a similar acid strength to sihcic acid. Calculated pH values based on the boric acid equihbrium constant are significantly higher than those observed experimentally. This anomaly has been attributed to secondary equilibria between B(OH)3, B(OH)4, and polyborate species. Interestingly, the aqueous solubihty of boric acid can be increased by the addition of salts such as potassium chloride and sodium sulfate, but decreased by the addition of others salts, such as the chlorides of lithium and sodium. Basic anions and other nucleophiles such as fluorides and borates significantly increase boric acid solubility. 

The similar shapes of the envelopes of vibrations associated with trigonal (1300-1500-cm ) and tetrahedral (850-1100-cm ) borons in partially hydrolyzed lithium tetraborate (Fig. 37b) and the 120-72 hour product species (Fig. 37c) indicate that the polyborate species formed during the latter stages of aging are structurally similar to partially hydrolyzed lithium tetraborate. The strong bands at about 930 and 1175 cm , present only in the partially hydrolyzed lithium tetraborate spectrum, were assigned to B-O-H out-of-plane and in-plane bending, respectively, on the basis of D2O and H2 0 hydrolyses [185]. 

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). 

Of the experimental techniques employed for studying polyborate equilibria, potentiometry has received the greatest attention although incorrect interpretation of the experimental data often led to the postulation of conflicting species. However, vibrational spectroscopy has proved to be a most useful technqiue for identifying the various species in solution.117... 

The pK value for this dissociation is about 9.14. Reports of lower values (15) may result from the formation of polyborate ions or from the presence of chloride ions. If a knowledge of Ka is important in a particular study, the possible effect of chloride ion should be investigated before it is introduced into the system. Dilute boric acid solutions probably contain only two boroxy species, trigonal boric acid and tetrahedral borate anion. 

The effect of calcium ions on polyborate equilibria is significant, but it is only recently that the detailed analysis of the ionic species in the Ca0-B20j-H20 system has been made (133, 377). 

Probably the most neglected field is that of metal borate complexa-tion in solution. Early proposals of ionic species were based on rather dubious evidence, and a great deal of new experimental work is required. With more sophisticated spectroscopic instruments becoming available, both this phenomenon and the related topic of polyborate ions in solution will be easier to observe. 

The structures of hydrated borates and polyborates (Chapt. 1) are principially different from those of anhydrous species (Chapt. 2), although there are transitions between both the groups. These latter arise if the borates are not fully hydrated, or, for example, if the BO3 group is associated with isolated OH groups (Chapt. 2.2.2), but also for boracites containing OH groups (Chapt. 2.8). 

The structures of the following hydrated borates and polyborates are unknown but X-ray powder data on the species have been reported ... 

Although B(0H)3 and B(OH)4 are monomeric in dilute solutions, at concentrations above about O.lmolL, condensed borate species that are often referred to as polyborates form. Titration of a boric acid solution with one molar equivalent of a strong base leads to formation of the tetrahydroxyborate anion, B(OH)4, as the principal species in solution. Mixtures of boric acid and its conjugate base, the tetrahydroxyborate anion, form what appears to be a classical buffer system where the pH is determined primarily by the acid salt ratio with [H+] = K[B(OH)3]/[B(OH)4 ]. This relationship is approximately correct for sodium and potassium borates with a sodium boron ratio of 1 2. Here the B(0H)3 B(0H)4 ratio equals one, and the solution pH remains near 9 over a wide range. However, for borate solutions with pH values significantly above or below 9,... 

It is important to note that the presence of polyborate anions, as shown in Fignre 2, is only significant in relatively concentrated solutions. This is relevant to the enviromnental and biological aspects of boron chemistry, since boron is present in both natural waters and biological systems at low concentrations. Under these conditions, only B(OH)3 and B(0H)4 are significant species and at near neutral pH, the concentration of the B(OH)4 anion is minor. Ocean waters contain an average boron concentration of about 4.6 g p,g g , which is almost entirely present in the form of naturally occurring boric acid. Boron in plants and animals is also present mainly as boric acid, even if the dietary source of boron is a borate salt. 

There is an extensive and complex structural chemistry of boron-oxygen anionic species (polyborates), in aqueous or nonaqueous solution and in the melt or solid state, Six-membered ring formation dominates, but the structural chemistry of the species is complicated since boron exists in either 3- or 4-coordinate environments, or various combinations of these. Thus most polymeric species consist of (6—0)3 rings joined by boron atoms linked to an intervening oxygen atom, or y rings sharing a common boron atom. 

In aqueous solution polyborates exist in complex equilibria among species of varying degrees of oligomerization depending upon pH and concentration -. At low pH molecular 6(OH)3 predominates. An increase in pH effects an increase in the tetrahedral-total boron ratio to the upper limit of 1 in [6(OH)4] , exemplified by the intermediate species ... 

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