Boric acid, complex formation with

Buffers of boric acid are useful in the pH range 8.5 to 10. Borate has the major disadvantage of complex formation with many metabolites, especially carbohydrates. 

Initially, BN was synthesized by the pyrolysis of boric acid and urea in the presence of ammonia [27]. In this reaction, boric acid presumably reacts with urea to form a urea-boric acid complex gel, which on pyrolysis in an ammonia atmosphere results in the formation of BN [28]. Precursors for BN recently received the greatest attention, with emphasis on borazine-derived polymers. It has been recognized that borazine (1) can form polymeric chains of B and N, which can form BN ceramics on thermal pyrolysis [29]. The polymerization gives a graphitelike hexagonal layered structure (2) ... 

Addition of four equivalents of boric acid to the starting ketose improves the original 6.5% yield of D-hamamelose up to 20% [55]. Thus, the product of the molybdic acid catalyzed rearrangement, D-hamamelose, is apparently being removed from its thermodynamic equilibrium with D-fructose by a competitive complex formation with boric acid. 

In Reference 108, Wang et al. introduced a di- -amyl L-tartrate-boric acid complex for the enantioseparation of propranolol, sotalol, esmolol, atenolol, bisoprolol, metopro-lol, terbutaline, clenbuterol, cycloclenbuterol, bambuterol, and tulobuterol. Uncoated fused-silica capillaries of 50 p,m i.d. with a total length of 53.0cm and an effective length of 45.0 cm were used. A nonaqueous BGE proved useful for the ion-pair formation and the addition of TEA enhanced the enantiomeric discrimination. When a 100-mM boric acid, 80-niM di-n-amyl L-tartrate, 50-mM TEA in MeOH BGE was used, propranolol, sotalol, esmolol, atenolol, bisoprolol, and metoprolol were baseline separated with resolutions ranging from 2.3 (sotalol) to 3.0 (propranolol). Terbutaline, clenbuterol, cycloclenbuterol, bambuterol, and tulobuterol could be baseline separated with resolutions ranging from 3.2 (bambuterol) to 4.2 (clenbuterol) using a 120-mM boric acid, 100-mM di-n-amyl L-tartrate, 50-mM TEA in MeOH BGE. 

Ellis Wilson (1991, 1992) examined cement formation between a large number of metal oxides and PVPA solutions. They concluded that setting behaviour was to be explained mainly in terms of basicity and reactivity, noting that cements were formed by reactive basic or amphoteric oxides and not by inert or acidic ones (Table 8.3). Using infrared spectroscopy they found that, with one exception, cement formation was associated with salt formation the phosphonic add band at 990 cm diminished as the phosphonate band at 1060 cm" developed. The anomalous result was that the acidic boric oxide formed a cement which, however, was soluble in water. This was the result, not of an add-base readion, but of complex formation. Infrared spectroscopy showed a shift in the P=0 band from 1160 cm" to 1130 cm", indicative of an interaction of the type... 

Figure 17-14. Formation of complexes of boric acid with glycerol. Three hydroxyl units form an ester and one unit forms a complex bond. Here a proton will be released that lowers the pH. The scheme is valid also for polyhydroxy compounds. In this case, two polymer chains are connected via such a link.

It was found in the first place that aliphatic, non-cyclic glycols without adjacent hydroxyl groups have no effect they cannot form cyclic complexes. But also most glycols with not more than two adjacent hydroxyl groups are inactive. I therefore assumed that only if two hydroxyl groups are favorably situated for the formation of a complex boric acid compound, will it be possible to observe an increase in the conductivity. 

Bachelier, N. and J.-F. Verchere, Formation of neutral complexes of boric acid with 1,3-diols in organic solvents and in aqueous solution. Polyhedron, 1995. 14(13) p. 2009-2017. 

We [47] further studied quantitatively the strain-induced complex formation in PVA films in dilute iodine solutions whose iodine concentration is lower than the threshold required for the complex formation. We were interested in the effects of degree of hydration D.H. of PVA films and the iodine concentration of the soaking solutions on the strain-induced complex formation. PVA films were stretched in iodine-KI soaking solutions whose iodine concentration was in the range of 2 x 10 9 x 10 mol/1. No boric acid was added to the solution. No complex forms in these solutions, and therefore films remain brown in color before extension. However, when stretched in the solution the color turns to blue at the points indicated by arrows on the stress-strain curves shown in Fig. 11, which shows the beginning of the formation of the complex. The strain-induced complex formation is also shown by the visible ray absorption spectra measured before and after extension in a solution of 3x 10 mol/1 iodine concentration at 30 °C as shown in Fig. 12. A broad peak of the complex with... 

Borate complexes have been utilized by Brigl and Griiner47 to effect partial esterification. Anhydrous D-glucose and metaboric acid dissolved in acetone give a complex which exhibits the analysis of a diborate. Reaction of the latter with an excess of benzoyl chloride gives 2,6-di-O-benzoyl-D-glucose (XL). D-Mannitol likewise forms a diborate, which produces the 1,6-di-O-benzoyl derivative (XLI) upon benzoylation. In the presence of boric acid, D-glucose diethyl thioacetal yields the 6-benzoate (XLII). In the non-aqueous medium the formation of complexes... 

Lopez Garcia et al. [2] have described a rapid and sensitive spectrophotometric method for the determination of boron complex anions in plant extracts and waters which is based on the formation of a blue complex at pH 1 - 2 between the anionic complex of boric acid with 2,6-dihydroxybenzoic acid and crystal violet. The colour is stabilised with polyvinyl alcohol. At 600 nm the calibration graph is linear in the range 0.3-4.5 xg boron per 25 ml of final solution, with a relative standard deviation of 2.6% for xg/l of boron. In this procedure to determine borate in plant tissues, the dried tissue is treated with calcium hydroxide, then ashed at 400 °C. The ash is digested with 1N sulfuric acid and heated to 80 °C, neutralized with cadmium hydroxide and then treated with acidic 2,6-dihydroxybenzoic acid and crystal violet, and the colour evaluated spectrophotometrically at 600 nm. Most of the ions present in natural waters or plant extracts do not interfere in the determination of boron complex anions by this procedure. Recoveries of boron from water samples and plant extracts were in the range of 97 -102%. 

A number of solvent mixtures were studied by Burrows, Grylls and Harrison.123 These included a series of acetone-acid-water mixtures, and methanol-ammonium hydroxide. The advantages of these solvents are rapid flow of solvent and higher RF values than may be obtained with the solvents of Hanes and Isherwood. Cohen and Scott124 used an ethanol-boric acid solvent to take advantage of the selectivity of borate-complex formation. 

Under alkaline conditions, boric acid (or at this pH, boron tetrahydrate) forms complexes with diols 11) The formation of such a complex between two polysaccharide molecules can lead to crosslinking. The use of borax can, therefore, be expected to increase the stability of the bonds between the hemicellulose and between the hemicellulose and cellulose fibers in the paper. 

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