Borate complexes tetraborates

Direct UV detection of alkylphosphonic acids through the formation of sodium borate complexes has been reported (9). While native alkylphosphonic acids exhibit negligible absorbance above 200 nm, borate complexation shifted their absorption maxima such that they were readily detected at either 214 nm or 254 nm. The buffer system consisted of 10 mM sodium tetraborate with complexation occurring in solution at pH 7 and higher. The reported detection limits on the order of nanograms. The UV, IR, NMR, and mass spectra of various borate/phosphonic acid esters were also reported. 

Fig. 4 Role of xylulose-borate complexation in the co-iimnobilized enzyme system. When sodium tetraborate (borax) is added to solution, it dissociates into tetrahydroxyborate (borate, B) ion and boric acid. In the pellet interior, higher pH favors tighter xylulose-borate binding, which effectively reduces the xylulose concentration in the interior and forces the isomerization forward. In the bulk, the lower pH has an uncoupling effect on the Xu-B complex, making the dissociated xylulose readily available to the yeast. Removal of xylulose via fermentation further forces dissociation of the xylulose-borate complex. Dashed lines represent transport of species solid lines represent reactions... 

Recently Isbell and coworkers have published the results of an extensive study of the behavior of solutions of sorbitol and D-mannitol in the presence of tetraborates. They found that sorbitol appears to form three complex borate compounds, whereas D-mannitol forms only two. Since the specific rotation in the tetraborate-D-mannitol system is a function of the ratio of the components and is independent of concentration at constant tetraborate-D-mannitol ratios, D-mannitol can be determined quantitatively by this method. However, sorbitol cannot be determined this way because the change in observed rotation at constant tetraborate concentration shows a reversal with increasing amounts of sorbitol. 

The principal source of compounds of boron is the complex borate minerals, including borax, sodium tetraborate decahydrate, Na2B407 IOH2O kemite, sodium tetraborate tetrahydrate, NaoB407 4H.20 (which gives borax when water is added) and colemanite, calcium hexaborate pentahydrate, CaoB. O, 51400. The main deposits of these minerals are in California. 

The borate buffer is necessary to complex the neutral carbohydrates to give them ionic properties that then allow separation by anion exchange chromatography. A sodium tetraborate-boric acid buffer (pH 8.5) whose composition varies from 0.169 to 0.845 Af in the borate ion is used as the eluent. The anion exchange separation column is maintained at a constant 55°C. 

The chemistry of borates is complex both in solution and in the melt. It is concluded from B n.m.r. and other studies that Na3B30g dissociates in solution to B(0H)4 ions and that at low concentrations tetraborates dissociate completely into B(0H)3 and B(OH)4, but that in more concentrated solutions of borates various polyborate ions coexist in equilibrium with one another. From melts extensive series of borates are obtained, the product depending on the composition of the melt, for example ... 

Each borate molecule should form the 1 2 complex. However, the viscosity of the pol5miers decreases at high borax concentrations. The authors attributed this fact to the increasing pH, while ZitUe (77) stated that the decrease must be due to the formation of 1 1-complexes. On the other hand it has to be realized that at high borax concentrations tetraborate anions are present to a large extent which might react in a completely different manner with the polysaccharides. 

The reaction of polyhydroxy compounds with boric acid or boronic acids has been used for derivatization and separation of carbohydrates and other compounds containing vicinal diols using different chromatographic and electrophoretic techniques (1,9,69). The mechanism of reaction is a complex between cis diol moieties and borate or boronate groups. It has been demonstrated that it is the borate ion, rather than boric acid, which is complexed by the polyol (70,71). The reaction is pH dependent and the optimum conditions are usually at pH > 8.0. In a pH ranging from 8 to 12, aqueous borate solutions contain tetrahydroxyborate ions and also more highly condensed polyanions such as triborate and tetraborate. Equilibrium between the different species depends on the pH and the total borate concentration. 

---