Boric acid ionization

Boric acid ionizes in water in a similar fashion ... 

C17-0069. Determine the percent ionization of a solution of boric acid that is 75 mM (see AppendixE for K values). 

Anions of weak acids can be problematic for detection in suppressed IEC because weak ionization results in low conductivity and poor sensitivity. Converting such acids back to the sodium salt form may overcome this limitation. Caliamanis et al. have described the use of a second micromembrane suppressor to do this, and have applied the approach to the boric acid/sodium borate system, using sodium salt solutions of EDTA.88 Varying the pH and EDTA concentration allowed optimal detection. Another approach for analysis of weak acids is indirect suppressed conductivity IEC, which chemically separates high- and low-conductance analytes. This technique has potential for detection of weak mono- and dianions as well as amino acids.89 As an alternative to conductivity detection, ultraviolet and fluorescence derivatization reagents have been explored 90 this approach offers a means of enhancing sensitivity (typically into the low femtomoles range) as well as selectivity. 

Buffers contain mixtures of weak acids and their salts (i.e., the conjugate bases of acids), or mixtures of weak bases and their conjugate acids. Typical buffer systems used in pharmaceutical dosage forms include mixtures of boric acid and sodium borate, acetic acid and sodium acetate, and sodium acid phosphate and disodium phosphate. The reason for the buffering action of a weak acid, HA (e.g., acetic acid) and its ionized salt, A" (e.g., sodium acetate) is that A" ions from the salt combine with the added hydrogen ions, removing them from solution as undissociated weak acid. 

Kryukov, P.A. Starostina, L.I. Tarasenko, S.Ya Pavlyuk, L.A. Smolyakok, B.S. Larionov, E.G. "Ionization Constants of Carbonic Acid, Hydrogen Sulfide, boric Acid, and Sulfuric Acid at High Temperatures," Mezhdunar. Geokhim., Koagr. (Dokl.) 1st, 1971, 186-98 C.A, 1974, 84 (69193). 

For all the other halides, Eh-pH conditions have no influence. Boron occurs in water mainly as boric acid H3BO3 and its progressive ionization products at increasing pH. Redox conditions do not affect the speciation state of boron. 

In concentrated solutions boric acid forms a complex mixture of polyborate ions. In such solutions data obtained from titrations are complicated by the presence of these complex ions. Aqueous boric acid solutions below 0.2M seem to have negligible amounts of polyborate ions (14). Much of the early data reported in the literature was collected using more concentrated solutions (3). Thus, the ionization of boric acid can be described by the following equation ... 

Tetra(Hydrogen Sulfato)Boric Acid-Sulfuric Acid. HB(HS04)4 prepared by treating boric acid, B(OH)3, with sulfuric acid ionizes in sulfuric acid as shown by acidity measurements7 [Eq. (2.19)]. 

The parent acid, H3B03, functions as a weak acid in aqueous solution, possessing an ionization constant of about 6 x 10-, at 25°C in dilute solution. As its concentration increases, the ionization constant increases markedly. Kolthoff (220) investigated this effect by electrical conductivity and emf measurements, obtaining values of 4.6 x 1010 and 408 x 10 1 for boric acid concentrations of 0.1 M and 0.75 M respectively at 18°C. These results were attributed to the formation of tetraboric acid. 

Other studies into the apparent ionization constant of boric acid and interpretation of data are given by Sprague (392). Of greater relevance... 

Until recently, very little had been reported on the important area of metal borate complexation in aqueous solution. The effect of salts on the ionization of boric acid (358, 375) has been mentioned above, and subsequent research suggests that complexation of borate with, for example, calcium ions can account for the enhanced acidity of H O Literature on cationic complexes of boron was reviewed in 1970 (376). 

Ward G.K. and Millero F.J. (1974) The effect of pressure on the ionization of boric acid in aqueous solutions from molal volume data. J. Sol. Chem. 3, 417-430. 

Boron rejection membranes exhibit up to 90+% rejection of boron, while standard membranes reject about 50-70%.(20, 29) These membranes are typically used for seawater applications where boron removal is a concern. Boron is difficult to remove with membranes because boron, which exists as boric acid, is not ionized a typical seawater pH, 7.0 - 8.0, whereas the pKa of boric acid is 9.14 - 9.25.20... 

Boron, having the electronic configuration s22s22p, has 3 valence electrons, and forms planar, tricovalent derivatives that are electron deficient, and which, as Lewis acids, accept two electrons from bases to complete the boron outer-shell octet and give tetrahedral adducts. Boric acid exemplifies this behavior by ionizing, in aqueous solution, not by direct deprotonation, but by hydration and subsequent ionization, to give the symmetrical borate anion ... 

The ammonia can be distilled into an excess of standard sulfuric acid, and the excess determined by back titration. Preferably, the distilled ammonia can be absorbed into a solution of boric acid or other weak acid. Direct titration of the boric acid distillate with standard acid, using bromophenol blue, with a blank determination gives excellent results. As a routine method for determining traces of nitrogen, Milner and Zahner distilled the ammonia into a dilute boric acid solution. Urban suggested an aqueous solution of p-hydroxybenzoic acid (ionization constant 2.9 X 10 ) as an ammonia-absorbing solution. 

Reactor waters (separation of ionized silica and boric acid)... 

Boric acid and boric oxide and probably many other boron compounds are converted in sulfuric acid to boron tri (hydrogensulfate), B(HS04)3, which behaves as a strong acid, HBfHSO ). The evidence for the formation of this acid and its salts provides an interesting example of the application of the cryoscopic and conductimetric methods for determining modes of ionization ( 4). 

Silica sol, sodium hydroxide, lithium hydroxide, boric acid and sodium aluminate solutions were used as starting materials to prepare the reaction mixtures. After different crystallization times, the products were separated by filtration, washed with de-ionized water, ail-dried at 105°C and rehydrated in air. 

Write net ionic equations for the three ionizations of boric acid (H3BO3) in water. Include H2O in the three equations. Identify the conjugate acid-base pairs. 

Aussenac et al. (1998) used capillary zone electrophoresis with UV detection at 260 nm to analyze isoflavones in soybean seeds of various varieties grown in various locations. Methanol was used for extraction. Total extraction was not affected by temperature but was affected by the composition of the solvent. Electrophoresis was conducted at pH 10.5, at which the isoflavones were weak acids and were ionized. Boric acid was added to form a negatively charged borate-isoflavone complex. A fast capillary electrophoresis method was also developed by Vanttinen and Moravcova (1999) to determine daidzein and genistein after enzyme hydrolysis in soy products. Photodiode array was used to detect the isoflavones at 254 and 268 nm, respectively. Minimum detection was 0.4 mg/L. p-Nitrophenol was used as an internal standard. 

Buffer ions have a twofold purpose in electrophoresis they carry the applied current, and they fbc the pH at which electrophoresis is carried out. Thus they determine the kind of electrical charge on the solute, the extent of ionization of the solute, and therefore the electrode toward which the solute will migrate. The buffer s ionic strength determines the thickness of the ionic cloud (buffer and nonbuffer ions) surrounding a charged molecule, the rate of its migration, and the sharpness of the electrophoretic zones. With increasing concentration of ions, the ionic cloud increases in size, and the molecule becomes more hindered in its movement. For the separation of serum proteins, the barbital or tris-boric acid-EDTA buffers remain the most popular. 

Acidic eluents are used in separating weak acids (such as carboxylic acids) to repress their ionization and give sharp chromatographic peaks. However, water alone is often a suitable eluent for very weak acids such as carbonic acid and boric acid. 

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