Boron borate buffers

At room temperature, the pH of a sodium borate solution is 9.3. The buffering capacity of boric acid-borate solutions was evaluated at 350°C and 3500 psia by measuring the pH change during titrations with sodium hydroxide and hydrochloric acid[13]. The borate buffer titration experiments were conducted at 350°C and 24.1 MPa (3500 psia) where the water density is 0.622 g/mL and the pKa of boric acid is approximately 9.6. The concentration of sodium borate in the feed solutions was fixed at 6.25 X 10 m, which corresponded to a total boron concentration of 0.025 m. At 350°C, the measurable pH ranges of the various optical indicators are 9.0-11.0 for 2-naphthol, 7.0-9.0 for 2-naphthoic acid, 3.0-5.0 for collidine, and 2.0-4.0 for acridine. The indicator 2-naphthol was used to measure pH for the titration of the borate buffer with NaOH which began at pH of 9.5 and ended at 10.5. For the titration with HCl, 2-naphthol, 2-naphthoic acid, and collidine were used in series to measure pH values between 9.5 and 3.0. 

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,... 

Borates are slightly volatile. Evaporation and condensation from the atmosphere may circulate boron geochemically and may have contributed to a rather uniform distribution of B in soils worldwide. Boron s ubiquitous nature and buffering of boron concentrations by soils has apparently allowed life to evolve with a narrow range of boron sufficiency. 

However, boron also accumulated at the interface and borates are known to act also as corrosion inhibitors, particularly with steel and so there is a possible dual effect. It has also been postulated that borates may act by buffering the environment at the rubber-metal interface and so help to inhibit corrosion mechanisms. 

The element boron forms strong bonds with oxygen. The resultant borates include boric acid, boric oxide, alkali, ammonium, and alkaline earth metal borates. All except for the alkaline earth borates are reasonably water soluble. When dissolved borates release boric acid, a Lewis acid, and its conjugate base, the tetrahydroxyborate anion. Borax, a form of di-sodium tetraborate, releases equimolar amounts of these species and consequently is an excellent mildly alkaline pH buffer. Borates exhibit several important behaviors in solution, including the sequestration of water hardness ions, the ability to form borate esters with polyols, and to deliver the oxidant hydrogen peroxide into solution from the dissolution of sodium perborate. 

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