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Physical & Chemical properties

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A melting point for diboron trioxide was determined to be higher than 906 °C. A boiling point not applicable for diboron trioxide. The relative density of diboron trioxide is 1.84 x 103 kg/m³ at 21.5 ± 0.5 ºC. Diboron trioxide is not flammable, is not a pyrophoric or self-heating substance and emits no flammable gases on contact with water. A saturated solution of diboron trioxide in water is a candidate for classification as a corrosive substance of UN Class 8, Packing group III (according to the UN Transport of Dangerous Goods Recommendations).

Studies on dustiness were performed according to CIPAC method MT 171 and showed that diboron trioxide (boric oxide ABA 60 mesh) which has a particle size (d50 mm) of 0.051, and boric oxide (ETI) which has a particles size (d50 mm) of 0.44, were dust category 1 - nearly dust free. This sample represents the most representative of those placed on the market. Diboron trioxide (Boric oxide 200 mesh) which has a particles size (d50 mm) of 0.044 was dust category 3 – dusty. This sample represents the finest grade placed on the market.

The dissociation constant for boric oxide as such cannot be determined because boric oxide is converted into boric acid/borate upon dissolution in water:

B2O3 + 3 H2O = 2B(OH)3. The dissociation constant found will be the dissociation constant for boric acid.

At low boron concentrations (B ≤ 0.025 M) the following equilibrium is found:

B(OH)3 + 2H2O ↔ B(OH)4- + H3O+ with pKa = 9.0 at 25 °C.

In dilute aqueous solutions (B ≤ 0.025 M) boric acid exists as undissociated boric acid B(OH)3 at pH < 7, at pH > 11 the metaborate ion becomes the main species in solution. In between values (pH 7 – 11) both species are present.

At higher boron concentrations (B > 0.025 M) an equilibrium is formed between B(OH)3, polynuclear complexes of B3O3(OH)4 -, B4O5(OH)42 -, B3O3(OH)52 -, B5O6(OH)4 - and B(OH)4 -. In short B(OH)3 ↔ polynuclear anions ↔ B(OH)4-.

In acid solutions at pH < 5, boron is mainly present as B(OH)3 and in alkaline solutions at pH > 12.5 boron is mainly present as B(OH)4-. At in between values (pH 5 – 12) polynuclear anions are found as well as B(OH)3 and B(OH)4-.

The dissociation constant depends upon temperature, ionic strength and presence of group I metal ions (Na, K, Cs).

In the presence of metal ions (e.g. Na, Mg, Ca) ion-pair complexes are formed, which further reduce the undissociated boric acid concentration:

Mn+ + B(OH)4- ↔ MB(OH)4(n-1)+

These ion pair complexes are expected to be present in solutions of disodium tetraborate, disodium octaborate and buffered solutions of boric acid and boric oxide (Ingri N (1963)).

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