Registration Dossier

Diss Factsheets

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Disodium octaborate tetrahydrate was determined to be a white odourless crystalline solid. No melting point for disodium octaborate tetrahydrate can be defined in the range 25 - 1000 °C due to the decomposition of the substance. The relative density of disodium octaborate tetrahydrate is 1.87 at 22 ºC. Disodium octaborate tetrahydrate has a vapour pressure of 9.9E-17 Pa at 25 ºC. Disodium octaborate tetrahydrate is water soluble (223.65 g/L at 20 ºC). Disodium octaborate tetrahydrate is not flammable, is not a pyrophoric or a self-heating substance and emits no flammable gases on contact with water. Disodium octaborate tetrahydrate is exempt from 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 disodium octaborate tetrahydrate Foliarel QS which had a particle size (d50 mm) of 0.062 and represents the most representative grade placed on the market was dust category 3 - dusty. Foliarel which had a particle size (d50 mm) of 0.026 and which represents the finest grade placed on the market, and Polybor were dust category 3 - dusty. Disodium octaborate tetrahydrate had a mean particle size distribution d50 of 12.729 μm.The dissociation constant for disodium octaborate tetrahydrate as such cannot be determined because disodium octaborate terahydrate is converted into boric acid/borate upon dissolution in water:

Na2B8O13·4H2O + 9H2O ↔ 2NaB(OH)4+ 6B(OH)3.

The dissociation constant found will be the dissociation constant for boric acid in the presence of sodium ions.


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)3at 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)3and 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)3and 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)).

Additional information