Reaction product of mixed inorganic base and acid resulting in dipotassium hydroxytetrafluoro triborontrioxide, potassium trihydroxy fluoroborate, dipotassium tetrahydroxy tetraboronpentaoxide dehydrate in powder form.

Endpoint summary

Additional information

Boron represents an essential plant micronutrient with an average total concentration of 10 mg/kg in the earth’s crust (Adriano, 2001). Dissolution of B-bearing minerals (e.g. tourmaline, muscovite), irrigation waters, fertilizers, atmospheric deposition of emitted B (e.g. coal fly ash) as well as the soils’ buffer capacity, affect the B concentration in soil. The natural level of B in soils largely depends upon the soil parent material. In general, soils derived from igneous rocks and those of tropical and semitropical regions of the world are considerably lower in B content compared with soils derived from sedimentary rocks and those of arid and semiarid regions. The content of total B in the latter group may range up to 200 mg/kg, particularly in alkaline, calcareous soils, while that for the former group is usually lower than 10 mg/kg (Swaine, 1955, cited in Adriano, 2001).

The oceans are the largest global reservoir for boron with a global average concentration of about 4.6 mg B/L (Argust, 1998, Park and Schlesinger, 2002). However, boron may range in concentration from 0.52 mg B/L in the Baltic Sea to 9.6 mg B/L in the Mediterranean Sea (Argust, 1998).

Natural events such as generation of seasalt aerosols over the ocean, biomass burning, rock weathering and volcanic activity are estimated to release 2 x 10^9 kg B/year (Park and Schlesinger, 2002) Formation of seasalt aerosols and their transfer to land represents the largest flux of boron from the sea to the terrestrial environment, estimated as 1.44 x 10^9 kg B/yr by Park and Schlesinger (2002). They estimate riverine transfer to the oceans to be about 0.58 Tg B/yr.

Most anthropogenic releases of boron to the environment are from global coal combustion, estimated as 2 x 10^8 kg B/yr (Park and Schlesinger, 2002). Boron produced from mining is estimated to be about 3.1 x 10^8 kg B/yr (Argust, 1998) with about half the processed boron being used in products that are unlikely to release boron to the environment (glass, fiberglass and ceramics) (Park and Schlesinger, 2002).

Most anthropogenic boron (excluding coal-related materials) in Europe originates from mines in Turkey and California. Ratios of the boron isotopes ¹¹B and ¹⁰B provide a tool to distinguish locally-derived boron from anthropogenic boron, although this has not been widely done (Vengosh et al., 1994; Chatelet and Gaillardet, 2005). ¹¹B separates preferentially into dissolved boron (i.e. boric acid), whereas ¹⁰B is preferentially incorporated into solid phase (Vengosh et al., 1994). The boron-11 isotope enrichment value (identified as δ¹¹B) ranges from about 39‰ in seawater, to about 0‰ in average continental crust, to -0.9 to +10.2‰ in sodium borate minerals from Turkey and California (Vengosh et al., 1994). The ratio has been used to identify anthropogenic boron fractions in surface waters (Chatelet and Gaillardet, 2005) and groundwaters (Vengosh et al., 1994, Kloppmann et al, 2005).

Justification for grouping of different borate compounds

This report covers dipotassium tetraborate (EC# 215-575-5).

In aqueous solutions at environmentally relevant concentrations and pH ranges, low concentrations of simple borates such as boric acid B(OH)3, potassium pentaborate (K2B10O16^.8H2O), potassium tetraborate (K2B4O7^.4H2O), disodium tetraborate decahydrate (Na2B4O7^.10H2O; borax), disodium tetraborate pentahydrate (Na2B4O7^.5H2O; borax pentahydrate), boric oxide (B2O3) and disodium octaborate tetrahydrate (Na2B8O13^.4H2O) will predominantly exist as undissociated boric acid. Above pH 9 the metaborate anion B(OH)4- becomes the main species in solution (WHO, 1998). The toxicokinetics and toxicological effects of boric acid, potassium tetraborate, potassium pentaborate, disodium tetraborate decahydrate, boric oxide and disodium octaborate tetrahydrate are likely to be similar on a boron equivalents basis. Therefore, the data obtained from studies with the borates can be read across in the health and environmental assessments for each individual substance.

For comparative purposes, exposures to borates are often expressed in terms of boron (B) equivalents based on the fraction of boron in the source substance on a molecular weight basis. As noted previously, only boric acid and the borate anion are present at environmentally and physiologically relevant concentrations. Read-across between the different boron compounds can be done on the basis of boron (B) equivalents (see section on dissociation constants). Conversion factors are given in the table below.

Conversion factors to boron equivalents

Substance	Formula	Conversion factor for equivalent dose of B (multiply by)
Boric acid	H3BO3	0.1748
Boric Oxide	B2O3	0.311
Disodium tetraborate anhydrous	Na2B4O7	0.2149
Disodium tetraborate pentahydrate	Na2B4O7•5H2O	0.1484
Disodium tetraborate decahydrate	Na2B4O7•10H2O	0.1134
Disodium octaborate tetrahydrate	Na2B8O13·4H2O	0.2096
Sodium metaborate (anhydrous)	NaBO2	0.1643
Sodium metaborate (dihydrate)	NaBO2·2H2O	0.1062
Sodium metaborate (tetrahydrate)	NaBO2·4H2O	0.0784
Sodium pentaborate (anhydrous)	NaB5O8	0.2636
Sodium pentaborate (pentahydrate)	NaB5O8∙5H2O	0.1832
Dipotassium tetraborate (anhydrous)	K2B4O7	0.185
Dipotassium tetraborate (tetrahydrate)	K2B4O7^.4H2O	0.1415
Potassium pentaborate (anhydrous)	B5KO8	0.244
Potassium pentaborate (tetrahydrate)	B5KO8^.4H2O	0.1843