Potassium metaborate

Water solubility

Administrative data

Endpoint:

water solubility

Data waiving:

study technically not feasible

Justification for data waiving:

the study does not need to be conducted because the substance is hydrolytically unstable at pH 4, 7 and 9 (half-life less than 12 hours)

Justification for type of information:

JUSTIFICATION FOR DATA WAIVING
According to REACH annex VII column 2, the study does not need to be conducted if i.a. the substance is hydrolytically unstable at pH 4, 7 and 9 (half-life less than 12 hours). Further, Annex XI addresses the waiving possibility if testing is technically not feasible: “Testing for a specific endpoint may be omitted, if it is technically not possible to conduct the study as a consequence of the properties of the substance: e.g. very volatile, highly reactive or unstable substances cannot be used, …. The guidance given in the test methods referred to in Article 13(3), more specifically on the technical limitations of a specific method, shall always be respected.” OECD TG 105 addresses the determination of the solubility in water of essentially pure substances which are stable in water.
In aqueous solutions at physiological and acidic pH, low concentrations of simple inorganic borates such as boric acid, disodium tetraborate decahydrate, disodium tetraborate pentahydrate, boric oxide and disodium octaborate tetrahydrate will predominantly exist as undissociated boric acid [WHO, Environmental Health Criteria 204, boron, World Health Organization, Geneva, 1998]. Most of the simple inorganic borates exist predominantly as undissociated boric acid in dilute aqueous solution at physiological pH [Hubbard SA, Biological Trace Element Research Vol. 66, 1998]. In aqueous solution, the metaborate ion is rapidly converted to the borate anion and the weakly dissociated boric acid by the sequential reactions shown by the following equations [Antia NJ, 1975, J. Fish. Res. Board Can. 32: 2487-2494]:
BO2- + 2 H2O → B(OH)4-
B(OH)4- + H3O+ ↔ B(OH)3 + 2 H2O
If metaboric acid (resp. borates) is dissolved in water, orthoboric acid is formed [Riedel, Anorganische Chemie, de Gruyter, 1999].
So summarizing, upon contact with water, potassium metaborate dissociates immediately into potassium and metaborate ions, whereas the latter is converted rapidly into boric acid. Hence, potassium metaborate is not stable in water with a half-life way below 12 hours, and hence, a proper conduction of a water solubility study according to OECD TG 105 is not possible. Hence, testing can be omitted.